I a (alpha) Average percent­

A age change in resistance per degree of pure metal resis­tance of device between O°C and 100°C. Usually desig­nated by the Greek letter

I alpha, with units of W /W / 0C.

! aberration Deviation from ideal behavior by ( a lens, optical system, or optical component.

: i. Aberration exists in all optical systems, and ~. i designers must make trade-offs among the \] different types depending on how much .;t aberration they will tolerate.

ABI Application Binary Interface; to run without porting.

'... abnonnally distributed uncertainty In the ISA recommended practice ISA-RP67.04.02­2000 this term denotes uncertainties that do not have a normal distribution.

abort In data processing, to terminate a com­puter operation before its normal conclusion.

abrasion 1. The act of removing surface mate­rial from a substance through sliding or roll­

.. ing contact with the hard particles made of i the same or another substance. The particles

themselves may be loose or part of another .surface brought into contact with the first. 2. A surface blemish caused by roughening or scratching.

abrasive 1. Particulate matter, usually having sharp edges or points, that can be used to shape and finish workpieces in grinding, honing, lapping, polishing, blasting, or tum­bling processes. Depending on the process, abrasives may be loose, formed into solid shapes, glued to paper or cloth, or sus­pended in a paste, slurry, or air stream. 2. Any substance capable of removing material from a surface through the act of abrasion. 3. A material formed into a solid mass, usually fired or sintered, and used to grind or polish workpieces; common forms are grinding wheels, abrasive disks, honing sticks, cones, and burrs.

., ABS Acrylonitrile butadiene styrene; fre­quently used for enclosures; good high-tem­perature resistance with high impact strength and cold-temperature impact, good overall chemical resistance; can be adversely affected by ultraviolet.

;absolute accuracy error The deviation of the analog value at any code from its theoretical value after the full-scale range has been cali­brated. Expressed in percent, ppm, or frac­tions of 1 LSB.

. absolute address An address that indicates the exact storage location at which the refer­

enced operand is to be found or stored in the actual machine code address numbering sys­tem. It is synonymous with specific address and actual address and related to absolute code.

absolute alann An alarm caused when a sys­tem has detected a variable that has exceed­ed a set of prescribed high- or low-limit con­ditions.

absolute altimeter See terrain clearance indica­tor.

absolute altitude Distance between an air­craft or spacecraft and the actual surface of a planet or natural ~atellite.

absolute code Coding that uses machine instructions with absolute addresses. Synon­ymous with specific code.

absolute encoder An electronic or electrome­chanical device that produces a unique digi­tal output (in coded form) for each value of an analog or digital input. In an absolute rotary encoder, for instance, you can deter­mine directly the position following any incremental movement, without having to refer to the starting position.

absolute feedback In numerical control, the action of assigning a unique value to each possible position of a machine slide or actu­ating member.

absolute humidity The weight of water vapor in a gas-water vapor mixture per unit volume of space occupied, as, for example, grains or pounds per cubic foot.

absolute instrument An instrument that determines the value of a measured quantity in absolute units by making a simple physi­cal measurement.

absolute measurement A measured value that is expressed in terms of fundamental standards of distance, mass, and time.

absolute pressure 1. The pressure measured relative to zero pressure (vacuum). 2. The combined local pressure that is induced by some source and the atmospheric pressure at the location of the measurement. 3. Gauge pressure plus barometric pressure expressed in the same units of measurement.

absolute programming In numerical control, the process of using a single point of refer­ence to determine all positions and dimen­sions.

absolute stability A linear system is abso­lutely stable if there exists a limiting value of the open-loop gain such that the system is stable for all lower values of that gain and unstable for all higher values.

absolute value error The magnitude of the error disregarding the algebraic sign or, if a vectorial error, disregarding its direction.

3

I absolute viscosity / accelerometer

absolute viscosity A measure of the internal shear properties of fluids. It is expressed as the tangential force per unit area at either of two horizontal planes separated by one unit thickness of a given fluid, with one of the planes fixed and the other moving with unit velocity.

absorbance An optical property expressed as log (l/T), where T is the transmittance.

absorptance The fraction of the incident light absorbed by something.

absorption [Sci) 1. The reduction in intensity of a beam of electromagnetic or particulate rJdiation as it passes through matter. This reduction is chiefly due to interactions with atoms or electrons or with their electric and magnetic fields. [Comm] 2. In fiber-optic cable, the loss of power resulting from the conversion of optical energy into heat. This loss is usually caused by impurities such as transition metals and hydroxyl ions. .

absorption band A region of the electromag­netic spectrum in which a given substance exhibits a high absorption coefficient com­pared to adjacent regions of the spectrum.

absorption coefficient An inherent material property that is expressed as the fractional loss in radiation intensity per unit mass or per unit thickness. It is determined over an infinitesimal thickness of the given material at a fixed wavelength and bandwidth.

absorption curve A graph showing the varia­tion of transmitted radiation through a fixed sample while the wavelength material of a given thickness is changed at a uniform rate.

absorption dynamometer A device for mea­suring mechanical force or power by con­verting the mechanical energy into heat in a friction mechanism or bank of electrical resis­tors.

absorption-emission pyrometer An instru­ment for determining gas temperature by measuring the radiation emitted by a cali­brated reference source both before and after the radiation passes through the gas, where it is partly absorbed.

absorption hygrometer An instrument for determining the water vapor content of the atmosphere by measuring the amount of water absorbed by a hygroscopic chemical.

absorption meter An instrument for measur­ing the quantity of light transmitted through a transparent medium by means of a photo­cell or other light-detecting device.

absorption spectroscopy The study of the wavelengths of light that are absorbed by materials and the relative intensities at which different wavelengths are absorbed. This

technique can be used to identify materials and measure their optical densities.

AC Altemating current; electric current that reverses its direction at regularly recurring intervals, s~ch as 60 times/second (60 Hz).

accelerated hfe test A method for estimating the .reli~bil~ty or dura~ility of a product by subjectIng It to operatmg conditions above its maximum ratings.

accelerating agent 1. A ?ubstance that increa­S~5.a chemical te,a.ction rate. 2. A chemical that h~stt~ns the curing of rubber, plastic, cement or adhesives, and may also improve their properties. Also known as accelerator.

accelerating electrode An auxiliary electrode in an electron tube that is maintained at an applied potential so as to accelerate electrons in a beam.

acceleration The time rate of change of veloc­ity; the second derivative of a distance func­tion with respect to time.

acceleration error The maximum difference at any measured value within the specified' range, between output readings taken with and without the application of specified con­stant acceleration along specified axes. Note: See transverse sensitivity when applied in the context of acceleration transducer.

acceleration limit The maximum vibration and shock acceleration that the transducer can accept in either direction along its sensi­tive axis without permanent damage. This limit is usually stated as ± -8's. The acceleration limits are usually much wider than the acceleration range and thereby rep­resent a measure of the overload capability of the transducer. [ISA/RP-37.2 (R1995)]

acceleration range The range of accelerations over which the transducer has the specified linearity. [ISA/RP-37.2 (R1995)]

acceleration time [Camp] 1. The amount of time it takes a mechanical component of a computer to go from rest to running speed. [Sci] 2. The measurement of the time required for any object to reach a predeter­mined speed.

accelerators A feature of some computer applications to create keyboard commands that provide altematives to screen menus. for selecting choices. The keystroke has speCIal meaning within that particular application,

. " which means more involved, but usually more "user- friendly," procedures can be saved for making requests or entries.

accelerometer 1. An instrument for measur­ing acceleration or an accelerating force su.ch as gravity. If the instrument includes'p!oVl­sions for making a recorded output, It 15

4

I acceptance angle / accumulator

called an "accelerograph," 2, A transducer used to measure linear or angular accelera­tion.

acceptance angle In fiber-optic cable, the half angle of cone within which incident light is totally reflected internally by the fiber core,

acceptance test A series of tests used to demonstrate the capabilities and workability of a new system and usually conducted by

: j the manufacturer to show the customer that , ~ the system is in working order.

~. access The state in which it is permissible to .;~ place information into, or retrieve informa­\, tion, from a storage device. ,r access, direct storage (DSA) The procedure

j,

whereby data are transferred to or from stor­age that is essentially coincident with the normal operation of a computer, without dis­turbing the central processing unit registers. [ISA/RP-55.1-1975 (R1983)] , access, random 1. The process of obtaining

, data from, or placing data into, storage such that the time required for such access is inde­pendent of the location of the data most recently obtained or stored, 2, A type of stor­age device in which the access time is effec­

;~ tively independent of the location of the ! c,

data. [ISA/RP-55.1-1975 (R1983)] access, serial The process of obtaining data

~

from, or placing data into, storage when ,! there is a sequential relation governing the t C time it takes to access successive storage I locations. [ISA/RP-55.1-1975 (R1983)] I, access code A group of alphanumeric charac­j, ters that identifies the user to the system so

information can be placed or retrieved byi' other devices in the system. ; J access line The portion of a leased communi­J cation line, such as in a telephone system,

that permanently connects the user with the

'I" serving central office or wire center.

~ access method Any of the data-management techniques available to the user for transfer­

t ring data between main storage and an input/output device.", access privilege The right or permission to i.

~ access (read or write) a file that is granted by1 I the processor after such permission has been 1 requested.

access procedures The procedure by which~ the devices attached to the network gain~ access to the medium. The access procedure" typically includes provisions for guarantee­

ing fairness in the sharing of the network bandwidth between attached devices. The most common access procedures for LANs are CSMA/CD, token bus, token ring, and slotted ring. See MAC.

access time 1. The interval between a request for stored information and the delivery of that information; often used to refer to the speed of memory. 2. The time interval that is characteristic of a storage unit. A measure of the time required to locate information in a storage position and make it available for processing or to return information from the processing unit to a storage location.

access unit interface (AUI) The optional interface between a data station that uses an IEEE 802.3 LAN and a transceiver or modem. The AUI makes it possible to transparently connect a data station to either baseband or broadband media.

accessible 1. A system feature that is view­able by and interactive with the operator, and allows the operator to perform user-per­mitted control actions, for example, make set point changes and auto-manual transfers or perform on-off action. 2. The capacity to be touched with a standard test finger or test pin, when used"as 'specified in Section 6.2 of ANSI!ISA-82.02.01-1999 (IEC 1010-1 Mod). [ANSI!ISA-82.02.01-1999 (IEC 1010-1 Mod)]

accessible area An area that is routinely or periodically entered by plant personnel in their performance of routine functions dur­ing normal plant operation and in accor­dance with applicable health physics procedures. [ISA-67.03-1982]

accessible isolation valve The isolation valve that is nearest the measured process on an instrument-sensing line, which is available to personnel during normal plant operation. The root valve mayor may not perform the function of the accessible isolation valve, depending on where it is located. [ANSI! ISA-67.02.01-1999]

accessible part A part that can be touched during normal use or operator servicing. [ANS/lSA-82.01.01-1999 (IEC 1010-1 Mod)

accessible surface A surface to which a flam­mable or combustible mixture has access. [ANSI/lSA-12.01.01-1999]

accessible terminal A node in an electronic network that is configured to allow it to be connected to an external circuit.

accessory A peripheral device that supports a main system function, such as a floppy disk drive or printer.

Accredited Standard Committee A standard committee accredited to ANSI.

accumulator 1. The register and associated equipment in the arithmetic unit of the com­puter in which arithmetical and logical oper­ations are performed. 2. A unit in the digital computer where numbers are totaled, that is,

5

I accuracy / accuracy rating

accumulated. Often the accumulator stores one operand and when it receives any sec­ond operand it performs the indicated opera­tions on them and then forms and stores the result. It is related to an adder. [Proc] 3. A pressure vessel containing water and steam that is used to store the heat of the steam for use at a later time and at some lower pres­sure: 4. A relatively large-volume chamber or other hydraulic device that receives fluid lmder low hydraulic power, stores it, and then discharges it at high hydraulic power,

, I

six-place numerals; nevertheless, a properly computed four-place numeral might be more accurate than an improperly computed six-place numeral. [ANSI/ISA-51.1-1979 (R1993)]

accuracy, mean (data processing) Mean accu­racy is precisely defined as (100 - E) % FR., where the mean error E is expressed as a per­centage of full range (FR.). It is common practice, however, to equate mean accuracy with the value of tl1e m.~an error. That is, mean accuracy is cornmonly stated as OJ %

~ after which it is ready to repeat the cycle. 5. A chamber or vessel for storing low-side liquid refrigerant in a refrigeration system. 6. An accumulator is also referred to as a receiver, a "reflux receiver", or a "reflux drum."

accuracy 1. The ratio of the error to the full­scale output or the ratio of the error to the

FR. whereas a more precise and acceptable statement is that mean accuracy is 99.9% FR. [ANSI/ISA-51.1-1979 (R1993)]

accuracy, measured The maximum positive and negative deviation observed when testing a device under specified conditions and by a specified procedure. Note 1: Measured accu­

l

j!j

Jj

jIoutput, as specified, expressed as a percent­ racy is usually measured as an inaccuracy and ~

age. Note 1: Accuracy may be expressed in terms of units of measurand or as being within ±[number] percent of full-scale out­put. Note 2: The term accuracy should be used only for generalized descriptions of characteristics. It should not be used in speci­fications. The term error is preferred in speci­fications and other specific descriptions of transducer performance. [ISA-37.1-1975 (R1982)] 2. In process instrumentation, the degree of conformity of an indicated value to a recognized, accepted standard value or ideal value. [ANSI/ISA-51.1-1979 (R1983); AI\;SI/ISA-75.05.01-2000] 3. The degree to which an indicated value matches the actual value of a measured variable. 4. Quantita­tively, the difference between the measured value and the most probable value for the

expressed a~ accu.racy. Note 2: Measured accuracy is fYPk4l11y expressed in terms of the measured vanable, percentage of span, per­centage of upper-range value, percentage of scale length, or percentage of actual output reading. [ANSI/ISA-51.1-1979 (R1993)]

accuracy, reference See accuracy, rating. accuracy, total (data processing) Total accu­

racy is precisely defined as (lao - Emax) % P.5. where the maximum error Emax is expressed as a percentage of full-scale value. Total accuracy is a measurement of the worst-case effect of all the errors present in the analog subsystem. [ISA/RP-55.1-1975 (R1983)]

accuracy measured test The procedure for this test is described in ISA Standard ANSI/ ISA-51.1-1979 (R1993).

II

same quantity, when the latter is determined from all available data and critically adjusted for sources of error. 5. The deviation or error by which an actual output varies from an expected ideal or absolute output. Each ele­ment in a measurement system contributes to errors, which should be separately speci­fied if they contribute significantly to the degradation of total system accuracy. 6. In an analog-to-digital converter, accuracy is tied to resolution. A 13-bit A/D, as used in the controller, for example, can resolve to one part in 213 or 8,192, so the best accuracy as a percentage of full-scale range is theoretically 1/8192, or about 0.0125 percent.

accuracy (data processing) The degree of freedom from error, that is, the degree of con­

accuracy rating In process instrumentation, a number or quantity that defines a limit that errors will not exceed when a device is used under specified operating conditions. Note 1: When operating conditions are not specified, reference operating conditions shall be assumed. Note 2: As a performance specifi­cation, accuracy (or reference accuracy) shall be assumed to mean the accuracy rating of the device when used at reference operating conditions. Note 3: Accuracy rating includes the combined effects of conformity, hystere­sis, dead band, and repeatability errors. The units being used should be stated explicitly. It is preferable to use a ± sign before the number or quantity. Accuracy rating can be expressed in a number of forms. The follow­

formity to truth or to a rule. Accuracy is con­trasted with precision. For example, four-place numerals are less precise than

ing five examples are typical: (a) Accuracy rating can be expressed in terms of the mea­sured variable. Typical expression: The accu­

a

6

I

1

I AC/DC coupling I acoustic inertance

j

! . !

~ .

