I label In data processing, a ladder logic Traditional language of

L set of symbols that is used to identify or describe an item,i;� record, message, or file. Occasionally, it may be the same as the address in stor­age.

f

labeled Equipment or materials to which have been attached the label, symbol, or other identifying mark of an approved organization. The approval to label the equipment or materials is given by a gov­ernment authority that has jurisdiction and is concerned withproduct evaluation. It peri­odically inspects the production of labeled equipment or materials, and this organiza­tion's label indicates that the manufacturer complies with the appropriate, specified standards or performance l,evels. Note: Some agencies use the terms approved, listed, or certi­fied equipment to indicate compliance with the applicable standard. [ANSI/ISA-RP12.6­1995; ANSI/ISA-12.01.01-2000]

labeled common Named data areas that are accessible to all computer programs that declare them.

labeled molecule A molecule of a specific chemical substance in which one or more of its component atoms is an abnormal nuclide. That is, a nuclide that is radioactive when the molecules are normally composed of stable isotopes, or vice versa.

lactometer A hydrometer that is designed to measure the specific gravity of milk.

ladder diagram A diagram used to program a programmable controller. Power flows through a network of relay contacts that are arranged in horizontal rows called "rungs" between two vertical rails on the side of the diagram that contains the symbolic power. The basic program elements are contacts and coils as in electromechanical logic systems.

DRAWN MANUALLY

t=:~07~~ FREE·FORMAT EaUlVALENT PLC DIAGRAM

-----1'rlf-----r~c~ __---IlL II-----~

LEGEND:

1PB = PUSH BUTTON 2CR. 4CR. seR = CONTROL RELAY SWITCH CONTACTS 3lS = lEVEL SWITCH SOL A • SOLENOID-ACTUATED DEVICE

A Relay Ladder Diagram

programmable controllers that originated� with electricians and the electrical mainte­�nance of electromechanical relay panels. It is� a form of symbolic representation that sche­�matically illustrates the functions of a control� circuit. The power lines form the sides of a� ladderlike structure, with the program ele­�ments arranged to form rungs. One of five� language? accepted under the IEC 61131� standar_~ ft:>[ PLCs..S~e function block diagram,� instruction list, sfr~t"ctured text, and SFC. See� also state logic.�

LADDER LOGIC SIMULATES HARD-WIRED RELAY LOGIC

- POWER RAILS ON EITHER SIDE

- CONTACTS ARE INPUTS TO LOGIC

-COILS ARE OUTPUTS

-RUNGS CARRY POWER (BOOLEAN DATA)

-BRANCHING AND LOOPING ARE PROVIDED BY LABELS

INA INS OUT 1

OUT1 = INA& INE: ". ~'3;;'

." OUT2 =INC OR IND�

..of---'� 1--1-:~:~ I OUT3 = NOT INE

Structured Text Compared to Ladder Logic

lag 1. A relative measure of the time delay between two events, states, or mechanisms. 2. In control theory, a transfer function term� in the form, l/(Ts + 1). A process for which a� lag is the dominant dynamic characteristic� approaches equilibrium exponentially.�

lagging 1. In an AC circuit, a condition in which peak current occurs at a later time in each cycle than does peak voltage. 2. A ther­mal insulation, usually made of rock wood and magnesia plaster, that is used to prevent heat transfer through the walls of process equipment, pressure vessels, or piping sys­tems.

lag time An interval of time between the ini­tiation of a discrete sample (particle, mole­cule, atom) at the sample tap to its termination at a specific volumetric flow rate through the sample line.

lambert A unit of luminance. It equals the uniform luminance of a perfectly di,ijusing surface that is emitting or reflecting:1ight at one lumen per square centimeter.·~

Lambert's cosine law The radiance of certain surfaces, known as Lambertian reflectors, Lambertian radiators, or Lambertian sources, is independent of the angle from which the surface is viewed.

laminar boundary layer A layer of a moving turbulent stream adjacent to the wall of a

283�

laminar flOlV / laser Doppler flowmeter

I

pipe or other conduit in which the motion approximates streamline flow.

laminar flow 1. A type of streamline flow that is most often observed in viscous fluids near solid boundaries. It is characterized by the tendency for fluid to remain in thin, parallel layers to maintain uniform velocity. 2. A non­turbulent flow regime in which the stream fil­aments glide along the pipe axially, with essentially no transverse mixing. Also known as viscous flow or streamlineflow. 3. Flow under conditions in which forces resulting from vis­cosity are more significant than forces result­ing from inertia, generally below Reynolds number of 2000.

lamp Any device for producing light, usually one that converts electric energy into light.

lamp cabinet A cabinet that contains visual displays only. [ANS1/ISA-18.1-1979 (R1992)]

lamp follower See auxiliary output. lamp test See test. lance door A door through which a hand

lance may be inserted in order to clean heat­ing surfaces.

land ban Under the Resource Conservation a~d Recovery Act (U.5.), a land ban prohibits disposal on land of certain hazardous wastes unless they meet applicable treatment stan­dards.

language In data processing, a set of repre­sentations, conventions, and rules that are used to convey information. See algorithmic language, art{ficiallanguage, machine language, natural language, object language, problem-ori­ented language (POU, procedure-oriented lan­guage, programming language, source language, and target language.

language extendability The ability to chano-e a programming language through source 0

statements written in that language. language translator A general term for any

assembler, compiler, or other routine that accepts statements in one language and pro­duces equivalent statements in another lan­guage.

lantern ring A rigid spacer that is assembled in the packing box, normally with packing above and below it. It is designed to allow the packing to be lubricated or a leak-off con­nection to be accessed. [ANSI/ISA-75.05.01­2000]

LAP Line Access Procedure. A CCITT-spec~­fied data link protocol.

lap joi~t A connection between two parts that is made by overlapping members at the junction and then welding, riveting, or bolt­ing them together.

Laplace transform For a function j(x), its ~aplace transform is the function F(y), which is defined as the integral over x from 0 to 00 of the function e-Yj(x).

Laplace transform, nilateral Of a function f(t), the quantity that is obtained by perform­ing the operation:

sr dt. FJs) =

ex:

f f(t) e­o

where: F(.s) =fUJ1ction of s; 5 =complex vari­able, _ + JCD; f(t) == function of t; t = time, sec­ond~; _ = real part of the complex variable 5; J= n; CD = angular frequency radians per second. [ANSI/ISA-51.1-1979 (R1993)]

lapped-in Mating contact surfaces that have been refined by grinding and/or polishing them together or separately in appropriate fixtures. [ANSI/ISA-75.05.01-2000]

lapping 1. A process of mating contact surfaces by grinding and/or polishing. [ANSI/ISA­75.05.01-2000] 2. Smoothing or polishing a surface by rubbing it with a tool made of cloth, leather, plastic, wood, or metal in the presence of a fine abrasive.

laptop A small, portable computer that usu­ally has a flip-up screen.

lap weld ~ lap joint that is made by welding. large core flber An optical fiber that has a ~omparatively large core, usually of a step mdex type. There is no standard definition of "large," but for the purposes of this defini­tion, diameters of 400 micrometers or more are designated as "large."

large-scale integration (LSI) 1. The process of fabricating integrated circuits with over 1,000 tran~istors per chip. 2. A computer chip that contams a large number of digital cir­cuits in a small area.

Laser "Light Amplification by Stimulated Emission of Radiation." A laser is a source of elec~romagnetic (E~) radiation generally in the mfrared (IR), Visible, or ultraviolet (UV) bands. It is characterized by small diver­genc~, coherence, and monochromaticity.

laser dIOde array A device in which the out­put of several diode lasers is brought together in one beam. The lasers may be inte­wated on th: same substra!~ or consist of discrete deVICes that are coupled optically and electronically. '.~

laser disk An optical medium that is capable of holding thirty minutes of moving video footage or up to 54,000 individual frames of still video per side. Individual segments of these frames can be accessed by computer.

laser Doppler flowmeter An apparatus for determining flow velocity and velocity pro­

284

file by measuring the Doppler shift in the laser radiation scattered from particles in the moving fluid stream. Contaminants such as smoke may have to be introduced into a gas stream to provide scattering centers. This technique can be used to measure velocities of 0.01 to 5,000 in./s (0.25 mmls to 125 m/s).

laser glass An optical glass that is doped with a small concentration of a laser mate­rial. When the impurity atoms are excited by light, they are stimulated to emit laser light.

laser interferometer A type of optical inter-· ferometer that uses a laser as the source of monochromatic light. Accuracies of better than 20 microinch (1.25 /lm) are achieved when measuring lengths up to 200 in. (5.08 m).

laser ionization mass spectrometer (LIMS) An instrument that analyzes sam­ple surfaces in a region 1 to 2 microns across. The surface is locally heated with a laser pulse to desorb or ionize atoms and then the liberated atoms or ions are mass analyzed.

laser line filter A filter that transmits light in a narrow range of wavelengths, centered on the wavelength of a laser. Light at other wavelengths is reflected. Such filters are used to remove light from nonlaser sources, which could interfere with the operation of a laser system.

laser printer A print-quality printer that uses a laser beam to electrostatically transfer an image to paper.

laser simulator A light source that simulates the output of a laser. In practice, the light source is a 1.06 micrometer LED that simu­lates the output of a neodymium laser at much lower power levels.

last in, first out (LIFO) In an ordered push­down stack, a discipline wherein the last transaction to enter a stack is also the first to leave it. Contrast withfirst in,first Ollt (FIFO).

latching digital output A contact closure out­put that holds its condition (set or reset) until the execution of a computer program changes it. See momentary digital Olltput.

latching relay A device or program element that retains a changed state when power is removed.

latch switch A control for preventing a fuel valve from opening if the burner is not secured in the firing position.