,

racy rating is ± (C, or ± iF. (b) Accuracy rating can be expressed in percentage of span. Typical expression: The accuracy rating is ±-0.5% of span. (This percentage is calcu­lated using scale units such as degrees F, psig, etc.). (c) Accuracy rating can be expressed in percentage of the upper-range value. Typical expression: The accuracy rat­ing is ± 0.5% of upper-range value. (This per­centage is calculated using scale units such as kJ'a,.degrees F, etc.). (d) Accuracy rati,og is expressed in percentage of scale length. Typi­cal expression: The accuracy rating is ± 0.5% of scale length. (e) Accuracy rating is expressed in percentage of actual output reading. Typical expression: The accuracy rating is ± 1% of actual output reading. [ANSI/ISA-51.1-1979 (R1993)]

AC/DC coupling A selection on the front panel of some readout instruments. The AC­coupling position switches a capacitor into the input conductor. This is used for piezo­electric accelerometers to remove the power supply bias voltage. The DC-coupling posi­tion removes this capacitor from the input line. This is used for transducers that have an output at zero frequency (such as piezoresis­tive accelerometers and proximity probes), and it is desired to make measurements below 1 Hz.

ACE Advanced Computing Environment ini­tiative; alliance of more than twenty firms who support software standards for MIP architecture based on common ABI and API. Also known as asynchronous communica­tions elements.

achromatic Optical elements that are designed to refract light of different wave­lengths at the same angle. Typically, achro­matic lenses are made of two or more components of different refractive index and are designed to be uses at visible wave­lengths only.

acid cleaning The process of cleaning the interior surfaces of steam-generating units by using an inhibitor so as to prevent corro­sion and then subsequently draining, wash­ing, and neutralizing the acid with a further wash of alkaline water.

acid wash A chemical solution that contains phosphoric acid, which is used to neutralize residues from alkaline cleaners and to simul­taneously produce a phosphate coating that protects a surface of metal from rusting and prepares it for painting.

acidity Represents the amount of free carbon­dioxide mineral acids and salts (espeCially sulphates of iron and aluminum) that

7

hydrolize to yield hydrogen ions in water. Acidity is reported as milliequivalents per liter of acid, ppm acidity is reported as cal­cium carbonate, and pH is reported as the measure of hydrogen ion concentration.

acid-resistant Able to withstand chemical attack by strongly acidic solutions.

AC input module An I/O module that con­verts process-switched AC into logic levels for use in the Pc.

ACK A. transmission.control character that is transmitted by a receiving device as an affir­mative response to a sending device. Also see acknowledge.

acknowledge 1. The sequence action that indicates that a new alarm has been recog­nized. [ANSI/ISA-18.1-1079 (R1992)] 2. A message sent between peer entities to indi­cate that data was properly received.

Acme screw thread A type of power-trans­mission thread tha t is made in four series­29° general purpose, 29° stub, 60° stub, and 10° modified square. The number of threads per inch is not standardized according to shank diameter.

acoustic Related to sound. acoustic compliance The reciprocal of acous­

tic stiffness. acoustic coupler A type of communications

device that converts digital signals into audio tones that can be transmitted over the public telephone network through a conven­tional handset.

acoustic dispersion The separation of a com­plex sound wave into its various frequency components. Acoustic dispersion is usually caused by variation of the wave velocity in the medium that has the sound frequency. It is usually expressed in terms of the rate of change of velocity with frequency.

acoustic generator A transducer for convert­ing electrical, mechanical, or some other form of energy into sound waves.

acoustic holography A technique for detect­ing flaws or regions of inhomogeneity in a part by subjecting it to ultrasonic energy. This produces an interference pattern on the free surface of water in an immersion tank. Laser holography can then read the interfer­ence pattern to produce an image of the test object.

acoustic impedance The complex quotient that is obtained by dividing the sound pres­sure on a surface by the flux through the sur­face.

acoustic inertance A property that is related to the kinetic energy of a sound medium, which equals Za/26j, where Za is the acoustic

_

acoustic interferometer I activatIon analY~l~

I

IfI'

j

I i'

reactance andf is sound frequency. The usual units of measure are g/cm4

. Also known as acoL/stic mass.

acoustic interferometer An instrument for measuring either the velocity or frequency of sound pressure in a standing wave that has been established in a liquid or gas medium between a sound source and a reflector as the reflector is moved or the frequency is varied.

acoustic radiometer An instrument that mea­sures sound intensity by determining unidi­rectional steady-state pressure when the­sound wave is reflected or absorbed at a boundary.

acoustic reactance The imaginary component of acoustic impedance.

acoustic resistance The real component of acoustic impedance.

acoustic sensitivity The output of a trans­ducer (not caused by rigid body motion) in response to a specified acoustical environ­ment. This sensitivity is sometimes expressed as the acceleration in g rms that is sufficient to produce the same output that is induced by a specified sound pressure-level spectrum with an overall value of 140 dB referred to as 0.0002 dyne per sq em rms. [ISA-RP37.2-1982 (R1995)]

acoustic signature In sonar applications, the profile that is characteristic of a particular undersea object (or class of objects), for example, the profile of a school of fish or a sea-bottom formation.

acoustic spectrometer An instrument for analyzing a complex sound wave by deter­mining the volume (intensity) of sound-wave components that have different frequencies.

acoustic stiffness A property related to the potential energy of a medium or its bound­aries that equals 2 <3 fla' where Z" is the acoustic reactance and f is sound frequency. The usual units of measure for acoustic stiff­ness are dyne/ems.

acoustical ohm The unit of measure for acoustic resistance, reactance, or impedance. It equals unity when a sound pressure of one microbar produces a volume velocity of one cubic centimeter per second.

acoustics 1. The technology associated with the production, transmission, and utilization of sound, and the science associated with sound and its effects. 2. The architectural quality of a room-especially a concert hall, theater, or auditorium-that influences the ability of a listener to hear sound clearly at any location.

acousto-optic An interaction between an acoustic wave and a lightwave passing through the same material. Acousto-optic devices can be used for the deflection or modulation of beams, for signal processing, and for Q switching.

acousto-optic glass Glass whose composition is designed to maximize the acousto-optic effect.

AC output module An I/O module that con­verts PC logic levels into output switch action for AC.load control.. ;" .

SIGNALFROM CPU ISOLATOR

AC OUTPU

acquisition time The interval in which a saml2kHmd;.hold circuit acquires an input signal to within specification.

acronym A word formed from the initial let­ters of a long or complex technical term. For example, ~M is the acronym for "random access memory."

acrylonitrile butadiene styrene (ABS) A plas­tic frequently used for enclosures because it provides good high-temperature resistance with high-impact strength and cold-tempera­ture impact as well as good overall chemical resistance. ABS can be adversely affected by ultraviolet. See ABS.

ACSE Association control service element; for ISO layer 7/ for MAP 3.0.

ACT Acoustic charge transport; technology that processes signals in their natural analog domain, taking advantage of the 10,000-to-1 simplification that results when using analog rather than digital circuits. Converts analog input signal into discrete- time signal, sam­pled in timet not quantitized in amplitude.

actinicity The ability of radiation to induce chemical change.

actinometer 1. An instrument for measuring the actinic quality of radiation-that is, its relative ability to induce chemical change. 2. An instrument for measuring the flux den­sity of solar radiation.

action, air-to-close See fail-open. action, air-to-open See fail-close. action object An object that supports the

instantiation and deletion of blocks and objects within a resource. [ISA-TR50.02, Part 9-2000]

activation analysis A method for determin­ing composition, especially the concentration

8

----------_._--- ~- -- ....

I active alarm point / actuator, diaphragm type

.. i'

, I'

' .. j

FI> t. .~

of trace elements. The composite substance is bombarded with neutrons, and the wave­lengths and intensities of the characteristic gamma rays emitted from the activated nuclides are measured.

active alarm point See alarm point. active application The software application

on a computer that the keyboard is now focused in a Windows environment. Usually, it is the "top" window that is "open."

active device Any component, device, or cir­cuit that introduces gain or has functional direction. The active device is usually consid­ered to be any device except pure capaci­tance, inductance, resistance, or combina­tions of these. In current loop applications, the active device is a device capable of sup­plying current for the loop.

active medium The material in a laser that produces the amplified stimulated emission. The name of the laser identifies the active medium.

active transducer A transducer whose output waves are produced by power derived from a source other than any of the actuating waves, but whose output power is controlled by the actuating waves.

active window The window that appears "on top" of other windows in a Windows com­puter environment, thereby overlapping and obscuring from view other windows (appli­cations) that are also open. The active win­dow is the window that is currently in use. See active application.

active-matrix liquid crystal displays (AMLCDs) A liquid crystal display (LCD) technique in which the pixels on the screen are controlled by voltage signals applied in rows and columns. An array of thin film transistors (TFIs), with one transistor per pixel, keeps the pixels energized at all times so there is no need to reenergize on each scan. Thus, the pixels respond faster and are brighter than passive-matrix liquid crystal displays (PMLCDs). Compare with passive­matrix liquid crystal displays.

ActiveX Short for "active component exten­sion." A binary reusable software object (COM component) that plugs into object linking and embedding (OLE) software, thereby allowing different software packages to communicate and interact with one another in a networked environment. This makes it possible to integrate plant floors using the Internet and intranets. Since the adv2nt of Internet, Microsoft has preferred the term ActiveX over OLE because of the

9

former's expanded scope (and marketing appeal).

activity [Comm]1. The presence of a signal or noise at the input terminals of a fieldbus device in which the signal or noise is higher than the receiver signal level threshold of that device. [ANSI/ISA-50.02, Part 2-1992] [Sci] 2. The ratio of the escaping tendency of the component in a solution to the escaping tendency of a standard state. The ion concen­tration multiplied by an activity coefficient is equal to the activity. .

activity-based costing An information sys­tem that maintains and reports data on the activities, products, and processes of a plant or company.

actual address See absolute address. actual flow The actual volume of liquid pass­

ing through the flowmeter in a unit time. It is computed by applying all necessary correc­tions for the effects of temperature, pressure, air buoyancy, and the like to the correspond­ing readings indicated by the calibrator. [ISA-RP31.1-1977]

actuate To put into action or motion. actuating error signal See signal, actuating

error. actuation signal The set point minus the con­

trolled variable at a given instant. Same as error.

actuator 1. A pneumatic, hydraulic, or electri­cally powered device that supplies force and motion so as to position a valve's closure member at or between the open or closed position. [ANSI/ISA-75.05.01-2000]2. A part of the final control element that translates the control signal into action by the final control device in the process. Typical examples are motors, solenoids, cylinders, and so on. 3. A device responsible for actuating a mechani­cal device such as a control valve. 4. A device that actuates.

actuator, bellows type A fluid-powered device in which the fluid acts upon a flexible convoluted member, known as the bellows, so as to provide rotary motion to the actuator stem. [ISA-75.01-1985 (R1995)]

actuator, diaphragm type A fluid-powered device in which the fluid acts upon a flexible member, termed the diaphragm, so as to pro­vide linear motion to the actuator stem. [ANSI/ISA-75.05.01-2000]

actuator, double-acting I adaptive control, adaptive tuning

PRESSURE CONNECTION ,>-<-_~I::...----.... (VENT FOR REVERSE ACTION)

DIAPHRAGM PLATE

DIAPHRAGM

SEAL VENT (PRESSURE CONNECTION FOR REVERSE ACTION)

ACTUATOR SPRING

I. --' - ACTUATOR STEM

Actuator, Diaphragm Type

actuator, double-acting An actuator in which the power supply acts both to extend and retract the actuator stem. [ISA-75.01-1985 (R1995)]

actuator, electric type A device that converts electrical energy into motion. [ANSI/ISA­75.05.01-2000]

actuator, electrohydraulic type A self-con­tained device that responds to an electrical signal by positioning an electrically operated hydraulic pilot valve so that it allows pre~­surized hydraulic fluid to move an actuahng piston, bellows, diaphragm or fluid motor in order to position a valve stem. [ANSI/ISA­75.05.01-2000]

actuator, electromechanical type A device that uses an electrically operated, motor­driven gear train or screw to position the actuator stem. Such actuators may operate in response to either analog or digital electrical signals. The electromechanical actuator is also referred to as a "motor gear train actua­tor." [ANSI/ISA-75.05.01-2000]

actuator, fluid motor type A fluid-powered device that uses a rotary motor to move the actuator stem. [ANSI/ISA-75.05.01-2000]

actuator, hydraulic type A fluid-powered device that converts the energy of an incom­pressible fluid into motio~. .

actuator, piston type A flUid-powered deVice in which the fluid acts upon a movable cylin­drical m:ember, termed a piston, so as to pro­vide linear motion to the actuator stem. [ANSI/lSA-75.05.01-2000]

actuator, pneumatic A device that converts the energy of a compressible fluid, usually air, into motion.

actuator, single-acting An actuator in which the power supply acts in only one direction. In a spring and diaphragm actuator, for example, the spring acts in a direction oppo­site to the diaphragm thrust. Single-acting spring and diaphragm actuators may be fur­

Il ther classified in terms of the direction of the

stem movement on the increasing fluid pres­sure: (a) air to extend the actuator stem and (b) air to retract the actuator stem.[ANSI/ ISA-75.05.01-2000]

actuator, vane type A fluid-powered device in which the fluid acts upon a movable piv­oted member, known as the vane, so as to provide rotary motion to the actuator stem. [ANSIjISA-75.05.01-2000]

..actuator effective area The net area of a pis­ton, bellpws, ,\~ne, or diaphragm that is acted on by fluid .pressure in order to gener­ate actuator output force. [ANSI/ISA­75.05.01-2000]

actuator environment 1. The temperature, pressure, humidity, radioactivity, and corro­siveness of the atmosphere surrOl.mding the actuator. 2. The mechanical and seismic vibration transmitted to the actuator through the piping or heat that is radiated toward the actuator from the valve body. [ANSI/ISA­75.05.01-2000]

~ctuator stem or actuator shaft The part that connects the actuator to the valve stem or shaft and transmits motion (force) from the actuator to the valve. Rotary-motion valves have shafts,·while linear-motion valves gen­erally have stems. [ANSI/ISA-75.05.01-2000]

actuator travel time See stroke time. ACU Automatic calling unit; Dialing device

that permits a business machine to automati­cally dial calls over a communications net­work (auto answer j dial modem).

Ada A Pascal-based, real-time systems pro­gramming language developed by CII-Hon­eywell Bull under a contract to develop weapons system tracking for the United States Department of Defense. It has since been used throughout the federal govern­ment for applications far beyond this initial purpose.

adapter Device that makes possible compat­ibility between different equipment.

adapting See self-adapting. adaptive control 1. A control system that

uses its previous experience to adjusts its response to its inputs. 2. Automatic means used to change the type or influence (or both) of control parameters in such a way as to improve the performance of the control system. [ANSI/lSA-51.1-1979 (R1993)]. See control, adaptive.

adaptive control, adaptive tuning 1. Contin­uously adjusting the gain (proportioning action) of the control loop from a signal external to that loop. Sometimes other parameters are (also) modified, particularly integral (reset action). 2. When referring to

10

adaptive dithering / adhesive bonding

advanced control techniques, the term has come to have the broader connotation of a system of advanced process control that is capable of automatically adjusting (adapt­ing) itself to meet a desired output despite shifting control objectives and process condi­tions or unmodeled uncertainties in the pro­cess dynamics. Such control is often performed through neural networks and/or fuzzy logic coupled with traditional PID­type algorithms.

adaptive dithering A form of dithering for displaying graphics on a system that has a different- sized palette. In this form of dither­ing, the program looks to the overall image on the system to determine the best set of colors or shapes.

adaptive optics Optical components that can be made to change the way in which they reflect or refract light. In practice, the term usually means mirrors with adjustable sur­face shapes.

adaptive system A system that displays the ability to learn, change state, or otherwise react to stimuli. It is capable of adapting itself to changes in its own environment.

adaptor bushing The part that attaches a close-coupled diaphragm actuator to the bonnet of the diaphragm valve body. [ANSI/ ISA-75.05.01-2000)

ADCCP Advanced Data Communications Control Procedure; bit-oriented communica­

'/j tions protocol standard defined by ANSI. AID converter (ADO A hardware device

that converts analog data into digital form; also called an "encoder."

add time The time required for one addition, not including the time required to get and return the quantities from storage.

ADD 1. See false add and OR. 2. See sum. adder A device that forms, as output, the

sum of two or more numbers that are pre­sented to it as inputs. Often such devices include no data-retention feature, that is, the output signal remains only as long as the input signals are present. Related to accumu­lator.