latency In data processing, the time between the completion of the interpretation of an address and the start of the actual transfer from the addressed location. Latencv includes the delay that is associated\vith

laser glass / LCP

accessing storage devices such as drums and delay lines.

latent heat The amount of heat absorbed or evolved by 1 mole, or a unit mass, of a sub­stance during a change of state (e.g., fusion, sublimation, or vaporization) at constant temperature and pressure. Latent heat does not cause a temperature change.

lateral displacement loss The loss of power that result;> from a lateral displacement of the optimum qlign.mentbetween two optical fibers or befween a fiber and an active device.

latest actuation (ACTN) point The value fur­thest from the normal operating point of the process variable at which a bistable or chan­nel could be expected to actuate. [ANSIjISA­TR67.04.08-1996]

latitude Of a photographic emulsion, the ratio of the exposure limits between whi.ch the film density curve (known as the "Hurter aoo Driffield curve," or H & D curve) is essentially linear.

lattice network An electronic network that is composed of four branches connected end to end to form a mesh. Two nonadjacent junc­tions are the input terminals, and the two remaining nonadjacent junctions are the out­put terminals.

lattice parameter In crystallography, the length of any side of the unit cell in a given space lattice. If the sides are unequal, all unequal lengths must be specified.

Lauritsen electroscope An electroscope in which the sensitive element is a metallized quartz fiber.

layer An element of a digital communication stack. One level of a hierarchy of functions or segments of a protocol that performs special­ized roles (physical, data link, application, user). In the OSI (Open System Interconnec­tion) reference model, a layer is one of seven basic layers, consisting of a collection of related network processing functions. See Open System Interconnection (OSI).

L-band In telemetry, the radio spectrum that is available for manned vehicles: 1,435-1,540 MHz.

LCCA Life cycle cost analysis. An orderly selection I elimination method that;ttcludes all of the cost factors (fixed and variable) of a project, product, or endeavor.

LCH Luminance, chroma, hue. A model for color-correction software for use in digital scanning, output devices, and displays.

LCP Liquid crystal polymer. A material that is frequently used to make electronic connec­tors.

I�

285�

LDAP / leakage

LDAP Lightweight Directory Access Proto­col. A standard Internet access protocol that has been embraced by many software ven­dors.

LOAR Leak detection and repair. Refers to programs maintained by the EPA (US) to ensure valve-stem leakage is extremely low.

LOR Land Disposal Restrictions. EPA-pro­mulgated rules that implement the land ban. See land ban.

LID (reflux-to-distillate ratio) A quantity that is used to analyze column operations. See reflux ratio.

Lead [Cont] 1. In control theory, a transfer function term in the form, (Ts + 1). [Eng] 2. The distance a screw mechanism will advance along its axis in a single rotation.

lead angle 1. In welding, the angle between the axis of the electr~de and the axis of the weld. 2. The angle between the tangent to a helix and a plane perpendicular to the axis of

• the helix.

I lead equivalent The radiation-absorption

rating of a specific material. It is expressed in terms of the thickness of lead that reduces the radiation dose by an equal amount under given conditions.

leader 1. A blank section of tape at the begin­ning of a reel of magnetic tape or at the beginning of a paper tape. 2. A system pro­gram that enables other programs to be loaded into the computer.

leaders In the typographical composition of screen displays and printing, leaders are rows of dashes or dots to guide the reader's eye across the screen or page. Leaders are typically used in tabular work, programs, tables of contents, and the like.

leading 1. In an AC circuit, a condition in which peak current occurs earlier in each cycle than does peak voltage. 2. In printing, the insertion of additional horizontal space between lines.

leading edge The first transition of a pulse, going in either a positive (high) or a negative (low) direction.

lead-lag network A compensating network that combines the characteristics of the lag and lead networks. In it, the phase of a sinu­soidal response lags a sinusoidal input at low frequencies and leads it at high frequencies. Also known as a "lag-Iead network."

lead time In industrial engineering, the amount of time that is required to design and develop a piece of equipment before it is ready for use.

lead wire Any wire that connects two points in an electrical circuit. In particular, a wire

INPUTTO� LEAD-LAG�

LEAD

LAG

OUTPUT OF� LEAD-LAG�

Lead-lag Compensator

that connects an electric device to a source of power or that connects an indicating or con­trolling instrument to a sensor.

leak An opening, however minute, that allows the undesirable passage of a fluid from its containing boundaries. [ISA-67.03­1982]

leakage class Classifications established by ANSI/FCI 70-2 to categorize the seat leakage allowances for different needs of seat tight­ness. [ANSI/ISA-75.05.01-2000]

leakage classification Categories for the seat leakage of control valve trim as defined in ANSI BI6.104.

leakage rate 1. The maximum rate at which a fluid is permitted or determined to leak through a seal. Note: To determine this rate, the type of fluid, the differential pressure across the seal, the direction of leakage, and the location of the seal must be specified. [ISA-37.1-1975 (RI982)] 2. In detecting cool­ant pressure boundary leaks in light water reactors, leakage is expressed in volumetric units per unit of time at 20'C and one atmo­sphere pressure. [ISA-67.03-1982]3. The amount of leakage across a boundary per unit of time. See also bubble tight.

leakage [Proc] 1. The quantity of fluid that passes through a valve when the valve is in the full closed position under stated closure forces, with the pressure differential and temperature as specified. Leqkage is usually expressed as a percentage o~he valve capac­ity at full rated travel. [ANS1/ISA-75.05.01­2000] 2. Undesirable loss or entry across the boundary of a system. The term is usually applied to the slow passage of a fluid through a crack or fissure, but may also be used to describe the passage of small quanti­ties of particles, radiation, electricity, or mag­netic lines of force beyond desired

286�

--boundaries. [Comp] 3. An error introduced into the fast-Fourier transform (FIT) process when finite-length time blocks are used that do not match at the ends. Its effects include smearing of the frequency lines at lower amplitudes. This error is minimized by using window functions.

leakage, packing The quantity of process fluid that escapes through the valve packing. [ANSI/ISA-75.05.01-2000]

leakage, seat The quantity of fluid that passes through a valve when the valve is in the fully closed position, with pressure dif­ferential and temperature as specified. [ANSI/ISA-75.05-.01-2000]

leak detector An instrument, such as a helium mass spectrometer, that is used to detect small cracks or fissures in a vessel wall. l

leak-off gland A packing box in which pack­ing is used above and below the lantern ring to provide a collection point for fluid leaking past the primary seal (lower packing). [A SI/ISA-75.05.01-2000]

leak pressure See pressure, leak. leased line A telephone line reserved for the

exclusive use of a single customer that does not go through the central office. Also called a "private line," it is used to connect two or more locations (such as company plants) on a private WAN (wide area network).

least significant bit (LSB) The smallest bit in a string of bits, usually at the extreme right.

least significant digit (LSD) The right-most digit of a number.

least-squares line The straight line for which the sum of the squares of the residuals (devi­ations) is minimized. [ISA-37.1-1975 (R1982)]

LEe Local exchange carrier. The carrier responsible for extending telephone service to the final user.

Ledoux bell meter A type of manometer whose reading is directly proportional to the flow rate sensed by a head-producing mea­suring device such as a pitot tube.

left justified A field of numbers (decimal, binary, etc.) in a memory cell, location, or register, that has no zeroes to its left.

leg 1. One of the members of a branched object or system. 2. The distance between the root of a fillet weld and the toe. See fillet weld. 3. Any structural member that supports an object above the horizontal.

legacy systems Software and/or hardware systems that have existed for some time and yet are still viable. They are often proprietary in nature and cannot practically be removed from an operation, perhaps because of the

leakage, packing I liberation

magnitude of use or their popularity. Legacy� systems usually need some interface in order� to operate with more current systems.�

length A fundamental measurement of the distance between two points, measured along a straight or curved path.

length of flame path (width of a joint) The shortest path through a joint from the inside to the outside of a flameproof .enclosure. Note: This definition does nor apply to threaded joints. [ANSIDSA-12.22.01-1998 (IEC 60079-1 Mod)]

lens joint ends Valves whose ~nds have been prepared for lens ring gaskets. [ANSI/ISA­75.05.01-2000]

letterspacing In the typographical composi­tion of screen displays and printing, the insertion of extra space between each letter of a word.

level 1. A measure of the logarithm of the ratio of some quantity to a similar reference quantity. The reference quantity must be identified. [ISA-37.10-1975 (R1982)] 2. Any bubble-tube device that is used to establish a horizontal line or plane. 3. To make the earth's surface even and roughly horizontal.

level indicator 1. An indicating instrument I for determining the position of a liquid sur­�face within a vessel. 2. An instrument that� shows audio voltage level in an operating� sound-recording system via a meter, neon� lamp, or cathode ray tube.�

leveling saddle A pipe clamp that anchors a swivel-joint 2-in. threaded socket so a 2-in. pipestand can be properly positioned.

levels (logic) The two voltages whose values have been arbitrarily chosen to represent the binary numbers 1 and 0 in a particular data­processing system.

lexical analysis In data processing, a stage in the compilation of a program in which state­ments, such as IF, AND, END, and so on, are replaced by codes.

LIF (reflux-to-feed ratio) A quantity used to analyze column operations.

L-glass In the construction of glass bulb-type pH sensors, L-glass is used for the mem­branes of electrodes for processes in which measurements are to be made in alk9line media with high process tempera~es. See E-glass, G-glass, and 5-glass. .

Liapunov's second method A method for determining the stability or instability of a process control system. It is analogous to the rate-of-change-of-energy method for mechanical systems. Also referred to as the "indirect method."

liberation See heat release.

287�

library / light key

library 1. A collection of information and standard programs that is available to a com­puter, usually on auxiliary storage. 2. A file that contains one or more relocatable binary modules, which are routines that can be incorporated into other programs.