This capability is one of the powerful bene­fits of a good distributed control system.

address 1. An identification, represented by a name, label, or number, of a register or loca­tion in storage. Addresses also are part of an instruction word along with commands, tags, and other symbols. 2. The part of an instruction that specifies an operand for the instruction.

address bus The highway that links the sub­components of the ~crocomputersystem along which addniss data are transferred.

address field That part of an instruction or word that contains an address or operand.

address format The arrangement of the address parts of an instruction.

addressing The means whereby the origina­tor or control station selects the unit to which it is going to send a message.

addTessing mode Method for addressing a location that is used for data storage.

address modification The hardware action in which an instruction's effective operand address is computed by some sequence of the following two operations (as prescribed within the instruction): (a) indexing--adding an index to the address; (b) indirect address­ing--using the intermediate computed address to obtain another address from memory.

address register A register in which an address is stored.

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address resolution The process of relating a logical address to a physical address.

add time The time required for one addition, not including the time required to get and return the quantities from storage.

adequately ventilated area An area that has a ventilation system (natural or artificial) that, at a minimum, prevents the accumulation of gases or vapors to an explosive level. Most standards and recommended practices rec­ommend having systems that prevent levels in excess of 25 percent of the lower flamma­ble (explosive) limit (LFL or LEL). Note: Adequate ventilation of an area is by itself not an effective means for preventing dust

I

'; adder-subtrador A device whose output is a representation of either the arithmetic sum or difference, or both, of the quantities repre­sented by its operand inputs.

additive primaries In color reproduction, red, green and blue. When lights of these col­ors are added together, they produce the sen­sation of white light.

add-Oil A component or device that is added to a system to increase storage capacity, mod­ify architecture, or upgrade performance.

explosions. [ISA-12.01.01-1999) adhesion 1. A bonding between two surfaces

that is usually applied to localized welding at high points under substantial contact pres­sures. 2. Bonding between two surfaces, assisted by an adhesive substance.

adhesive Any substance capable of bonding two surfaces together.

adhesive bonding A commercial process for fastening parts together in an assembly by

11

I adhesive strength / aerosol

using only glue, cement, resin, or other adhe­sive.

adhesive strength The strength of an adhe­sively bonded joint, usually measured in ten­sion (perpendicular to the plane of the bonded joint) or in shear (parallel to the plane of the joint).

adiabatic Taking place without any exchange of heat between the process system and another system or its surroundings.

adiabatic temperature The theoretical tem­perature that would be attained by the prod­ucts of combustion provided that the entire chemical energy of the fuel, the sensible heat content of the fuel, and the combustion air above the datum temperature were trans­ferred to the products of combustion. This assumes that (a) combustion is complete, (b) there is no heat loss, (c) there is no dissocia­tion of the gaseous compounds formed, and (d) inert gases play no part in the reaction.

adjacent channel In FM/FM telemetry, the modulated signal bandwidth immediately below or above the channel of interest. See FM/FM.

adjacent equipment The auxiliary equip­ment that may be located adjacent to the valve or actuator. [ANSI/ISA-75.05.01-2000]

can function independently and simulta­neously. The asymmetrical component means that the upload speeds are slower than the download speeds, but most delays occur during downloading.

adsorption The concentration of molecules of one or more specific elements or compounds at a phase boundary. The concentration usu­ally occurs at a solid surface that bounds a liquid or gaseous"medium containing the

... specific element or compound. advanced process control (APC) Process con­

trol strategies beyond straightforward PID loop control, which is usually defined as "classical" advanced control. These advanced strategies involve a combination of PID loops, dead-time compensators, lead/ lag feedforward function blocks, and single­variable constraint controllers.

advisory alert An alert object that has an assigned priority value of less than 8. [ISA­TR50.02, Part 9-2000]

AE Application entity; active element within an ISO layer. Also known as: architectural engineer; application enabler.

A&E Architecture and Engineering; com­pany that designs and builds the process plant. l

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adjustment The process of altering the value AEB Australian Electrotechnical Board; stan­of some circuit element or some component dards association of Australia, a member of of the mechanism of an instrument, control­ lEe.

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ler, or auxiliary device so as to bring the indi­cation to a desired value within a specified tolerance. This desired value usually corre­sponds to an independently determined value of the measured variable within a spec­ified tolerance.

adjustment, span A means provided in an instrument to change the slope of the input­output curve. See span shift. [ANSI/ISA-51.1­1979 (R1993)]

adjustment, zero A means provided in an instrument to produce a parallel shift of the input-output curve. See zero shift. [ANSI/ ISA-51.1-1979 (R1993)]

ADO Ampex Digital Optical; in video development, a traditional video editing sys­tem component that allows video frames to appear to be in motion.

ADPCM Adaptive differential pulse code modulation; encoding format for compress­ing and storing audio information in digital format.

ADSL Asymmetric digital subscriber line; a technique that compresses the amount of data needed to send computer, voice, and video data over normal phone lines (a com­pared to coaxial cable) so those three services

iAECMA Association Europeenne des Constructeurs de Materiel Aerospatial (also known as the European Association of Aero­space Industries). European standards group for aerospace industry.

AENOR Asociaci6n Espanola de Normaliza­ci6n y Certificaci6n; standards group in Spain.

aeolight A type of glow lamp whose inten­

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1sity of light output varies with a signal volt­age applied to it. Its construction employs a cold cathode and an envelope filled with a mixture of gases.

aerator Any device for injecting air into a material or process stream.

aerodynamics A branch of mechanics that deals with the motion of gases, such as air, and with the forces acting on solids in rela­tive motion with respect to a gas.

aerograph Any self-recording instrument carried aloft to take meteorological data.

aerometer An instrument for determining the density of air or other gases.

aerosol A dispersion of fine liquid or solid particles in a gas. For instance, both smoke and fog are aerosols.

1

I III

12

I Aex / air bind

Aex A required prefix for apparatus that meet one or more types of protection as

i defined in the National Electric Code: Sec­. i

\. tion 505-2(c) for installation in accordance '. with Article 505 of the National Electrical

Code, NFPA 70: 1999. [ANSI/ISA-12.01.01­1999]

AFNOR Association Fran<;aise de Normalisa­tion; standards group itt france.

afterglow Luminosity that persists in a gas after an electrical discharge passes through it. The'phenomenon is sometimes utilized to measure flow.

AGA Advanced Graphics Architecture; chip set for driving high-resolution multimedia tools. Also known as: American Gas Associa­tion, a national trade association founded in 1918 in the United States, composed of about three hundred gas distribution and transmis­sion companies to act as clearinghouse on gas energy information, and act as a catalyst in technical and energy policy matters, and as a voice for gas industry.

age hardening Raising the strength and hard­ness of an alloy by heating a supersaturated solid solution at a relatively low temperature so as to induce precipitation of a finely dis­persed second phase. Also known as aging and precipitation hardening.

agent 1. A system that acts on behalf of another system. 2. A computing program

. ~- that performs some information-gathering or processing task in the background, usually some very small, well-defined task. Some believe that the human mind consists of thousands or millions of agents working in parallel, so "true" artificial intelligence (AI) machines should also contain many agents along with some system for arbitrating among the competing results of these agents' activity.

I agglomeration Any process for converting a

mass of relatively fine solid material into a mass of larger lumps.

aggregate 1. Natural sand, gravel, and crushed stone that is mixed with cement toJ make mortar or concrete. 2. A structured col­lection of data elements. [ISA-TR50.02, Part l. . 9-2000]

! ~ aging 1. The alteration of the characteristics

of a device because of use. 2. Operating a product before shipping it so as to stabilize component functions or detect early failures. 3. Any time-dependent change in the proper­ties of a material, but especially age harden­ing at room or slightly elevated temperatu­res. 4. Curing or stabilizing parts or materials

through long-term storage outdoors or under closely controlled storage conditions.

agitator A device for mixing, stirring, or shaking liquids or liquid-solid mixtures so as to keep them in motion.

AGP Accelerated graphics port; provides the capability for three-dimensional and full­motion video graphics in workstations.

AGVS Automated Guided Vehicle System; controls vehicles that proceed along prede­terminedro~tesar guide-paths, performing scheduled material-handling tasks without operators.

AHP Analytical Hierarchy Process; approach to using multiple criteria in decisions that aids in the meaningful organization of infor­mation, provides the ability to easily change weights and ratings for "what-if" analysis, and verifies that the approach to evaluation is logical and consistent.

AlA Application integration architecture; method of Digital Equipment Corporation for allowing programs to be portable between VMS and ULTRIX.

AIFF Audio Interchange File Format; method for saving digital audio in electronic mem­ory, used for exchanging data between com­puters. Developed by Apple and used in Macintosh computers with a compression standard called Macintosh Audio Compres­sion/Expansion (MACE).

air 1. The mixture of oxygen, nitrogen and other gases that, with varying amounts of water vapor, forms the atmosphere of the earth. [ISA-77.41-1992] 2. Air implies the use of any suitable and normally clean, dry, safe gas. [ISA-RP60.9-1981]

air, dry 1. Air with which no water vapor is mixed. This term is used comparatively since in nature air always has some water vapor, which, being a gas, is dry. 2. A papermaking term used to describe dry pulp that contains about 10 percent moisture.

air, saturated Air that contains the maximum amount of the vapor of water or other com­pound that it can hold at its temperature and pressure.

air-atomizing oil burner A burner for firing oil in which the oil is atomized bv com­pressed air, which is forced into ~nd through one or more streams of oil. This action breaks the oil into a fine spray.

air bearing A device that lubricates motion with flowing air. A linear air bearing floats a table on air as it travels a straight line.

air bind An air pocket in a pump, conduit, or piping system that prevents liquid from flowing past it. Also known as liquid trap.

13

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air binding / air cushion

l air binding The inclusion of air in a space, placement of air from the measuring vessel

hindering the flow of some other gas or liq­ during the calibration run. The standard air uid. (50% R.H.) for correcting to weights in vac­

air blast The flow of air at a high velocity, usually for a short period.

airborne Carried in the atmosphere either by being transported in an aircraft or by being dispersed in the atmosphere.

air-bubbler liquid-Ievel.:detector A device for indirectly measuring the level of liquid in a vessel-especially a corrosive liquid, vis­cous liquid, or liquid containing suspended solids. This detector consists of a standpipe, open at the bottom and closed at the top, that is connected to an air supply whose pressure is maintained slightly above the maximum head of the liquid in the vessel. Air bubbles out of the bottom of the pipe, ensuring that the internal pressure is equal to the head of the liquid in the vessel (this pressure is mea­sured by a simple gauge or transducer). Also known as purge-type liquid-level detector.

air-bubbler specific-gravity meter Any of several devices that measure specific gravity by determining the differential pressure between two air-purged bubbler columns. The devices ordinarily use either of two prin­ciples for determining specific gravity-com­paring sample density with the density of a known liquid, or comparing the pressure between two bubbler columns immersed at different depths in the process liquid.

i.eN Rf/';"Ul()i

(T'P)

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uum has a density of 1.217 kg/m3 at 288.7 K and 1.013 250E + 05 Pa. When weighings are made against weights, the buoyancy force on these weights must also be considered. For brass weights, the net effect of air buoyancy in air at standard conditions is about 0.015 percent. [ISA-RP31.1-1977]

air compressor A machine that raises the pressur~ of air a~ve atmospheric pressure and normally delivers it to an accumulator or distribution system.

air condenser 1. A heat exchanger for con­verting steam into water in which the heat-transfer fluid is air. Also known as air-cooled condenser. 2. A device for removing oil or water vapors from a compressed-air line.

air-conditioned area See area, air conditioned. air conditioning Controlling the atmospheric

environment in a confined space by measur­ing and continually adjusting factors such as temperature, humidity, air motion, and con­centrations of dust, gases, odors, pollen, or microorganisms.

air consumption The maximum rate at which air is consumed by a device within its operat­ing range during steady-state signal condi­tions. Note: Air consumption is usually expressed in cubic feet per minute (f~ /min) or cubic meters per hour (m3 /h) at a stan­dard (or normal) specified temperature and pressure. [ANSl/lSA-51.1-1979 (R1993)]

air-cooled engine An engine, such as an internal combustion engine, whose waste heat is removed directly by a flowing stream of air-either a stream blown across the engine's external surfaces or one blown through internal cooling passages.

air-cooled heat exchanger A device for removing heat from a process fluid by pass­ing it through a bank of finned tubes that are cooled by blowing or drawing a stream of air across the tube exteriors. [ANSl/lSA-51.1­1979 (R1993)]

air curtain A stream of high-velocity, condi­tioned air that is directed downward across an opening such as a door or window so as to exclude insects and exterior drafts, pre­

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vent heat transfer through the opening, andI....J_

space. air buoyancy The lifting effect or buoyancy air cushion 1. A mechanical device that uses

"-­ make it possible to air condition the interior

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of the ambient air that acts during a weigh­ trapped air to absorb shocks or arrest motioning procedure involving open gravimetric without shock. 2. The partly confined stream calibrations. This effect is caused by the dis- of low-pressure, low-velocity air that sup­

14

I air cylinder I air resistance

ports a vehicle known as an air-cushion vehi­cle, ground-effect machine, or hovercraft. This air allows the vehicle to travel equally well over water, ice, marshland, or relatively level ground.

" air cylinder A cylindrical body for storing compressed air, for compressing air with a piston, or for driving a piston with com­pressed air,

air deficiency Insufficient air, in an air-fuel mixture, to supply the oxygen that is theoret­ically required to completely oxidize the fuel.

air ejector A device for removing air or non­condensable gases from a confined space, such as the shell of a steam condenser, through the eduction created by a fluid jet.

air entrainment The artificial infusion of a semisolid mass such as concrete or a dense

r-'··

slurry with minute bubbles of air, especially by mechanical agitation.

air filter A device for removing solid parti­cles such as dust or pollen from a stream of air, especially by causing the airstream to pass through layered, porous material such as cloth, paper, or screening.

airfoil-vane fan A device for creating a stream of moving air by drawing it into a fan casing near the hub and propelling it centrif­ugally with a rotor whose vanes are curved backward from the direction of rotation.

air furnace Any furnace whose combustion air is supplied by natural draft or whose internal atmosphere is predominantly heated air.

air gap The space between two ferromagnetic elements of a magnetic circuit.

air gauge 1. A device for measuring air pres­sure. 2, A device for precisely measuring physical dimensions by measuring the pres­sure or flow of air from a nozzle against a workpiece surface and then relating the mea­surement to the distance between the nozzle and the workpiece,

air hoist A lifting or hauling tackle whose power is provided by air-driven pistons (for reciprocating motion) or by air motors (for rotary motion).

air infiltration The leakage of air into a set­ting or duct.

,., air knife A device that uses a thin, flat J'et off air to remove excess coating material from sheet stock such as paper.

air lance A device for directing a high-veloc­ity stream of pressurized air into a process vessel or against a surface such as a boiler wall to remove unwanted deposits.

air lock 1. An intermediate chamber between an environmentally controlled confined

space and the outside atmosphere that allows personnel and materials to enter by sealing a door between the chamber and the confined space. After the door is opened to the outside to admit personnel or materials, it is closed and sealed. The environmental conditions in the chamber are changed to match those in the confined space, then an interior door is opened to permit entry into the confined space. The process is reversed when exiting the confined space. 2. See air bind. ',' .'

air meter A device for measuring the flow of air or other gas and expressing it as weight or volume per unit time.

air moisture Water vapor suspended in air. air monitor A warning device that detects

airborne radioactivity or chemical contami­nation and sounds an alarm when the radia­tion, gas, or vapor level exceeds a preset value.

air motor An engine that produces rotary motion by using compressed air or other gas as the working fluid.

air nozzle An air port that has direction and appreciable length so as to direct an air stream.

air permeability A method of measuring the fineness of powdered materials, such as port­land cement, by determining the ease with which air passes through a defined mass or volume.

air port An opening through which air passes.

air preheater A heat exchanger for transfer­ring some of the waste heat in flue gases from a boiler or furnace to incoming air, thus increasing the efficiency of combustion.

air purge 1. A flow of air through a furnace, boiler gas passages, and associated flues and ducts that effectively removes any gaseous combustibles and replaces them with air. Purging may also be accomplished by an inert medium. [ISA-77.41-1992] 2. The removal of undesired matter by replacing it with air.

air purging Removing airborne contami­nants, gases, or odors from a confined space by introducing fresh, clean air.

air regulator A device for controlling air­flow-for example, a damper to control the flow of air through a furnace or a register to control the flow of heated air into a room.

air reheater A device in a forced-air heating system that adds heat to air circulating in the system.

air resistance The opposition offered to the passage of air through any flow path.