Lichtenberg figure camera A device for indi­cating the polarity and approximate crest value of a voltage surge. It consists of a pho­tographic film or plate that is backed by an extended-:plane electrode. The film or plate's emulsion contacts-a small electrode that is connected to the circuit in which a surge occurs.

life, cycling The specified minimum number of full-range excursions or specified partial range excursions over which a transducer will operate as specified without changing its performance beyond;,the specified toler­ances. [ISA-37.1-1975 (R1982)]

I life, operating The specified-minimum

length of time over which the specified con­tinuous and intermittent rating of a trans­ducer applies without a change in the transducer's performance beyond specified tolerances. [ISA-37.1-1975 (R1982)]

life, storage The specified minimum length of time over which a transducer can be exposed to specified storage conditions with­out changing its performance beyond speci­fied tolerances. [ISA-37.1-1975 (R1982)]

life-cycle cost analysis An orderly selection or elimination method that includes all the cost factors (fixed and variable) of a project, product, or endeavor.

COSTS

_ - --MAINTENANce & EXPANSION COSTS

LIFE CYCLE COST ANALYSIS

life test A destructive test in which a device is operated under conditions that simulate a lifetime of use.

lift A nonstandard term for "travel." See travel. [ANSI/ISA-75.05.01-2000]

ligament The minimum cross section of solid metal in a header, shell, or tube sheet between two adjacent holes.

light 1. An electromagnetic radiation Whose wavelength is between approximately 10_2

and 10_6 cm. Note: St_rictly defined, only visi­ble radiation (4 x 10-' to 7 x 10-5 em) can be considered as light. [ISA-37.1-1975 (R1982)] 2. Electromagnetic radiation with a wave­length that lies in the range over which it can

. be detected with the unaided human eye. light-beam galvanometer A type of sensitive

galvanometer whose null-baIance point is indicat.e,d by the position Qf ~ beill11 of light that is reflected'fro!T1 a mirror carried in the moving coil of the instrument. Also known as d'Arsonval galvanOineter.

light-beam instrument A measurement device that indicates measured values by means of the position of a beam of light on a scale.

light-coupled switch A switch in which the switching signal is transmitted to the activat­ing device-by means of a light beam.

light curtain . An arrangement whereby a wide, thin beam of invisible modulated light is used to detect the passage of objects through a plane up to about 8 by 78 in. (200 mm by 2m).

light-duty cable Generally, a type of fiber­�optic cable that is designed to withstand the� conditions encountered in a building-not� outdoor conditions.�

lighted dial A dial or indicating scale and pointer in which a small lamp within the assembly illuminates the scale and pointer so they can be viewed in darkness. Compare with luminous dial.

light-emitting diode (LED) A semiconductor diode that emits visible or infrared light. Light from an LED is incoherent spontaneous emis­sion, as distinct from the coherent stimulated emission that is produced by diode lasers and other types of lasers. The indicator lights on most I/O modules are LEDs. An LED is often a light source for fiber-optic transmission and is also used as annunciator lamps in red, amber, green, and blue.

light ends The fraction of a petroleum mix­�ture that has the lowest boiling point.�

lighting-off torch A torch that is used to ignite fuel from a burner. The torch may con­sist of asbestos that has been ......i!rapped around an iron rod and saturated with oil or it may be a small oil or gas burner.

lighter-than-light key The remaining compo­nents in the overhead stream other than the light key. See light key

light key The component in multicompo­�nent distillation that is removed in the over­�head stream and has the lowest vapor�

j .1

288 J" 1 -----'

I�

pressure of the components in the overhead. If the reboiler head is decreased or the reflux flow increased, the light key component is the first component to fall into the bottoms product.

light meter A small, handheld instrument for measuring the intensity of illumination.

light modulator An apparatus that produces a sound track by means of a source of light, an appropriate optical system, and a device for inducing controlled variations in light-beam characteristics. : ,':.. :. '. '

light oil Any oil whose boilin,e; poii1't is in the temperature range 110 to 210 C, especially a coal tar fraction that is obtained by distilla­tion.

light pen A device by which an individual can communicate with an information sys­tem through a cathode ray tube.

light valve A device whose ability to trans­mit light can be made to vary by applying an external electrical quantity such as a current, voltage, electric field, electron beam, or mag­netic field.

limit check The comparison of data from a specific source with preestablished allowable limits for that source.

limit checking Internal program checks used to detect signals that indicate the undesirable or unsafe operation of a plant. Signals are checked against limits on high value, low value, rate of change, and deviation from a reference.

limit control A sensing device that shuts down an operation or terminates a process step when a prescribed limiting condition is reached.

limit cycle A sustained oscillation of finite amplitude.

limited-distance modem A signal converter that conditions and boosts a digital signal and allows it to be transmitted over much longer distances than a standard RS-232 sig­nal.

limiter A device that applies limits to a sig­nal.

limiting The action that causes a transducer output to become constant even though its input continues to rise above a certain value.

limiting safety system setting (LSSS) For nuclear reactors, these are settings for auto­matic protective devices that are linked to the variables that have significant safety func­tions. (See CFR, 50.36[c][l][i][A].) [ANSI/ ISA-67.04.01-2000; ISA-RP67.04.02-2000]

limiting temperature The maximum permis­sible temperature for apparatuses or parts of apparatuses. It is equal to the lower of the

light meter / line driver

tvvo temperatures determined by (a) the dan­�ger of ignition of the explosive gas atmo­�sphere and (b) the thermal stability of the� materials used. Note: This temperature may� be the maximum surface temperature or a� lower value. [ANSI/ISA-12.16.01-1998 (IEC� 60079-7 Mod)]�

limit of detection In any instrument or mea­surement system, the smallest value of the measured quantity that causes the indicator to move discernib)y, '.

limit of error . In ~ninstrumentor control device, the maximum error over the entire scale or range of use unde/; specific condi­tions.

limit of measurement In any instrument or measurement system, the smallest value of the measured quantity that can be accurately indicated or recorded.

limit priority A priority specification that is associated with every task in a multit'.lsk operation. It represents the highest dispatch­ing priority that the task may assign to itself or to any of its subtasks.

limits The prescribed maximum and mini­mum values of a dimension or other attribute. I

limits of error The tolerance band for the thermoelectric response of thermocouple wire. It is expressed in degrees or percentage, as defined by ISA-MC96.1-1982.

limit switch 1. An electromechanical device that is positioned to be actuated when a cer­tain motion limit is reached. 2. A nonstand­ard term for "position switch." See position switch. [ANSI/ISA-75.05.01-2000]

Lindemann electrometer An electrometer in which a metallized quartz fiber is mounted on a quartz torsion fiber perpendicular to its axis. The metallized quartz fiber is posi­tioned within a system of electrodes to pro­duce a visual indication of electric potential.

line [Comp] 1. In word processing, a string of characters that terminates with a vertical tab, form feed, line feed, or carriage return. [Comm] 2. A data transmission link. [Mfg] 3. In process plants, a collection of one or more associated units and equipment modules, arranged in serial and/ or parallel paths, that are used to make a complete batch,.f mate­rial or finished product. See also prOduction line or train.

lineal-scale length The distance from one end of an instrument scale to the other, mea­sured along the arc if the scale is curved or circular.

line driver A signal converter that conditions the digital signal transmitted by the RS-232

289�

_li_n_e_f_e_e_d_'_l_i_n_e_ar_i_ty_,_i_n_d_e_p_e_n_d_e_n-t---------­ F� interface in order to ensure reliable transmis- trolled condition and the command signal' sion beyond the S0-100-ft. limit recom- independent of the amplitude of the com- IS

mended for RS-232. mand signal. line feed A control character that is normally linear editing In video development, a pro-

used to command a printer to skip to the cess cen~r.al to the old paradigm of using ~

next line. tape: edltmg from data stored on media that ~ line mixer See flow mixer. is not instantly accessible because of the con- i~;;; line power The main power source supplied tinuous and linear layout of the data. This ~

by a power company or the central generator lack of instant accessibility means linear edit- i of a seE-sufficient site. It is the U.s. equiva- ing is sl.ower than nonlinear editing. ~ lent of what is called "mains" in the United. . linearity 1. Tht;:clQseness of a calibration ; Kingdom. See mains. • curve to 'a specified straight line. Not~: Lin-, l

line pressure See reference pressure. earity is expressed as the maximum devia­line printer A computer printer that operates tion of any calibration point on a specified

on a line-by-line (rather than character-by- straight line, during anyone calibration character) basis for high-speed systems. cycle. It is expressed as "within ±_ percent

line regulation In power supplies, a change of full scale output." [ISA-37.1-1975 (R1982)] in the value of DC output voltage resulting 2. The closeness to which a curve approxi­from a change in AC input over a specified mates a straight line. Note 1: Linearity is usu­range, from low line to high line. Line regula- ally measured as a nonlinearity and tion is normally specified as a plus or minus expressed as linearity, for example, a maxi­

.Ii

II change of nominal AC input voltage. mum deviation between an average curve

line replaceable unit (LRU) An avionics and a straight line. The average curve is ~ equipment module that is packaged in such determined after making two or more full­ . l' a way as to be rapidly replaceable on the range traverses in each direction. The value ~ i

flight line. of linearity is referred to the output unless . l'line spacing, line depth, line feed In the otherwise stated. Note 2: As a performc.nce itypographical composition of screen dis- specification, linearity should be expressed i plays and printing, the distance from the as "independent linearity," "terminal-based baselines of two lines of typed text. linearity," or "zero-based linearity." When

line spectrum The spectrum of a complex expressed simply as linearity it is assumed to wave that consists of several components be independent linearity. [ANSI/lSA-Sl.l­with discrete frequencies. 1979 (R1993)] 3. A characteristic of a device