15

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air ride / alarm-only instrument

air ride An air-suspension system installed on some moving-van trailers to provide shock and vibration control for moving deli­cate equipment. [ISA-RP60.11-1991]

air separator A device for separating materi ­als of different density, or particles of differ­ent sizes, by means of a flowing current of air.

air set A device that is used to control the supply air pressure to the valve actuator and its auxiliaries. [ANSI!ISA-75.05.01-2000]

airspeed The speed of an airborne object with respect to the surrounding air mass. In calm air, airspeed is equal to ground speed. True airspeed is a calibrated airspeed that has been corrected for pressure and temperature effects caused by altitude and for compress­ibility effects at high airspeeds.

air spring A device commonly used in place of a mechanical spring in heavy vehicles to support the vehicle's body on its running gear. The energy-storage element is an air-filled container with an internal elasto­meric bellows or diaphragm.

air supply (AS) 1. The supply of air used in pneumatic instrumentation as a power sup­ply. 2. Plant air supply (PA). 3. Instrument air (IA). 4. The energy supply for pneumatic instrumentation.

air thermometer A device for measuring temperature in a confined space by detecting variations in the pressure or volume of air in a bulb placed inside the space.

airtight Sealed to prevent the passage of air or other gas; impervious to leakage of gases across a boundary.

air vent A valve opening in the top of the highest drum of a boiler or pressure vessel

18.1-1979 (R1992)] See annunciator. 5. An audi­ble, visual, or physical presentation that is designed to alert the instrument user that a specific level of gas concentration has been reached or exceeded. [ISA-92.02, 03, 04 and 06.01-1998] 6. An instrument, such as a bell, light, printer, or buzzer, that indicates when the value of a variable is out of limits. 7. A piece of equipment that generates a visual or audible signal that attracts attention. [ISA­RP12.4-1.99..6] 8. A warning signal presented whenever a criticar deviation state frofu. nor­mal conditions occurs in a process. Techni­cally, alarm is a condition (not an event). An event is when the alarm condition begins; another event is when that co;n.dition ends. 9. The detection of a block leaving a particular state as well as its returns back to that state. [ISA-TR50.02, Part 9- 2000]

alarm, maintained An alarm that returns to normal after p.eing acknowledged. [ANSI! ISA-18.1-1979 (R1992)]

alarm, momentary An alarm that returns to normal before being acknowledged. [ANSI! ISA-18.1-1979 (R1992)] An indication that is .used to alert an operator about an abnormal operating condition. [ANSI!ISA-77.13.01­1999]

alarm extensions, electrically operated Usually a highly sensitive induc­tion-type device for signaling high or low flows or deviations from any set flow. The device consists of a sensing coil positioned around the extension tube of the rotameter. When the metering float moves into the field of the coil it causes a low-level signal change that is usually amplified to a level suitable for performing annunciator or control func­

for the purpose of venting air. tions. [ISA-RP16.4-1960] air vessel An enclosed chamber, partly filled alarm extensions, magnetically actuated A

1 with pressurized air, that is connected to a piping system to counteract water hammer or promote the uniform flow of liquid. [ISA­RP60.9-1981]

airy disk The central bright spot produced by a theoretically perfect circular lens or mirror. The spot is surrounded by a series of dark and light rings, which are produced by dif­fraction effects.

alarm 1. A device or function tha t signals the existence of an abnormal condition by making an audible or visible discrete change, or both, so as to attract attention to that condition. [ANSI!ISA-5.1-1979] 2. An abnormal process condition. 3. The sequence state when an abnormal process condition occurs. 4. A device that calls attention to the existence of an abnormal process condition. [ANSI!ISA­

device attached to the meter body that con­tains an electrical switch and is magnetically actuated by the metering float extension so as to signal a high or low flow. The switch is adjustable with respect to the float position over a range that is equal to the travel of the metering float. Standard switch ratings are usually 0.3 amperes for a nO-volt, 60-cycle AC supply (five amperes or more if relays are used). [ISA-RP16.4-1960]

alarm module (point or sequence module) A plug-in assembly that contains the sequence logic circuit. Some alarm modules also con­tain visual display lamps or lamps and win­dows. [ANSI/ISA-18.1-1979]

alarm-only instrument An instrument that provides alarm(s) but does not have an inte­

16

I alarm point I alignment

gral display device to indicate gas concentra­tion levels. [ISA-92.02, 03, 04 and 06.01-1998]

1. alarm point The sequence logic circuit, visual

display, auxiliary devices, and internal wir­

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ing that are related to one visual display. [ANSI/ISA-18.1-1979]

alarm point, active An alarm point that is wired internally and completely equipped. The window is labeled to identify a specific monitored variable. [ANSI/ISA-18.l-1979) .

alarm point, future (blank) An alarm point that is wired internally and equipped except for the plug-in alarm module. The window is not labeled to identify a monitored variable. [ANSI/ISA-18.1-1979]

alarm point, spare An alarm point that is wired internally and completely equipped. The window is not labeled to identify a mon­itored variable. [ANSI/ISA-18.1-1979]

alarm set point 1. The selected input value at which an indication, alarm, or other output function is initiated. The value of the process variable and the corresponding value of the transmitted signal at which an alarm is initi­ated. Example: An electronic pressure trans­mitter is calibrated 0-200 psi. A high­pressure alarm needs to be sounded if the process reaches 175 psi. The transmitter out­put is a 4-2 mA signal. The alarm set point would be 175 psi with an increasing pres­sure. To calibrate a switch activated by the mA signal, the following formula is used:

(PValarmsetpointJ.(TransmitterlRVl) 16 Jl 40 I ."----'-'-_-'-.CC.-"'-'-'----'--_C--.:. X mA + . = a arm scrpoJOl[( Transm itta span

The alarm set point would be 175 psi, which is represented by an 18 mA increasing signal. 2. The selected gas concentration level(s) at which an indication, alarm, or other output function is initiated. [ANSI/ISA-12.13.01­2000] or activated [ISA-92.06.01-1998]

alarm severity A selection of levels of prior­ity for the alarming of each input, output, or rate of change.

alarm system An integrated combination of detecting instruments and visible or audible warning devices that actuates when an envi­ronmental condition or process variable exceeds some predetermined value.

, alarm valve A device that detects water flow and sounds an alarm when an automatic sprinkler system is activated.

ALARP As low as reasonably practical; acceptable control system failure designation based upon IEC 1508 specification.

ALE Application link enabling; allows a mes­sage exchange between different applica­tions within or between computer systems.

alert Alarms and events. [ISA-TR50.02, Part 9-2000] See process condition; see also sequence state.

alert box In data processing, a window that appears on a computer screen to alert the user of an error condition.

alg~braic addero .. An ~ctronic or mechanical device that can ~titomaticallyfind the alge­braic sum of two quantities. .

algorithm 1. A finite set of well-defined rules for solving a problem in a finite number of steps. [ISA-TR50.02, Part 9-2000] 2. Detailed procedures for giving instructions to a com­puter. 3. Contrast with heuristic and stochastic. 4. A recursive computational procedure. 5. A step-by-step procedu,re for solving a problem or accomplishing an end. 6. Sometimes used to refer to a firmware or a software program.

algorithmic language A language designed for expressing algorithms.

algorithmic-oriented language (ALGOL) Computer language designed by a commit­tee of the Association for Computing Machinery and European computer industry representatives. Useful for mathematical problem-solving and the first block-oriented computer language.

alias 1. When varying signals are sampled at equally spaced intervals, two frequencies are considered to be aliases of one another if they canr..ot be distinguished from each other by an analysiS of their equally spaced values. 2. False signals in the frequency domain caused by an excessive sampling interval for digi tizing.

aliasing 1. A peculiar problem in data sam­pling, where data are not sampled enough times per cycle, and the sampled data cannot be reconstructed. 2. In digital bitmapped graphics, a jagged boundary along the edges of shapes and different-colored shapes within an image.

aliasing error An inherent error in time-shared telemetry systems where improper filtering is employed prior to sam­pling.

alidade 1. An instrument used in the plane-table method of topographic survey­ing and mapping. 2. Any sighting device for making angular measurements.

alignment The condition of proper orienta­tion of machine-drive components to mini­mize vibratory forces unnecessary to power transmission.

17

alkaline cleaner / alternate code complement

alkaline cleaner An alkali-based aqueous solu­tion for removing soil from metal surfaces.

alkalinity The amount of carbonates, bicar­bonates, hydroxides, and silicates or phos­phates in the water. Reported as grains per gallon, or ppm, as calcium carbonate.

Allen screw A screw or bolt that has a hexag­onal socket in its head and is turned by inserting a straight or bent hexagonal rod into the socket.

alligatoring 1. Cracking in a film of paint or varnish that is characterized by broad, deep cracks extending through one or more coats. Also known as "crocodiling." 2. The rough­ening of the surface of very coarse-grained sheet metal during forming. 3. The longitudi­nal splitting of flat slabs in a plane parallel to the rolled surface that occurs during hot roll­ing. Also called "fishmouthing."

allobar A form of an element in which the distribution of isotopes is different from the distribution in the naturally occurring form. Thus, an allobar has a different apparent atomic weight than the naturally occurring form of the element.

allocation A form of coordination control that assigns a resource to a batch or unit. Note: An allocation can be for the entire resource or for portions of a resource. [ANSI/ISA-88.01-1995]

allowable response time The limiting response time established in the safety analy­sis and documented in the plant's technical specifications. [ISA-67.06-1984]

allowable value A limiting value that the trip set point may have when it is periodically tested, beyond which appropriate action shall be taken. [ANSI/ISA-67.04.01-2000]

allowable working pressure See design pres­sure.

allowance The specified difference in limit­ing sizes-either minimum clearance or maximum interference between mating parts-that is computed mathematically from the specified dimensions and tolerances of both parts.

alloy A solid material that has metallic prop­erties and is composed of two or more chem­ical elements.

alloy steel An alloy of iron and carbon that also contains one or more additional ele­ments intentionally added to increase harde­nability or to enhance other properties.

all-pass network A network that is designed to introduce phase shift or delay into an elec­tronic signal without appreciably reducing amplitude at any frequency.

Alnico Any of a series of commercial iron-base permanent magnet alloys that con­tain varying amounts of aluminum, nickel, and cobalt as the chief alloying elements. The Alnicos are characterized by their ability to produce a strong magnetic field for a rela­tively small magnet mass and to retain their magnetism, with relatively insignificant loss in field strength when the magnetizing field is removed:

alphabet . The specific character set used by a computer. .

alphabetic word 1. A word consisting solely of letters. 2. A word consisting of characters from the same alphabet.

alpha counter 1. A system for detecting and counting energetic alpha particles. It consists of an alpha counter tube, amplifier, pulse­height discriminator, scaler, and recording or indicating mechanism. 2. An alpha counter tube and necessary auxiliary circuits alone. 3. A term sometimes loosely used to describe just the alpha counter tube or chamber itself.

alpha emitter A radionuclide that disinte­grates by emitting an alpha particle from its nucleus.

alphanumeric 1. Pertaining to a character set that contains both letters and numerals, and usually by other characters. Synonymous with "alphameric." [ISA-RP55.1-1975 (R1983)] 2. The sequencing or ordering of a list using both initial letters and numbers.

alpha particle A positively charged, energetic atomic particle consisting of two protons and two neutrons that is identical in all measured properties with the nucleus of a helium atom. It may be produced by radioactive decay of certain nuclides or by stripping a helium atom of its electrons.

alpha-ray spectrometer An instrument used to determine the energy distribution in a beam of alpha particles.

alpha test Trying out a new product at a ven­dor's own company before subjecting it to a beta test. Software undergoes alpha testing as a first step in gathering user feedback.

altazimuth A sighting instrument that has both horizontal and vertical graduated cir­cles so that both azimuth and declination can be determined from a single reading. Also known as astronomical theodolite and universal instrument.

alteration switch A manual switch on the computer console or a program-simulated switch that can be set on or off to control coded machine instructions.

alternate code complement In a frame syn­chronization scheme, a frame synchroniza-

I :. I, 18

i

I alternate immersion test I ambient temperature

.' i

tion pattern is complemented on alternate frames to give better synchronization.

alternate immersion test A type of acceler­ated corrosion test in which a test specimen is repeatedly immersed in a corrosive medium, then withdrawn and allowed to drain and dry.

alternating-current bridge A bridge circuit that utilizes an AC signal source and AC null detector. Generally, both in-phase (resistive) and quadrature (reactive) balance conditions must be established to balance the bridge. Some bridges require only one balance (resis­tive or reactive) and use a phase-sensitive detector.

alternating current plasma display panel (ACPDP) A type of liquid crystal display (LCD) that relies upon the emission of pho­tons from gas that has been ionized by elec­tric charge. Electrodes that are covered by insulation layers to protect them from work­ing gas and therefore assuring longer life. Compare with direct current plasma display panel (DCPDP).

alternating-current thin-film electrolumines­cent (ACTFEL) A type of liquid crystal dis­

; play (LCD) flat-panel display device. See electroluminescent displays.

,altigraph A recording pressure altimeter. altimeter An instrument for determining the

height of an object above a fixed level or ref­erence plane sea level, for example; the aner­oid altimeter and the radio altimeter are the most common types.

altitude 1. The vertical distance above a stated reference level. Unless otherwise spec­ified, this reference is mean sea level. [ISA­37.1-1975 (R1982)] 2. Height above a speci­fied reference plane, such as average sea level. Usually given as a distance measure­ment in feet or meters regardless of the method of measurement.

altitude signals Reflected radio signals returned to an airborne electronic device from the land or sea surface directly under­neath the vehicle.

ALU Arithmetic logic unit, which is the por­tion of the central processing unit (CPU) that performs arithmetic and logic functions (rather than memory organization and data transfer functions).

alum A general name for a class of double sulfates that contain aluminum and another cation such as potassium, ammonium, or iron.

Alumel Aluminum nickel alloy lIsed in the negative leg of a type K thermocouple. Trade name of Hoskins Manufacturing Company.

19

alumina The oxide of aluminum-AI20 3•

aluminizing 1. Applying a thin film of alumi­num to a material such as glass. 2. Forming a protective coating on metal by depositing aluminum on the surface or a reacting sur­face material with an aluminum compound. The aluminum is then diffused into the sur­face layer at elevated temperature.

aluminum A soft, white metal that in pure form exhibits excellent. electrical conductiv­ity and oxidation resistan,:=e. It is the base metal for'an extensive series of lightweight structural alloys used in such diverse appli­cations as aircraft frames.and skin panels, automotive body panels and trim, lawn fur­niture, ladders, and domestic cookware.

ambient A surrounding or prevailing condi­tion, especially one that is not affected by a body or process contained in it.

ambient air 1. Air to which the sensing ele­ment is normally exposed. [ISA-92.02, 03, 04 and 06.01-1998] 2. The air that surrounds the equipment. The standard ambient air for per­formance calculations is air at 80

o

p, 60 per­cent relative humidity, and a barometric pressure of 29.921 in. Hg. This gives a spe­cific humidity of 0.013 lb of water vapor per lb of air.

ambient compensation The design of equip­ment or a measuring instrument such that changes in ambient conditions do not affect the performance of that equipment or the readings of that instrument.

ambient conditions 1. The conditions (pres­sure, temperature, etc.,) of the medium that surrounds the case of the transducer. [ISA­37.1-1975 (R1982)] 2. The environment of an enclosure (room, cabinet, etc.) surrounding a given device or equipment.

ambient pressure See pressure, ambient. ambient pressure effects The change in sen­

sitivity and the change in zero-measurand output caused by subjecting the transducer to a specified ambient pressure change. [ISA­37.8-1982 (R1995)]

ambient pressure error The maximum change in output at any measured value within the specified range when the ambient pressure is changed between speCified val­ues. [ISA-37.1-1075 (1982)]

ambient temperature 1. The temperature of the atmosphere encompassing the entire area of the instrument air system installation, including the compressor, piping, dryer, and the instruments themselves. 2. The tempera­ture of the medium surrounding a device. [ANSI/ISA-7.0.01-1996]

II American standard pipe thread I amplitude linearity, shock

American standard pipe thread A series of specified sizes for tapered, straight, and dry­seal pipe threads established as a standard in the United States. Also known as Briggs pipe thread.

American standard screw thread A series of specified sizes for threaded fasteners, such as bolts, nuts, and machine screws, established as a standard in the United States.

AM/FM Automated mapping/facility management; electronic mapping, a branch of the Geographic Information System (GIS). See GiS.

Amici prism Also known as a "roof prism." A right-angle prism in which the hypotenuse has been replaced by a roof, where two flat faces meet at a 90° angle. The prism performs image erection while deflecting the light by 90°. This is the same as rotating the image by 180°'Vreversing it left to right and at the same time inverting it top to bottom.

ammeter An instrument for determining the magnitude of an electric current.

sometimes referred to as Q. [ISA-RP37.2-1982 (R1995)]

amplifier 1. In process instrumentation, a device that enables an input signal to control power from a source independent of the sig­nal. This makes the device capable of deliver­ing an output that bears some relationship to, and is generally greater than, the input signal. [AN5.I/ISA51.1-1979 (R1993)] 2. Any device that can increase the magnitude of a

j

IIIjiiI, l

chemical deposition or by condensation. The method employed is dictated by the mate­rial's composition and ultimate use.

amp or ampere 1. A unit of measure used to define the rate of flow of electricity (current) in a circuit; the electric current produced by one volt acting through a resistance of one ohm. 2. The current that will deposit silver at the rate of 0.001118 grams per second, with the current flowing at 1 coulomb (6.25 x 108

electrons) per second. ampere-hour A quantity of electricity equal

to the amount of electrical energy passing a given point when a current of one ampere flows for one hour.

ampere-hour meter An integrating meter

amplitude of a second signal supplied from another source. 2. The ratio of the output-sig­nal amplitude from an amplifier circuit to the input-signal amplitude from the control net­work, both expressed in the same units.

amplification factor The j.1 factor for the plate and control electrodes of an electron tube when the plate current is held constant.

amplification factor at resonant frequency The ratio of the maximum sensi­tivity'Of a transducer (at its resonant fre­quency) to' its ref~.rence sensitivity. The 1amplification factor at resonant frequency is 1

amorphous film A film of material deposited physical quantity, such as mechanical force on a substrate for corrosion protection, or or electric current, without significantly dis­ 1 insulation, for its conductive properties, or torting the wave shape of any variation with for a variety of other purposes. It is noncrys­ time associated with the quantity. 3. A com­ Italline and can be depOSited by evaporation ponent used in electronic equipment to raise

the level of an input signal so that the corre­sponding output signal has sufficient power to drive an output device such as a recorder orloudspeake~ .

amplifier (laser) A laser amplifier is a device that amplifies the light produced by an exter­nallaser but lacks the mirrors needed to sus­tain oscillation and independently produce a laser beam.

amplifying Having an integral output ampli­fier. [ISA-S37.1-1975 (R1982)]

amplitude 1. A measure of the departure of a phenomenon from any given reference. 2. The maximum departure measured from the mean position of a wave form. 3. The dif­ference between the average value of a sinu­

j II 1

II!

IIi

i that measures the electric current flowing in soidal variation and the maximum (or a circuit and indicates the integral of current minimum) value. 4. The maximum depar­with respect to time. ture of motion measured from the mean

j ar I

ampere per meter The SI unit of magnetic position to an extreme. The units of ampli­field strength; it equals the field strength tude are peak, peak-to-peak, and root mean

lao developed in the interior of an elongated, square. uniformly wound coil excited with a linear amplitude distortion A condition in an current density in the winding of one ampere amplifier or other device when the ampli­per meter of axial distance. tude of the output signal is not an exact lin­

(IHjA;\'

ampersand (&) In the typographical composi­ ear function of the input (control) signal. ation of screen displays and printing, the sym­ amplitude-frequency response See frequency bol for the Latin word et, meaning "and." response.

amplification 1. Increasing the amplitude of amplitude linearity, shock The closeness of

p ir

a signal by using a signal input to control the sensitivity to reference sensitivity over a AJ~

c(

20

amplitude linearity, vibration l analog electronic controller

stated range of acceleration amplitudes, process operator to manually position one or under shock conditions, usually specified as "within ±__percent for acceleration rise times longer than __ microseconds." [ISA­RP37.2-1982 (R1995)]

,lmplitude linearity, vibration Closeness of sensitivity to reference sensitivity over a stated range of acceleration amplitudes, at a stated fixed frequency. This linearity is usu­ally specified as within "± [number] percent." [ISA-RP37.2·1982 (1995)] .

, .lmplitude modulation (AM) The process (or the results of the process) of varying the amplitude of the carrier at the same time as and in proportion to the variation in the modula ting signal.

MODULATING,' SIGNALI

~ \ CARRIER

WAVE

) J.

~ t v v v v v v v v V V 1

I , ,.,.

AMPLITUDE MODULATED

WAVES

MAX1UM M!NIMUM I INTERMEDIATE I AMPLITUDE AMPLITUDE AMPLITUDEI

amplitude noise Random fluctuations in the output of a light source or signal from other

, generating or detecting means. : . amplitude ratio A factor that expresses the ~' . ratio of the output amplitude to the input . ,amplitude when the input is sinusoidal. ; < amplitude response A measure of the time it ~. . takes for a defined change of amplitude to

occur. , AM rejection The removal of unwanted

amplitude modulation of a signal. Usually performed by using signal clipping or limit­

" ing circuitry. : AIM station (automatic/manual station) In . control systems, a device that enables the

more valves. A single-loop station enables the operator to manually position a single valve, a shared station makes possible con­trol of multiple valves, and a cascade station provides control of paired loops. •

analog 1. Having the form of continuously variable physical quantities, as in data. Con­trast with digital. 2. The representation of numericql quantities by means of physical vaTia,bles, such as translation, rotation, volt­age, or resIstance. '3. A wa¥eform is aI}alog if it is continuous and varies over an arbitrary range. Contrast with digital., .

analog backup Conventional analog instru­.:~mentation that is provided to control a pro­

cess if the computer system fails. analog channel A channel on which the

information transmitted over it can take any value between the limits defined by that channel. Voice channels are analog channels.

analog computer 1. A computer in which data are mainly represented in analog form. 2. A computer that operates on analog data by performing physical processes on them. Contrast with digital computer.

analog control The implementation of auto­matic control loops with analog (pneumatic or electronic) equipment. Contrast with direct digital control.

analog control system Classically, a system that consists of electronic or pneumatic sin­gle-loop analog controllers, in which each loop is controlled by a single, manually adjusted device.

analog data Data represented in a continuous form, as contrasted with digital data, which is represented in a discrete, discontinuous form. Analog data are usually represented by means of physical variables, such as voltage, resistance, rotation, and so on.

analog DC current signal A signal used for transmission that varies in a continuous manner according to one or several physical quantities. [ANSI/ISA-50.1-1982 (R1992)]

analog device A mechanism that represents numbers by physical quantities, for example, by lengths, as in a slide rule, or by voltage or currents, as in a differential analyzer or a computer of the analog type.

analog electronic controller Any of several adaptations of analog computers to perform control functions. They may produce an out­put signal tha tis directly related to the differ­ence between a measured value and a predetermined set point or an output signal that is modified by rate-of-change or other feedback signals.

21

I

I analog hardware description language I Anderson bridge

.'

analog hardware description language (AHDL) A modeling language that is capa­ble of representing both the structural and behavioral properties of analog circuits. Structural refers to the connectivity or net-list properties of a circuit; behavioral refers to the mathematical equations for individual com­ponents.

analog input 1. A continuously variable input. 2. A termination panel used to connect field "Yiring from the input device. See input, analog.

analog input module An I/O module that converts a process voltage or current signal into a multiple-bit form for use in the Pc. The signal is the analog of some process vari­able.

analog input point An alarm point for use with an analog-monitored variable signal, usually current or voltage. The logic circuit initiates an alarm when the analog Signal is above or below a set point. [ANSI/ISA-18.1­1979 (RI992)]

analog output 1. Transducer output that is a continuous function of the measurand, except as modified by the resolution of the transducer. [ISA-37.1-1975 (RI982)] 2. A con­tinuously variable output (generally 4-20 rna or 3-15 psi). See also output, analog.

analog output module An I/O module that converts a multiple-bit number calculated in the PC into a voltage or current output signal for use in control.

analog sampling The process by which the computer selects individual analog input sig­nals from the process, converts them into an equivalent binary form, and stores the data in memory.

analog signal An analog signal is a continu­ously variable representation of a physical quantity, property, or condition such as pres­sure, flow, temperature, and so on. The sig­nal may be transmitted as pneumatic, mech­anical, or electrical energy. See signal, ana­log.

analog simulation The solution of a set of simultaneous equations that are similar in form to the set of equations that describe the dynamic behavior of the subject system.

analog-to-digital (AID) 1. A device, or sub­system, such as a microprocessor that changes real-world analog data (as from transducers) in to a form that is compatible with binary (digital) processing. 2. The con­version of analog data into digital data. See analog-to-digital converter.

analog-to-digital converter (ADC) Any unit or device that is used to convert analog infor­

ma~ion into ~pproximatecorresponding digi­ A~

tal mformatIon. See converter, analog to digital. ]

analysis The quantitative and/ or qualitative r. A~ determination of the constituent parts of some entity, object, or problem.

analysis, ultimate The chemical analysis of solid, liquid, or gaseous fuels. In the case of an coal or coke, the determination of the constit­uent carbon, hydrogen, sulfur, nitrogen, oxy­gen, and ash.

analysis of varianc.~ A method for partition­ ... ing the total vatiance.in experimental data ;

into components that can be assigned to spe­cific sources.

analytical balance Any weighing device that an has a sensitivity of at least 0.1 mg.

analytical curve A graphical representation of some function of relative intensity in ~pec­troscopic analysis plotted against some func­tion of concentration. an

analytical gap The separation between the source electroo@6 in a spectrograph.

analytical limit The limit of a measured or an calculated variable established by the safety analysis to ensure that a safety limit is not exceeded. [ANSI/ISA-67.04.01-2000] an

analytical line The spectral line of an element that is used to determine its concentration in an spectroscopic analysis.

analytical scale In spectroscopic analysis, the an scale that results when an analytical curve is projected onto the intensity axis. It is often used in lieu of an analytical curve to permit ar. direct reading of spectral intensity as element concentration.

analyzer 1. Unattended instrumentation that continuously monitors a process stream. 2. Any of several types of test instruments, ordinarily one that can measure several dif­ ar ferent variables either simultaneously or sequentially. 3. In an absorption refrigeration system, the component that allows the mix­ture of water and ammonia vapors leaving ar the generator to come into contact with the relatively cool ammonia solution entering the generator. In this component, the mixture loses some of its vapor content.

AND 1. Logic output D exists if and only if ar all logic inputs A, B, and C exist. [ANSI/ISA­5.2-1976 (RI992)]2. A logic operator that has the property that if P is an expression, Q is an expression, R is an expression ..., then the AND of P, Q, R ... is true if all expressions are true, false if any expression is false. ar

Anderson bridge A type of AC bridge that is especially suited to measuring the character­istics of extremely low Qcoils.

22

ANUF / angular momentum flowmeter

ANDF Architecture- Teutral Distributed For­ reflection and the angle of incidence are ma t; an OSF/1 term.

.AND gate A basic electronic circuit used in microprocessor systems. A logical 1 value on

. output is produced only if all of the inputs have logical 1 values.

anechoic chamber 1. A test room in which all the surfaces are lined with a sound-absorb-

I . ing material. Also known as a dead room. 2. A , room that is lined with a material that . absorbs radio waves of a particular fre­~ quency or band of frequencies. It is used \ chiefly for tests at microwave frequencies, I such as a radar-beam cross section.

aremobiagraph A recording pressure-tube anemometer, such as a Dines anemometer, in which springs are used to make the output from the float manometer linear with wind speed.

anemoclinometer An instrument for deter­mining tlTe inclination of the wind to a hori­zontal plane.

anemometer A device for measuring wind speed. If it produces a recorded output, it is known as an "anemograph."

anemoscope A device for indicating wind 'direction.

aneroid 1 ot containing or using liquid, as of a device or system.

angle beam In ultrasonic testing, a longitudi­nal wave from an ultrasonic search unit that

. enters the test surface at an acute angle. a:ngle modulation A type of modulation in ; which carrier-wave angle is varied in accor­

dance with some characteristic of a modulat­.ing wave. Angle modulation can take the form of either phase modulation or fre­quency modulation.

angle of elevation The angle between a hori­zontal plane and the observer's line of sight to an object that lies above the plane of the observer.

angle of extinction The phase angle of the stopping instant of anode current flow in a gas tube with respect to the starting instant of the corresponding half cycle of anode volt­age.

angle of ignition The phase angle of the starting instant of anode current flow in a gas tube with respect to the starting instant of anode current flow in a gas tube with respect

'- to the starting instant of the corresponding positive half cycle of anode voltage.

. angle of incidence The angle between the direction of propagation of a ray of incident radiation and a normal to the surface it strikes. For a reflected wave, the angle of

23

equal. angle of repose A characteristic of bulk sol­

ids that is equal to the maximum angle with the horizontal at which an object on an inclined plane will retain its position without •tending to slide. The tangent of the angle of repose equals the coefficient of static friction.

angle valve A valve design in which one port is collinear with the valve stem or actuator, and the other.porqs at a right angle to the valve stem. [AN'5I/I5A-75.05.01-2000j

angstrom (A) A unit of length defined as 1/6438.4696 of the wavelength of the red line in the Cd spectrum. It equals almost exactly 10-10 meters. Angstrom was once used almost exclusively to express the wavelengths of light and x-rays, but it has now been lar~ely

replaced by the 51 unit nanometer, or 10­meters.

angular accelerometer' A device for measur­ing the rate of change of the angular velocity between two objects.

angular frequency A frequency expressed in radians per second. It equals two times the frequency in Hz.

angular misalignment In fiber-optic cables, the loss of optical power that is caused by a devi­ation from the optimum alignment of the fiber to the fiber at the coupling.

angular momentum The product of a body's moment of inertia and its angular velocity.

angular momentum flowmeter A device for determining mass flow rate. An impeller turning at constant speed imparts angular momentum to a stream of fluid passing

I angular velocity / antialias filter

through the meter. A restrained turbine located just downstream of the impeller removes the angular momentum, and the reaction torque is taken as the meter output. Under proper calibration conditions, the reaction torque is directly proportional to mass flow rate. Also called an "axial flowme­ter."

angular velocity The rate of motion along a circular path, measured in terms of angle tra­versed per unit time.

animation The process or making an object move across a video screen by rapidly dis­playing a series of static pictures of it (icons), each in a slightly different position.

anisotropic Exhibiting different values of a property when characteristics of that prop­erty are measured along different directions or axes.

anisotropy Exhibiting different properties or other characteristics-strength or coefficient of thermal expansion, for instance--in differ­ent directions with respect to a given refer­ence, such as a specific lattice direction in a crystalline substance.

annotate To add explanatory text to com­puter programming or any other instruc­tions.

annotation A comment, note, or descriptive remark added to a printout, screen view, or even in memory itself.

annular nozzle A nozzle whose inlet opening is ring shaped rather than an open circle.

annulus 1. Any ring-shaped cavity or open­ing. 2. A plate that protects or covers a machine.

annunciator 1. A device or group of devices that call attention to changes in process con­ditions. An annunciator usually calls atten­tion to abnormal process conditions, but may be used also to show normal process status. Usually included in this category are sequence logic circuits, labeled visual dis­plays, audible devices, and manually oper­ated push buttons. [ANSI/ISA-18.1-1979 (R1992)] 2. An electromagnetic, electronic, or pneumatic signaling device that either dis­plays or removes a signal light, metal flag, or similar indicator or sounds an alarm, or both, when it detects the occurrence of a specific event. In most cases, the display or alarm is single-acting and after being tripped must be reset before it can indicate another occur­rence of the event.

anode 1. The metal plate or surface that acts as an electron donor in an electrochemical circuit. Metal ions go into solution in an elec­

electrochemical corrosion. 2. The negative electrode in a storage battery, or the positive electrode in an electrochemical cell. 3. The positive electrode in an X-ray tube or vac­uum tube, where electrons leave the inter­electrode space.

anode circuit A circuit that includes the anode-cathode path of an electron tube con­nected in series with other circuit elements.

anode supply voltage The voltage across the terminals of an electric power source con­nected in series in the anode circuit

anodic coating An oxide film that is pro­duced on a metal by treating it in an electro­lytic cell with the metal as the cell anode.

anodic protection Reducing the corrosion rate of a metal that exhibits active-passive behavior by imposing an external electrical potential on a part.

anodize To form a protective pilssive film (conversion coating) on a metal part, such as a film of AIZOJ on aluminum. This is done by making the part an anode in an electrolytic cell and by passing a controlled electric cur­rent through the cell.

anodizing A method of producing film on a metal surface that is particularly well suited for aluminum.

anomalous dispersion Inversion of the deri\'ative of the refractive index with respect to wavelength in the vicinity of an absorption band. .