line, train See train. [ANSI/ISA-88.01-199S] or system that can be described by a linear line turnaround In digital communication, differential equation with const:mt coeffi­

the reversing of the transmission direction cients. [ISA-26-1968] 4. The closeness to from the sender to the receiver or vice versa which a curve relating to two variables when a half-duplex circuit is used. approximates a straight line. [ANSI/ISA­

linear The type of relationship that exists 75.05.01-2000]5. The nearness with which between two variables when the ratio of the the plot of a signal or other variable plotted value of one variable to the corresponding against a prescribed linear scale approxi­value of the other is constant over the entire mates a straight line. [ANSI/ISA 77.44.01­range of possible values. 2000; ANSI/ISA 77.44.02-2001]

linear actuator A device for converting linearity, differential Any two adjacent digi­power into linear motion. tal codes should result in measured output

linear characteristic An inherent flow charac- values that are exactly 1 LSB (least significant teristic that can be represented by a straight bit) apart. Any deviation of the measured line on a rectangular plot of flow coefficient "step" from the ideal difference is called "dif­(Cv or Kv ) versus rated travel. Therefore, ferential nonlinearity" and expressed in mul­equal increments of travel provide equal tiples of 1 LSB. ~'<t increments of flow coefficient (C" or Kv)' linearity, end-point Linearity that is referred [ANSI/ISA-75.05.01-2000] to the end-point line. [ISA-37.1-1975 (R1982)]

linear circuit A circuit that handles signals linearity, independent 1. The maximum 1 that vary in proportion to their value and/or deviation of the calibration curve (i.e., the \where the input and output have a propor- average of upscale and downscale readings) 1 tiona1relationship. from a straight line that is positioned so as to 1,linear control system A control system in minimize the maximum deviation. [ANSI/ which the transfer function between the con- ISA-51.1-1979 (R1993)] 2. Linearity that is i

290 i

--

linearity, least-squares / linear variable differential transformer

-OUTPUT. PERCENT

20 60 80 >00

LINEAR: TYPICAL FOR PRESSURE OR LEVEL MEASUREMENT. AND OTHERS

NONLINEAR. TYPICAL FOR DIFFERENTIAL-PRESSURE- TYPE FLOW MEASUREMENT

, referred to the best straight line. [ISA-37.1­1975 (R1982)]

linearity, least-squares . Linearity that is referred to the least-squares line. [ISA-37.1­1975 (R1982)]

linearity, terminal Linearity that is referred to the terminal line. [ISA-37.1-1975 (R1982)]

linearity, terminal-based The maximum deviation of the calibration curve (i.e., the average of the upscale and downscale read­ings) from a straight line that coincides with a calibration curve at upper- and lower­range values. [ANSI/ISA-51.1-1979 (R1993)]

linearity, theoretical slope Linearity that is referred to the theoretical slope. [ISA-37.1­1975 (R1982)]

linearity, zero-based The maximum devia­tion of the calibration curve (i.e., the average of the upscale and downscale readings) from a straight line that is positioned so as to coin­cide with the calibration curve at the lower range-value and to minimize the maximum deviation. [ANSI/ISA-51.1-1979 (R1993)]

linearity of a turbine flowmeter The maxi­mum percentage deviation from the average sensitivity (K) across the linear range. [ISA­RP31.1-1977]

linearity range of a turbine flowmeter The flow range over which the output frequency is proportional to flow (constant K factor) within the limits of the specified linearity. [ISA-RP31.1-1977]

linearization The process of converting a nonlinear (nonstraight line) response into a linear response.

linear meter An instrument whose indicated output is proportional to the quantity being measured.

linear optimization See linear programming (LP).

linear polarization The polarization of an electromagnetic wave in which the electric field vector points in only a single direction.

linear position sensing detector An optical detector that can measure the position of a light spot along its length.

linear potentiometer A variable resistance device whose effective resistance is a linear functic1n of the positiOD ofa control arm or other adjustment. Most often, the device is constructed so that a single length of straight or coiled wire whose resistance varies uni­formly along its length is in contact with a shoe or similar sliding member. Effective resistance is varied by connecting the circuit to one end of the wire and to the shoe and then varying the position of the shoe. The use of a wire-wound resistor, thin film, or printed circuit element allows greater volt­age drop per unit lengtr. along the potenti­ometer, and therefore stronger and more useful output signals.

linear power supply Also known as "series pass power supply." The most commonly used power supply design. It consists of a I transformer, rectifiers, and filter capacitors, followed by a pass element that varies the voltage drop so as to maintain constant out­put voltage. Linear power supply provides the best performance in regulation, ripple, transient response, output impedance, and cost, but is larger, heavier, and has lower effi­ciency than other power supplies.

linear programming (LP) The analysis or solution of problems in which the linear function of a number of variables is to be maximized or minimized and when those variables are subject to several constants in the form of linear inequalities.

linear-quadratic-Gaussian problem An opti­mal-state regulator problem that contains Gaussian noise in both the state and mea­surement equations and in which the expected value of the quadratic performance index is to be minimized. Abbreviated as "LQG problem." It is the workhorse of opti­mal control theory.

linear system See system, linear. ~.j linear transducer A type of transducer for

which a plot of input signal level versus out­put signal level is a straight line.

linear variable differential transformer (LVDT) A type of position sensor that con­sists of a central primary coil and two sec­ondary coils wound on the same core. A moving-iron element that is linked to a

291

linear variable reluctance transducer / Link Master�

mechanical member induces changes in self­induction that are directly proportional to the movement of the member. See linear vari­able reluctance transformer (LVOT).