ANSI American National Standards Insti­tute; nonprofit, independent organization supported by trade organizations, industry, and professional societies for standards development and coordination in the United States. They represent the United States to the ISO; they defined ASCll.

ANSI screen control An ANSI standard that specifies a specific set of character sequences that instruct the computer to perform certain actions on the computer screen.

ANSI X3J3 Name given to the ANSr pL/r Language Standardization Committee.

antenna A device for sending or receiving radio waves. The term does not refer to the means for connecting the device to a trans­mitter or receiver. See also dipolt! antenlla and horn antellna.

antenna array A single mounting that con­tains two or more individual antennas cou­pled together to give specific directional characteristics.

antialias filter A low-pass filter that is designed to block frequencies greater than one-half the measuring rate.

trolyte at the anode during electroplating or

24 l

I antialiasing / aperiodic

antialiasing In digital graphics, a technique for reducing the jagged appearance of aliased bitmapped images, usually by insert­

It ing pixels that blend boundaries, especially j .. color boundaries.

; I

I. anti-cavitation trim A trim style for control valves that by its geometry reduces or elimi­nates the tendency of the controlled liquid to cavitate. [ANSI/ISA-75.05.01-2000] It is a combination of plug and seat ring or plug and cage that by its geometry permits opera­tion without cavitation or reduces the ten­dency to cavitate. This minimizes damage to the valve parts and to the downstream pip­ing. [ANSI/ISA-75.05.01-2000]

anticipatory action See rate action. anticipatory control Changing a control sig­

nal to one of the inputs to a process in response to a change in another input of the same process. Such control does not have self-correcting action as does closed loop (feedback) control. Anticipatory control is also called fecdforward control or open-loop con­trol.

anticoincidence circuit A circuit with two inputs and one output that produces an out­put pulse only if either input terminal receives a pulse within a specified time inter­val. It does not produce a pulse if both input terminals receive a pulse within that interval.

o anticorrosive Containing, as of paint or grease, a chemical that counteracts corrosion

oo. or produces a corrosion-resistant film by reacting with the underlying surface.

. antifriction Used to describe a device, such as a bearing or other mechanism, that employs rolling contact with another part rather than sliding contact.

antihunt circuit A circuit designed to prevent oscillation in a feedback process control loop, thereby stabilizing it.

antimagnetic Made of nonmagnetic materi­als or employing magnetic shielding that blocks the influence of magnetic fields dur­

._ ing operation, as of a device. antinodes The points, lines, or surfaces in a

medium that contains a standing wave " where some characteristic of the wave field is

at maximum amplitude. Also known as loops.

anti-noise trim A trim style for control valves that by its geometry reduces the noise gener­ated by fluid flowing through the valve. [ANSI/ISA-75.05.01-2000]

, . .antireflective coating A coating designed to ~ ,. suppress reflections from an optical surface.

'; anti-reset windup A device, circuit, or soft­ware that prevents the saturation of the inte­

gral mode of a controller, which occurs when control cannot be achieved. It helps to pre­vent the controlled variable from overshoot­ing its set point when the obstacle to control is removed.

antiresonance A condition that exists between an externally excited system and the external sinusoidal excitation, such that any small increase or decreas.e in the frequency of the excitation signal causes the peak-to-peak amplitude of a speCified response to increase.

antiresonant Having 'very high (approach­ing infinity) impedance, as of an electric, acoustic, or other dynamic system.

antiresonant frequency A frequency at which antiresonance exists between a system and its external sinusoidal excitation.

antiskid Used to describe a material, surface, or coating that has been roughened or that contains abrasive particles in order to increase the coefficient of friction and pre­vent sliding-or slipping. Also known as "antislip." .

antisurge control Control that avoids the unstable operating mode of compressors known as "surge." See surge.

anvil 1. The part of a machine that absorbs the energy of a sharp blow. 2. A heavy block made of wrought iron, cast iron, or steel and used to support metal being forged in a smith. 3. The base of a forging press or drop hammer that supports the die bed and lower die. 4. The stationary contact of a micrometer caliper or similar gauging deo/ice.

AOE Application operating environment; design for UNIX by AT&T.

AOX Adsorbable organic halides, a consider­ation in EPA (United States) regulations.

AP Application process. [ISA-TR50.02, Part 9­2000] Also application platform; part of soft­ware systems management services that pro­vide the environment for management application development, debugging, and execution.

APC Advanced process control; process con­trol strategies beyond straightforward PIO loop control, which are usually defined as "classical" advanced control; APC involves a combination of PIO loops, dead time com­pensators, lead/lag feed forward function blocks, and single-\'ariable constraint controllers.

APD Avalanche photodiode; diode that exhibits internal amplification of photocur­rent through avalanche multiplication of car­riers in the junction region. See avalanche.

aperiodic Varying in a way that is not period­ically repea ted.

25

aperiodically damped / application layer

aperiodically damped Reaching a constant value or stec.dy state of change without intro­ducing oscillation.

aperture A hole in a surface through which light is transmitted. Apertures are sometimes called spatial filters, a more descriptive term when the aperture is placed in the Fourier (focal) plane.

aperture time The time required in a sam­ple-and-hold circuit for the switch to open after the "hold" command has been given.

IIi(

! I ' II

API Application programming interface; a set of formalized software calls and routines that can be referenced by some application program so as to access underlying network services. Programs that use API-compliant calls can communicate with any others that use that same API; the interface between applications software and the application platform. Also known as American Petro­leum Institute.

APL "A Programming Language;" a com­puter language developed by Kenneth Iver­son and used mainly in scientific applications; known for its scope compact­ness, and facility with arrays, it has a highly specialized character set that can be mapped to keyboard.

APL "A Programming Language;" A power­ful systems programming language devel­oped by the International Business Machines Corporation.

APM Advanced Power Management; a stan­dard for saving power by automatically turn­ing off computer hardware when it is not in use.

APP Application Portability Profile; devel­oped by NIST, includes X Windows, POSIX, SQL, Information Resource Dictionary Sys­tem (see 1RDs) for database systems, Open Systems Interconnections (see 051), NFS (see NFs), COBOL, C, and Ada.

apparatus and systems of category "ia" Electrical apparatus and systems that contain intrinsically safe circuits that are incapable of causing ignition. This category has the fol­lowing safety factor: the circuits remain safe when up to two countable faults are applied and, in addition, those non-countable faults give the most onerous condition. [ISA­12.02.01-1999 (IEC 60079-11 Mod)]

apparatus and systems of category "ib" Electrical apparatus and systems that contain intrinsically safe circuits that are incapable of causing ignition. This category has the fol­

give the most onerous condition. [ISA­12.02.01-1999 (IEC 60079-11 Mod)]

apparent density The density of loose or compacted particulate matter as determined by dividing actual weight by volume occu­pied. The apparent density of the matter is always less than the true density of the mate­rial that comprises the particulate matter because the volume occupied includes the space devoted to pores or cavities between partil:les.

apparent flow' ·The uncorrected volume flow as indicated by the calibrator. [ISA-RP31.1­1977]

apparent viscosity The resistance to continu­ous deformation (viscosity) in a non-Newto­nian fluid that is subjected to shear stress.

APPC Advanced peer-to-peer communica­tions; network architecture definition by IBM that is specified as featuring high-level pro­gram interaction capabilities on a peer-to­peer basis.

appearance pot~ntial The minimum elec­tron-beam energy required to produce ions of a particular type in the ion source of a mass spectrometer.

applet A small application program fre­quently received by Internet users as part of a web page they are viewing.

applicable uncertainty (AU) That portion of the channel uncertainty that is applicable to a calculation of the minimum separation between set points. [ANSI/ISA-TR67.04.08­1996]

application 1. The system or problem to which a computer is applied. Computation, data processing, and control are often described as the three categories of applica­tion. 2. A program that provides functional­ity to end users. 3. A software functional unit that consists of an interconnected aggrega­tion of function blocks, events, and objects, which may be distributed, may have inter­faces with other applications, and may con­tain other applications. [ISA-TR50.02, Part 9­2000]

application enabler A software product that allows a software application to be devel­oped rapidly by using productivity tools and standard components and by reusing previ­ously developed software.

application layer A logical entity of the OSI digital communication model. It is the top­most of seven layers and the one that inter­faces with the network user. The application

iIIIIIII

I !

lowing safety factor: the circuits remain safe when up to one countable fault is applied and, in addition, those non-countable faults

layer performs network services like file transfer and e-mail.

26

!

I

I application-oriented language / area

application-oriented language A problem­oriented programming technique that employs statements that resemble the termi­nology of the user rather than of the pro­grammer.

application program A program that per­forms a task specific to a particular end-user's needs. Generally, an application program is any program written on a pro­gram development operating system that is not part of a basic operating system. ...

application program interface (API) A set of formalized software calls and routines that can be referenced by an application program to access underlying network services. Pro­grams that use API-compliant calls can com­municate with any others that use that same API. APIs are the interface between applica­tions software and the application platform.

application software Software that is specific to the user application in that it is the Safety Instrumented System functional description programmed into the PES (programmable electronic system) to meet the overall Safety Requirement Specifications. In general, application software contains logic sequen­ces, permissives, limits, expressions, and so

- on, that control the appropriate input, out­put, calculations, and decisions necessary to meet the safety functional requirements. [ANSIIISA-84.01-1996]2. Programs that are unique to a specific process control system installation or other specific installations rather than of a general purpose and a broad applicability.

application-specific software A computer program that is adapted or tailored to spe­cific user requirements for the purpose of collecting, manipulating, or archiving data or for process control.

. applied load 1. The weight carried or force sustained by a structural member in service. In most cases, the load includes the weight of the member itself. 2. Material carried by the load-receiving member of a weighing scale, not including any load necessary to bring the scale into initial balance.

applied shock Any rapidly applied load or , other form of excitation that produces shock

motion within a system. . approach idler The last idler passed before the

material on a belt reaches the weighbridge. [ISA-RP74.01-1984]

approved Acceptable to the authority that . has jurisdiction. The term is considered syn­

onymous with listed and certified. [ANSII ISA-RP12.6-1995]

apron The part of the control center that encloses the area below the console mount­ing panel.

APT Automatically programmed tools; com­puter-aided part programming system for numerically controlled machine tools devel­oped for multiaxis milling machines and for point-to-point and turning work.

aramid A liquid crystal polymer with excep­tional tensile strength and a coefficient of expansion near tha.t 6f glass. Widely used in fiber-optic cables. .,

arbitration A form of coordination control that determines how a resource should be allo­cated when there are more requests for the resource than can be accommodated at one time. [ANSIIISA-88.01-1995]

arc 1. A segment of the circumference of a cir­cle. 2. The graduated scale on an instrument for measuring angles. 3. A discharge of elec­tricity across a gap between electrical con­ductors.

arc lamp A high-intensity lamp in which a direct-current electric discharge produces light that is continuous, as opposed to a flashlamp, which produces pulsed light.

arc line A spectral line in spectroscopy. architecture 1. The structure, functional, and

performance characteristics of a system, specified in a way that is independent of the system's implementation. 2. The arrange­ment and interconnection of the hardware components or modules that comprise the Safety Instrumented System. [ANSIIISA­84.01-1996]

archival (archive) Long-term storage of data, usually onto some auxiliary storage medium, such as a separate disk or tape.

archival file In data processing, a store of sel­dom used data that must be retained for sev­eral years.

arcing device An electrical make/break com­ponent that is generally interpreted as being capable of producing an arc with enough energy to ignite a specific ignitable mixture. [ISA-12.01.01-1999]

ARCnet /I Attached Resource Computer net­work./I A token-passing network developed by Datapoint in 1977 that uses an active hub star at 2.5 Mbs, specifies only the bottom few layers of the ISO model, and combines a token-passing scheme with star, bus, or tree topologies rather than ring topology (such as token ring).

area 1. A component of a batch manufactur­ing site that is identified by physical, geo­graphical, or logical segmentation within the site. Note: An area may contain process cells,

27

area, air-conditioned / argentometer

units, equipment modules, and control mod­ules. [A SI/ISA-88.01-1995]2. A physical, geographical, or logical grouping deter­mined by the site. An area may contain pro­cess cells, production units, and production lines. [ISA-95.00.01-2000]

area, air-conditioned A location that has a temperature at a nominal value that is main­tained constant within a narrow tolerance at some point in a specified band of typical comfortable room temperatures. Humidity is maintained within a narrow specified band. Note: Air-conditioned areas also feature clean air circulation and are typically used for instrumentation, such as computers or other equipment, that requires a closely con­trolled environment. [ANSI/ISA-51.1-1979]

area, control room A location with heating and/or cooling facilities and whose condi­tions are maintained within specified limits. Such rooms mayor may not provide features for automatically maintaining constant tem­perature and humidity. Note: Control room areas are commonly provided for the opera­tion of those parts of a control system that require ongoing operator surveillance. [ANSI/ISA-51.1-1979]

area, environmental A basic qualified loca­tion in a plant that has specified environmen­tal conditions dependent on severity. Note: Environmental areas include air-conditioned areas; control room areas, heated and/or cooled; sheltered areas (process facilities); and outdoor areas (remote field sites). [ANSI/ISA-51.1-1979]

area, outdoor A location in which equipment is exposed to outdoor ambient conditions, including temperature, humidity, direct sun­shine, wind, and precipitation. [ANSI/ISA­51.1-1979]

area, sheltered An industrial process loca­tion, area, storage, or transportation facility that provides protection against direct expo­sure to the elements, such as direct sunlight, rain or other precipitation, or full wind pres­sure. The minimum and maximum tempera­tures and humidity in such areas may be the same as outdoors. Condensation can occur, and ventilation, if any, is provided by natural means. [ANSI/ISA-51.1-1979]

area classification The classification of haz­ardous (classified) locations by Class 1, II ,or III depending upon the presence of flamma­ble gases or vapors, flammable liquids, com­bustible dust, or ignitable fibers or flyings. The classification can also be by Division 1 or 2 depending upon whether these materials

exist in an ignitable concentration under nor­mal or abnormal conditions.

area classification (class) 1. Class I locations are those in which flammable gases or vapors are or may be present in the air in quantities sufficient to produce explosive or ignitable mixtures. 2. Class II locations are those that are hazardous because of the pres­ence of combustible dust. 3. Class III loca­tions are those that are hazardous because of the presence of easily ignitable fibers or fly­

- -ings, but where such fibers or flyings are not likely to be suspended in the air in quantities sufficient to produce ignitable mixtures.

area classification (division) 1. Division 1 (hazardous) locations are those where con­centrations of flammable gases or vapors exist (a) continuously or periodically during normal operations, (b) frequently during repair or maintenance or because of leakage, or (c) due to equipmef1t breakdown or faulty operation, which could cause the simulta­neous failure of electrical equipment. 2. Divi­sion 2 (normally nonhazardous) locations are those in which the atmosphere is normally nonhazardous and may become hazardous only if the ventilating system fails, pipe lines are opened, or other unusual situations occur.

area classification (group) Identified groups of chemicals and compounds whose air mix­tures have similar ignition and explosive characteristics for the purpose of testing, approval, and area classification. Group A: atmospheres containing acetylene. Group B: atmospheres containing butadiene, ethylene oxide, propylene oxide, acrolein, or hydro­gen (or gases or vapors equivalent in hazard to hydrogen). Group C: atmospheres such as cyclopropene, ethyl ether, ethylene, or gases or vapors of equivalent hazard. Group D: atmospheres such as acetone, alcohol, ammonia, benzene, benzol, butane, gasoline, hexane, lacquer solvent vapors, naphtha, natural gas, propane, or gases or vapors of equivalent hazard. Group E: atmospheres that contain metal dusts. Group G: atmo­spheres that contain combustible dusts that have a resistivity of <105 ohm-cm2.

area meter A device for measuring the flow of fluid through a passage of fixed cross-sec­tional area, usually by using a weighted pis­ton or float that is supported by the flowing fluid.

argentometer 1. A hydrometer is used to find the concentration of a silver salt in water solution.

28 ___ -4

I

4

argument I artificial intelligence (AI)

argument 1. In a mathematical function, the variable that, when a certain value is substi­tuted for it, determines the value of the func­tion. If y =j(x), then x, the independent variable, is the argument of the function. 2. An operand in an operation on one or more variables. See also parameter.

arithmetic ability The capability of perform­! ing (at least) addition and subtraction on the

personal computer. arithmetic check See nzatlzematiq1l check. arithmetic element The portion of a mechan­

1· ical calculator or electronic computer that performs arithmetic opera tions.

arithmetic expression An expression that contains any combination of data names,.. numeric literals, and named constants that is joined by one or more arithmetic operators in such a way that the expression as a whole can be reduced to a single numeric value.

arithmetic logic unit (ALD) A component of the central processing unit of a computer in which data items are compared, arithmetic operations performed, and logical operations executed.

arithmetic operation A computer operation , in which the ordinary elementary arithmetic

operations are performed on numerical quantities. Contrast with logical operation.

arithmetic operator Any of the operators + and - or the infix operators +, -, *, I, and **.

~ arithmetic unit The unit of a computing sys­1. ' tern that contains the circuits that perform

arithmetic operations. . arm To make it possible for a hardware inter­

rupt to be recognized and remembered. Con­trast with disarm. See enable.

J armature 1. The core and windings of the rotor in an electric motor or generator. 2. The portion of the moving element of an instru­ment that is acted upon by magnetic flux to produce torque.

armored meter tube A variable-area meter tube (rotometer) of all-metal construction that utilizes a magnetic coupling between the

, float and an external follower. t ARPA Advanced Research Projects Agency;

operates within United States Department of Defense, which developed the first major packet- switched digital computer network.

~. ARP Address Resolution Protocol; TCP lIP process that maps Internet Protocol (IP) addresses to Ethernet addresses; required by TCP lIP for use with Ethernet.

ARPANet Advanced Research Projects Admin­istration Network. A precursor to the Inter­net, developed in the late 1960s and early

1970s by the United States Department of Defense.

ARQ Automatic request for retransmission; in digital communications, where a receiver asks a transmitter to resend a block or frame, generally because of errors detected by the receiver.

array 1. An arrangement of elements in one or more dimensions. See also matrix and vec­lor. [Comp]2. In a computer program, a numbered, ordered c.ollection of elements, all of which have identica'l dataattributes. 3. An aggregate whose elements are of the same data type and may be uniquely referenced by subscripting. [ISA-TR50.02, Part 9-2000] [Eng] 4. A group of detecting elements that are usually arranged in a straight line (linear array) or in two-dimensional matrix (imag­ing array). 5. A series of data samples, all from the sam~ measurement point. Typically, an array is'assErihbled at the telemetry ground station for frequency analysis.

array dimension The number of subscripts needed to identify an element in the array.

array processing The capability of a com­puter to operate at a variety of data locations at the same time.

array processor A hardware device that pro­cesses data arrays. Fast Fourier transforms (FIT) and power-spectral density (PSD) are typical processes.

arrester A device that impedes the flow of large dust particles or sparks from a stack, usually through screening at the top.

arrow keys Keys on a computer keyboard tha t will move the cursor.

articulated arms (waveguides) A beam­direction arrangement in which light passes through a series of jointed pipes containing optics.

articulated structure A structure that is either stationary or movable, such as a motor vehi­cle or train, that is permanently or semiper­manently connected so that its different sections can move relative to the others, usu­ally by using pinned or sliding joints.

artifact 1. Any component of a signal that is extraneous to the variable represented by the signal. 2. In video development, the area within an image or characteristic of an image that is caused by system limitations, such as weird shimmering, jaggies, or other undesir­able distortion. Also, in digital graphics, image imperfections caused by data com­pression.

artificial intelligence (AI) That branch of com­puting that studies capabilities that resemble human thought processes, such as reasoning,

29

artificial language / aspiration

learning, vision, aural recognition, and even self- improvement.

artificial language 1. A language specifically designed for ease of communication in a par­ticular area of endeavor, but one that is not yet natural to that area. This is contrasted with a natural language which has evolved through long usage. 2. A programming language that is based on a prescribed set of rules estab­lished ahead of time, such as BASIC, COBOL, Pascal, and the like.

artificial radioactivity Radioactivity that.is induced by bombarding a material with a beam of energetic particles or with electro­magnetic radiation.

artificial weathering Producing controlled changes in materials, such as surface appear­ance, under laboratory conditions that simu­late outdoor exposure.

as built 1. A document revision that includes all the modifications that were performed as a result of actual fabrication or installation. As-built documents may have various issues that reflect various milestones such "as pur­chased," "as manufactured," "as insured," and "as commissioned.". [ISA-RP60.4-l990] 2. During construction, changes that are made in design and installation and marked up on drawings as built.

ascender In typography, that part of a lower­case letter that rises above the main body, as in the characters band d.

ASCII American Standard Code for Informa­tion Interchange; a widely used code in which alphanumerics, punctuation marks, and cer­tain special machine characters are repre­sented by lmique, seven-bit binary numbers. One hundred and twenty-eight different binary combinations are possible (27 = 128), thus 128 characters may be represented. ASCII was defined in ANSI X3.4-l986 and is normally used for asynchronous transmis­sion.

ASCII file A text file that uses the ASCII character set.

as-fabricated The condition of a structure or material after assembly, without any condi­tioning treatment such as a stress-relieving heat treatment. Specific terms such as "as-welded," "as-brazed," or "as-polished" are used to designate the nature of the final step in fabrication.

as-fired fuel Fuel in the condition it was as it was fed to the fuel-burning equipment.

as found The condition in which a channel, or portion of a channel, is found after a period .. of operation and before recalibration (if

l"

needed). [ANSI/ISA-67.04.0l-2000; ISA­RP67.04.02-2000]

ash The noncombustible inorganic matter in the fuel.

ash content The incombustible residue that remains after burning a combustible material completely.

ash-free basis The method for reporting fuel analysis whereby ash is deducted, and other constituents are recalculated to total 100 per­cent...

ASI Actu,flt.oi: sensor interface; European "fieldbus" for binary sensors and actuators.

as left The condition in which a channel, or portion of a channel, is left after calibration or final setpoint device setpoint verification. [ANSl/ISA-67.04.01-2000; ISA-RP67.04.02­2000]

ASN.l Abstract Syntax Notation One. An ISO standard (DIS 8824) that specifies a c~m.onical method of data encoding. This standard is an extension of CCITT standard X.409.

ASN.l Abstract Syntax otation One; ISO IS 8824 and IS 8825 encoding and decoding structures.

ASPC Algorithmic statistical process control; closed loop version of normally open loop SPC.

aspect ratio [Comp] 1. The ratio of a symbol's height to its width. [ISA-S.5-l985] [Comm] 2. The ratio of frame width to height for a television picture. It is 4:3 in the United States, Canada and United Kingdom. [Eng] 3. In any rectangular structure, such as the cross section of a duct or tubular beam, the ratio of the longer dimension to the shorter. 4. A ratio of width to depth used to calculate resistance to flow in a rectangular elbow.

aspheric Not spherical or flat, as of the sur­face of optical elements. Lenses with aspheric surfaces are sometimes called "aspheres."

aspirating burner A burner in which the fuel is in a gaseous or finely divided form and is burned in suspension. The air for combus­tion is supplied by bringing into contact with the fuel air that has been drawn through one or more openings by the lower static pres­sure created by the velocity of the fuel stream.

aspiration Using a vacuum to draw up gas or granular material. This is often accomplished by passing a stream of water across the end of an open tube or through the run of a tee joint, where the open tube or branch pipe extends into a reservoir that contains the gas or granular material.

30

- ~ _.,.,........- ----------_._--._---~--'_._-----~-_ ... _---­

I as-received fuel/astronomical theodolite

as-received fuel Fuel in the condition it is in as it received at the plant.

assemble To prepare a machine-language program from a symbolic language program

~ ~~ by substituting absolute code for symbolic operation codes and absolute or relocatable addresses for symbolic addresses.

assembler A program that translates sym­bolic source code into machine instructions by replacing symbolic operation codes with binary operation codes and symbolic addresses with absolute or relocatable addresses.

assembly [Eng] 1. A unit constructed of many parts or components and that func­tions in service as a single device, mecha­nism, or structure. [Comp] 2. A mid-level computer language.

assembly language A computer program­ming language, similar to a computer lan­guage, in which the instructions usually have a one-to-one correspondence with computer instructions in machine language and that utilizes mnemonics to represent instructions.

assembly list A printed list that is the by-product of an assembly procedure. It lists in logical instruction sequence all details of a routine, showing the coded and symbolic notation next to the actual notations estab­lished by the assembly procedure. This list­

» ing is highly useful when debugging a .- . routine.-{-,\

assembly program See assembly system. assembly system A system comprised of two

elements, a symbolic language and an assembly program. The assembly program translates the source programs written in the symbolic language into machine language.

assign To designate a part of a system for a specific purpose.

assignable Permitting the channeling (or directing) of a signal from one device to another without the need for switching, patching, or changes in wiring. [ANSl/lSA­5.1-1984 (R1992); lSA-5.3-1983]

,: assignment statement A program statement . that calculates the value of an expression and

assigns it a name (e.g., x = x + 5, Y=8). associated apparatus An electrical apparatus

in which there are both intrinsically safe cir­cuits and non-intrinsically safe circuits. The latter can affect the safety of the former. Note: An associated apparatus may be either (a) an electrical apparatus that has an alter­native type of protection for use in the appro­priate explosive gas atmosphere or (b) an electrical apparatus not so protected and that should not therefore be used within an

31

explosive gas atmosphere. An example of the latter is a recorder that is not itself in an explosive gas atmosphere but is connected to a thermocouple that is situated within an explosive gas atmosphere where only the recorder input circuit is intrinsically safe. [ISA-12.02.01-1999 (IEC 60079-11 Mod)]

associated electrical apparatus An electrical apparatus in which the circuits are not all intrinsically safe but that contains circuits that can affect the safety of intrinsically safe circuits connected to it.

association [Sci] 1. The combining ot ions into larger ion clusters in concentrated solutions. [Comp] 2. A program-to-program logical relationship which may be dynamically established and torn down; may be assumed, may not be required. See connection.

associative memory A neural network archi­tecture used in pattern recognition applica­tions, in which the network is used to associate data patterns with specific classes or categories it has already learned.

associative storage A storage device in which storage locations are identified by their con­tents, not by names or positions. Synony­mous with content-addressed storage. Contrast with parallel search storage.

astable circuit A circuit that alternates auto­matically and continuously between two unstable states at a frequency that is depen­dent on circuit constants, for example, in a blocking oscillator.

astable multivibrator A multivibrator in which each active device alternatelv con­ducts and is cut off for intervals of time as determined by circuit constants, without using external triggers.

astatic Without polarity; independent of the earth's magnetic field.

astigmatism A defect in an optical element that causes rays from a single point in the outer portion of a field of view to fall on dif­ferent points in the focused image.

ASTM American Society for Testing and Materials; scientific and technical organiza­tion that develops material standards and testing methods.

astrodynamics A practical application of fun­damental science to the problem of planning and controlling the trajectories of space vehi­cles.

astrolabe An instrument formerly used to find the altitudes of celestial bodies; a prede­cessor of the sextant.

astronomical theodolite See altazimuth.

I asymmetric rotor / attached equipment

asymmetric rotor A rotating machine ele­ment whose axis of rotation is not the same as its axis of symmetry.

asymmetrical compression A data compres­sion system that requires more processing capability to compress an image than to decompress an image. Such systems are typi­cally used for the mass distribution of pro­grams on media such as CD-ROMs.

asymmetry potential The difference in potential between the inside and outside pH­sensitive glass layers when they are both in contact with 7 pH solutions. This difference is caused by the deterioration of the pH-sen­sitive glass layers or by contamination of the internal fill of the measurement electrode. .

asynchronous [Comp]1. Circuitry or opera­tion without common clock or timing sig­nals. 2. A mode of operation in which an operation is started by a signal before the operation on which this operation depends is completed. When referring to hardware devices, it is the method in which each char­acter is sent with its own synchronizing information. The hardware operations are scheduled by "ready" and "done" signals rather than by timer intervals. This implies that a second operation can begin before the first operation is completed. [Eng] 3. Not synchronous with the line frequency as applied to rotating a.e. machinery.

asynchronous transmission 1. Transmission in which each information character, or sometimes each word or small block, is indi­vidually synchronized, usually by using start and stop elements. The gap between each character (or word) is not of a necessarily fixed length. (Compare with synchronol/s transmission.) Often called "start-stop trans­mission." 2. A data transmission mode in which the timing is self-determined and not controlled by an external clock.

ATM Asynchronous transfer mode; type of packet switching that transmits fixed-length units of data, and being asynchronous the recurrence of cells does not depend on the bit rate of the transmission system, only on the source requirements (packets include address of their destinations). Provides a very fast and efficient transfer mode for mul­timedia applications (up to ten thousand text pages per second), allowing, for example, real-time video transfer and groupware slide projection. Also, abbreviation for atmo­spheres, a unit of pressure measurement.

atmometer A generic name for any instru­ment that measures evaporation rates. Also

knJwn as an "atmidometer," "evaporime­ter," or "evaporation gauge."

atmospheric air Air under the prevailing atmospheric conditions.

atmospheric communication Sending sig­nals in the form of modulated light through the atmosphere, without the use of fiber optics to contain and direct the beam.

atmospheric corrosion Corrosion that occurs naturally due to exposure to climatic condi­tions. Cprro;;ion rates vary by global location because ofvariatiohs in average tempera­ture, humidity, rainfall; because of airborne substances such as sea spray, dust, and pol­len; and because of airborne pollutants such as sulfur dioxide, chlorine compounds, fly ash, and other combustion products.

atmospheric monochromator A monochro­mator in which the optical path is through air. This is the standard type of monochro­mator used for visible and infrared wave­lengths transmitted by air.

atmospheric pressure The barometric read­ing of pressure exerted by the atmosphere. At sea level it is 14.7 lb per sq in. or 29.92 in. 6f mercury.

atomic mass unit A unit for expressing atomic weights and other small masses. It equals exactly 1/12 the mass of the carbon 12 nuclide.

atomic number An integer that designates the position of an element in the periodic table of the elements. It equals the number of protons in the nucleus and the number of electrons in the electrically neutral atom.

atomic weight The weight of a single atom of any given chemical element. It is usually taken as the weighted average of the weights of the naturally occurring nuclides, expressed in atomic mass units.

atomization Mechanically producing fine droplets or mist from a bulk liquid or molten substance.

atomizer A device by means of which a liq­uid is reduced to a very fine spray.

atom probe An instrument that consists of a field-ion microscope with a probe hole in its screen that opens into a mass spectrometer. It is used to identify a single atom or molecule on a metal surface.

ATRAC Adaptive transform acoustic cod­ing; coding method to create minidisks using a varying number of bits per sample depend­ing upon "critical" frequencies encountered.

attached equipment The auxiliary equip­ment that must be located on the valve or actuator. [ANSI/ISA-75.05.01-2000]

32

I attachment plug / authority having jurisdiction

attachment plug A connecting device for a flexible cord that, when it is inserted into a receptacle, establishes supply circuit connec­tions between the flexible cord and the recep­

~'. tacle. . attemperation Regulating the temperature of

a substance, for instance, by passing super­heated steam through a heat exchanger or by injecting water mist into it to regulate final steam temperature.

attemperator A mechanical device used for maintaining and controlling the temperature of superheated steam. [ANSI/ISA-77.44­1995]

.attemperator (direct contact type) A mechani­cal device in which the steam and the cooling medium (water) are mixed. [ANSI/ISA­77.44-1995; ANSI/ISA-77.44-1995]

attenuate To weaken or make thinner-for example, to reduce the intensity of sound or ultrasonic waves by passing them through an absorbing medium.

attenuation 1. A decrease in signal magni­tude between two points or between two fre­quencies. 2. The reciprocal of gain, when the gain is less than one. [ISA-RP55.1-1075 (RI983)] Note: Attenuation may be express­ed as a "dimensionless ratio" or "scalar

" ratio" or expressed in decibels as "20 times the 10glO of that ratio." [ANSI/ISA-51.1-1979 (RI993)] 3. The loss of amplitude in a signal as it is transmitted through a conductor. The opposite of "gain."

attenuator 1. An optical device that reduces the intensity of a beam of light passing through it. 2. An electrical component that reduces the amplitude of a signal in a con­trolled manner.

attitude 1. The relative orientation of a vehi­cle or object as represented by its angles of inclination to three orthogonal reference axes. [ISA-37.1-1975 (RI982)]2. The position of an object in space as determined by the angles between its axes and a selected set of planes.

attitude error The error caused by the orien­tation of the transducer relative to the direc­tion in which gravity acts upon it. [ISA-37.1­1975 (RI982)] See acceleration error.

~. . attribute 1. A property or characteristic of an entity. For instance, value and status are

; attributes of an output parameter. [ISA­TR50.02, Part 9-2000] 2. A characteristic qual­ity of a data type, data structure, element of a data model, or system. In object-oriented programming, an attribute is some piece of information that describes a characteristic of the object.

attribute sampling A type of sampling inspection in which an entire production lot is accepted or rejected if the number of items in a statistical sample has at least one charac­teristic (attribute) that does not meet specifi­cations.

auctioneering device See signal selector. audible device A device that calls attention

by emitting a sound when abnormal process condition~ 9ccur. An audible device may also call attention t<Ja return.to.Dormal condi­tions. [ANSI/ISA-18.1-1979 (RI992)]

audible device follower See auxiliary output. audio Pertaining to audible sound-usually

taken as referring to sound frequencies in the range 20 to 20,000 Hz.

audio analyzer An instrument that displays audio signal voltage or power as a function of the signal's frequencies.

audio frequencies Frequencies that can be heard by the human ear, usually between 15 and 20,000 cps (cycles per second).

audiometer An instrument used to measure the ability of people to hear sounds. It con­sists of an oscillator, amplifier, and attenua­tor, and may be adapted to generate pure tones, speech,'or bone~conductedvibrations.

audit The action of corroborating the evi­dence regarding the authenticity and validity of data or procedures.

auger 1. A wood-boring tool that consists of a shank with a T-shaped handle. 2. A feeding device that consists primarily of a set of spi­ral blades mounted on a central shaft or fas­tened together to make a spiral rotating assembly. The auger may rotate in a tube, trough, or housing to move powdered, gran­ular, or semisolid material axially. In some applications, the auger may be constructed of two counter-spiraled zlUgers, which feed material toward the midpoint or outward from the midpoint of the axis, depending on the direction of rotation.

AUI Access unit interface; attachment unit interface; twisted-pair telephone wire IEEE standard for Ethernet.

authoring system Software that helps devel­opers design interactive courseware easily, without the painstaking detail required in computer programming.

authoring tools Software capabilities that make it possible to create applications with­out involving oneself in the tedious details of programming.

authority having jurisdiction The organiza­tion, office, or individual that has the respon­sibility and authority to approve equipment, installations, or procedures. Note: The term

33

auto answer / automatic/manual station

I

I ! I

I II

. ! I,

"authority having jurisdiction" is used broadly since jurisdiction and approval agencies vary, as do their responsibilities. Where public safety is primary, the authority that has jurisdiction may be federal, state/ provincial, local, other regional department, or an individual such as an inspector from a labor or health department, electrical inspec­tor, or others with statutory authority. An insurance inspection agency, rating bureau,

.or other. insurance company representative may be the authority with jurisdiction. An owner or his designated agent may also assume the role. At government-owned installations, the commanding officer, departmental official, or designated agent may be the authority with jurisdiction. [ISA­12.01.01-1999; ANSI/ISA-RP12.6-1995]

auto answer A modem that can automati­cally answer incoming telephone calls from computers and provide data to that system.

autoclave An airtight vessel for heating its contents and sometimes agitating them. It usually uses high-pressure steam to process, sterilize, or perform cooking steps using moist or dry heat.

autocollimator A telescopic sight that includes a light source and a partially reflect­ing mirror, focused to infinity. It is used to measure small angular motion and check alignment.

autocorrelation In a time series, the relation­ship between values of a variable taken at certain times in the series and values of the same variable taken at other, usually earlier times.

auto dial A modem capable of connecting to a telephone system and dialing a number. Modem and communications software that performs proper procedures so computers may exchange data.

autoexec.bat The name of the file in MS-DOS that contains the commands to be executed when the computer is booted.

autogenous ignition temperature (AIT) The minimum uniform temperature that is required to initiate or cause the self-sus­tained combustion of a solid, liquid, or gas­eous substance, independent of any other ignition source. [ANSI/ISA-12.01.01-1999] Formerly known as "auto-ignition tempera­ture."

auto-manual station Synonym for control sta­tion. [ANSI/ISA-5.1-1984 (R1992)]

automate To convert a procedure, a process, or equipment into an automatic operation.

automatic 1. Self-acting or operating by its own mechanism when actuated by some imper­

sonal influence, as, for example, a change in current strength, pressure, temperature, or mechanical configuration. [ISA-12.01.01­1999] 2. A machine that operates automati­cally. 3. Functioning without intervention by a human operator under specified condi­tions, as of a process or device.

automatically programmed tools A numeri­cal language.

automatic control The type of control in which there is no direct human action on the controlling device.

automatic control engineering The branch of science and technology that deals with the design and use of automatic control devices and systems.

automatic control panel A panel of indicator lights and switches that display an indication of process conditions and from which an operator can control the operation of the pro­cess.

automatic control system See control system, automatic.

automatic controller Any device that mea­sures the value of a process variable and gen­erates a sign'll or some controlling action to ensure that the value corresponds with a ref­erence value, or set point.

automatic error correction A technique for detecting and correcting errors that occur in data transmission or data handling. Such correction usually requires the use of special codes or automatic retransmission, which detects and corrects errors occurring in trans­mission. The degree of correction depends upon the coding and equipment configura­tion.

automatic frequency control A device or cir­cuit designed to maintain the frequency of an oscillator within a preselected band of fre­quencies. In a FM radio receiver, the circuitry that senses frequency drift and automatically controls an internal oscillator to compensate for the drift.

automatic gain control An auxiliary circuit that adjusts the gain of the main circuit in a predetermined manner when the value of a selected input signal varies.

automatic lighter A means for igniting fuel without manual intervention. Usually applied to liquid, gaseous, or pulverized fuel.

automatic/manual station A device that enables an operator to select an automatic signal or a manual signal as the input to a controlling element. The automatic signal is normally the output of a controller, while the

34

I automatic pilot / auxiliary output (auxiliary contact)

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manual signal is the output of a manually operated device. [ANSI/ISA-51.1-1979]

automatic pilot An automatic control system that is adapted to maintain an aircraft in sta­ble, level flight or to execute selected maneu­vers.

automatic reset See reset. automatic send/receive (ASR) A teletype­

writer .unit with keyboard, printer, paper tape, readerftransmitter, and paper tape punch. This combination of units may be based on line or off line and, in some cases, on line and off line simultaneously.

automatic test equipment (ATE) Equipment that analyzes the response of an instrument under test to a well-defined electrical stimu­lus in order to evaluate its performance with minimum human intervention.

automatic test program generator (ATPG) Computer software and/or hard­ware that automatically generates programs for automatic test equipment based on device characteristics and test parameters.

automatic tracking The action of a control system to automatically track a set point or the process variable without any other cor­rective mechanisms. [A SI/ISA-77.13.01­1999]

automatic utility translator (AUTRAN) A process control language and system offered by Control Data Corporation.

automatic zero- and full-scale calibration Zero and sensitivity stabiliza­tion by servos for the purposes of comparing demodulated zero- and full-scale signals with zero- and full-scale references.

automation 1. The implementation of pro­cesses by automatic means. 2. The theory, art, or technique of making a process more auto­matic. 3. The investigation, design, develop­ment, and application of methods for rendering processes automatic, self-moving, or self-controlling. 4. The conversion of a procedure, a process, or equipment to auto­matic operation.

autonomous system A system with no inputs.

autoradiography A technique for producing a radiographic image by using the ionizing radiation produced by radioactive decay of atoms within the test object itself.

autoranging The automatic selection of an appropriate range setting so as to measure the value of a signal applied to an instru­ment's input.

auto-tracking antenna A receiving antenna that always points to the transmitting site,

35

automatically tracking all movements of the vehicle being telemetered.

autotransformer A type of transformer in which certain portions of the windings are shared by the primary and secondary circuits.

auto restart The capability to perform auto­matic initialization functions so as to resume operations after an equipment or power fail­ure.

auto tuning A technique within a controller that analyzes the effects of a change in the set point of a closed toOp or iIi the control'output of an open loop and adJusts or recommends tuning parameters based upon that analysis. The change is necessary because it allows the tuner to learn the magnitude and period of process response, which it is uses to calculate new parameters. See self-adaptive tuning.

auto-tuning controller A controller feature that calculates PID settings based on calcula­tions using measured process dynamics and combining those with the parameters of a PID controller. Depending on who manufac­tured the controller, open or closed loop tun­ing may be used. Calculations may be based on transient responses, frequency responses, or parametric models. .

auto-zero An automatic internal correction for offsets and/or drift at zero signal input.

auto-zero logic module A component of a digital controller whose function is primarily to establish an arbitrary zero-reference value for each individual measurement.

auxiliary contact See auxiliary output. auxiliary device 1. Generally, any device that

is separate from a main device but is neces­sary or desirable for the effective operation of the system. 2. Specifically, any device used in conjunction with an instrument to extend its range, increase its accuracy, otherwise assist in making a measurement, or perform a function not directly involved in making the measurement.

auxiliary location A location for panel instru­ments that is somewhere other than the con­trol room.

auxiliary means A device or subsystem, usu­ally placed ahead of the primary detector, that alters the magnitude of the measured quantity to make it more suitable for the pri­mary detector without changing the nature of the measured quantity.

auxiliary output (auxiliary contact) 1. An output signal that is operated by a single alarm point or group of points so it can be used with a remote device. [ANSI/ISA­S18.1-1979 (R1992)] 2. A secondary output.

I auxiliary output, audible device follo·w"er / average resolution

,, , I· I

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auxiliary output, audible device follower (hom relay contact) An auxiliary output that operates while the common alarm audi­ble device operates. [ANSI/ISA-S18.1-1979 (R1992)]

auxiliary output, field contact follower An auxiliary output that operates while the field contact indicates an abnormal process condi­tion. [ANSI/ISA-S18.1-1979 (R1992)]

auxiliary output, lamp follower An auxiliary output that operates while the visual display lamps indicate an alarm, silenced, or ack­nowledged state. [ANSI/ISA-S18.1-1979 (R1992)]

auxiliary output, reflash An auxiliary output that operates when anyone of a group of alarm points indicates an abnormal process condition. The output usually returns to nor­JTIal briefly when each alarm point changes to an abnormal process condition and returns to normal when all alarm points in the group indicate normal process condi­tions. [ANSI/ISA-S18.1-1979 (R1992)]

auxiliary panelLA panel that is not in the main control room. The front of an auxiliary panel is normally accessible to an operator, but the rear is normally accessible only by maintenance personnel. 2. Located at an aux­iliary location.

auxiliary storage A storage device in addi­tion to the main storage of a computer, for example, magnetic tape, disk, magnetic drum, or core. Auxiliary storage usually holds much larger amounts of information than the main storage, but the information is accessible less rapidly. Contrast with main storage.

availability The ratio of time in which a sys­tem is operating correctly to the total hours of scheduled operation. Before 1962, avail­ability was calculated as the value of MTBF -0­

(MTBF + MTTR). After 1962, it was defined by military specifications. as MTTF -0- (MTTF + MTTR), which is a calculation of those times, not a tested value. Very much depen­dent on environmental conditions, the MTBF of electronic equipment will drop by about half for every lOoC increase. All other ambi­ent factors will have a similar impact.

availability factor The fraction of the time during which the unit is in operable condi­tion.

available capability The portion of the pro­duction capability that can be attained but is not committed to current or future produc­tion. [ISA-95.00.01-2000]

available draft The draft that may be utilized to cause the flow of air for combustion or the flow of prod ucts of combustion.

available energy Energy that theoretically can be converted into mechanical power.

available heat In a thermodynamic working fluid, the amount of heat that could be trans­formed into mechanical work under ideal conditions by reducing the temperature of the working fluid to the lowest temperature available for heat discard.

available power" . AI).attribute of a linear source of electric pmver. It is defined as V /4R, where V is the open-circuit rmsrms rms voltage of the power source, and R is the resistive component of the internal imped­ance of the power source.

available power gain An attribute of a linear transducer that is defined as the ratio of power available from the output terminals of the transd~lcerJ.o the power available from the input circuit.under specified conditions of input termination.

available work The capacity of a fluid or body to do work if applied to an ideal engine.

avalanche. The production of a large number of ions by cascade action in which a single charged particle, accelerated by a strong elec­tric field, collides with neutral gas molecules and ionizes them.

avalanche photodiode (APD) A photodiode that is designed to take advantage of ava­lanche multiplication of photocurrent. As the reverse-bias voltage approaches the break­down voltage, hole- electron pairs created by absorbed photons acquire sufficient energy to create additional hole-electron pairs when they collide with substrate atoms, producing a multiplication effect.

average outgoing quality limit The average percentage of defective units that remain undetected in all lots tha t pass final inspec­tion. It is a measure of the ability of sampling inspection to limit the probability of ship­ping defective product. Here, a defective unit is considered to be one that contains at least one attribute that does not meet specifica­tions.

average-position action A type of control system action in which the final control ele­ment is positioned in either of two fixed positions. The average time at each position is determined from some function of the measured value of the controlled variable.

average resolution The reciprocal of the total number of output steps over the unit range multiplied by 100 and expressed in % VR.

36

I averaging pitot tube I azimuth circle�

[ISA-37.6-1982 (R1995); ISA-37.12-1982� (R1995)]�

averaging pitot tube An adaptation of the pitot tube in which a multiple-ported pitot tube spans the process tube. Total pressure is measured as a composite of the pressures on several ports facing upstream, while static pressure is measured using one or more ports facing downstream. The device works best for clean liquids, vapors, and gases, but

., can be used for streams that contain sus­pended solids or viscous contaminants if the purging flow is supplied to the measuring

I tube. AVI Audio video interleaved; digital file for­

ma t by Microsoft developed for dynamic graphics.

AWG American Wire Gauge; United States standard system used for designating the size of electrical conductors; gauge numbers are inverse to size.

axial In the direction parallel to the shaft centerline.

axial fan Consists of a propeller or disk type of wheel within a cylinder in which the air is

. discharged parallel to the axis of the wheel. axial-flow Describing a machine such as a ,. pump or compressor in which the general .; direction of fluid flow is parallel to the axis . of its rotating shaft. axial hydraulic thrust In single-stage and

multiple-stage pumps, the axial component ,: of the summation of all unbalanced impeller 1 forces. axial runout For a rotating member, the total

amount that a specific surface deviates from a plane that is perpendicular to the axis of rotation in one complete revolution. It is usu­ally expressed in 0.001 in., or in some other suitable unit of measure, taken at a specific radial distance from the axis of rotation.

axle A rod, shaft, or other supporting mem­�ber that carries wheels and either transmits� rotating motion to the wheels or allows the� wheels to rotate freely about it.�

azimuth angle An angular measurement in a� horizontal plane about some arbitrary center� point, in which true north or some other arbi­�trary direction is used as a reference direc­�tion (0).�

azimuth circle A ring scale graduated from 0� to 360 and used with a compass, radar plan� position indicator, direction finder, or other� device to indicate compass direction, relati\'e� bearing, or azimuth angle.�

37