ARM

MOVEABLE CORE

AC~VOLTAGE OUTPUTS SOURCE

2

linear variable reluctance transducer (LVRT) A type of position sensor that con­sists of a center-tapped coil and an opposing moving coil attached to a linear probe. The winding is continuous over the length of the core, instead of being segmented as in an lin­ear variable differential transformer (LVDT). The chief disadvantage of an LVRT is that the overall length must be at least double the stroke, whereas its chief advantage is its excellent linearity over an effective stroke up to 24 in. (610 mm). See linear variable differen­tial transformer (LVOT).

linear velocity A vector quantity whose mag­nitude is expressed in units of length per unit time and whose direction is invariant. If the direction varies in circular fashion with time, the quantity is known as "angular" or "rota­tional velocity." If it varies along a fluctuat­ing or noncircular path the quantity is known as "curvilinear velocity."

line-class valve A valve that is qualified by its design characteristics to be used as the first valve off the process line.

lined body A body that has a lining that makes an interference fit with the disk in the closed position, thus establishing a seal. [ANSI/ISA-75.0S.01-2000]

lined valve body A valve body in which a coat­ing or liner has been applied to internal sur­faces to provide protection against corrosion/erosion or for flow shutoff. [ANSI/ISA-7S.0S.01-2000]

liner, slip-in An annular-shaped liner that makes a slight interference fit with the body bore and may be readily forced into position through the body end. Slip-in liners may be plain or reinforced. [ANSI/ISA-7S.0S.01­2000]

lining The material used on the furnace side of a furnace wall. It is usually of high-grade

refractory tile or brick or plastic refractory material.

link 1. Any specified relationship between two nodes in some network. A communica­tions path between two nodes. 2. A link is the logical medium by which HI Fieldbus devices are interconnected. It is composed of one or more physical segments that are inter­connected by bus repeaters or couplers. All of the devices on a link share a common schedule, which is administered by that link's current Link Active Scheduler (LAS).

Link Active Scheduler (LAS) A determinis­tic, centralized bus scheduler that maintains a list of transmission times for all the data buffers in all the devices that need to be cycli­cc:ly transmitted. Only one Link Master (LM) device on an HI Fieldbus link can be functioning as that link's LAS.

linkage [Comp] 1. A technique for providing interconnections between routines. [Eng] 2. A mechanism consisting of bars, slides, piv­ots, and rotating members that transfers motion from one part of a machine to another.

linkage editor A computer program that pro­duces a load modu-Ieby transforming object modules into a format that is acceptable to fetch. It also combines separately produced object modules and previously processed load modules into a single load module and resolves symbolic cross references among them. A linkage editor also replaces, deletes, and adds control sections automatically on request, and provides overlay facilities for modules that request them.

linked list In data processing, a method for organizing data so they are retrievable in an order that is not always the same as the order in which the data are stored.

linker A computer program that binds together independently assembled pro­grams. The program is developed in mod­ules, which are then linked together to form the whole.

link layer Layer 2 of an OSI reference model. See data link layer (OLL).

link library A generally accessible parti­tioned computer data set. Unless otherwise specified, it is used in fetch~iload modules that are referred to in execute..(EXEC) state­ments and in attach, link, load, and transfer control (XCTL) macro instructions.

Link Master (LM) A Link Master (LM) is any device that contains Link Active Scheduler (LAS) functionality and can control commu­nications on an HI Fieldbus link. An HI link

I -292

link objects / liquid-pressure recovery tactor

I I

must have at least one LM. One of those LM devices will be elected to serve as LAS.

link objects Contains information to link function block (FB) input/output (I/O) parameters in the same device and between different devices. The link object links directly to a virtual communication relation­ship (VCR). See virtual communication rela­tiOllShip (VCR).

link(s) In the context of the Internet, links are highlighted (with underlines, color, etc.) words in a hypertext document that act as pointers to more information on that specific subject. A mouse click on them can transport the user to another Web site.

link segment In data communications, an electronically continuous piece of bus that consists of the same cable with only two devices in point-to-point configuration. Also called inter-repeater link.

Linux A UNIX-based operating system for PCs. Linux was devised by Linus Torvalds. It is a relatively compact, highly reliable open system.

liquid, combustible A liquid that has a flash point at or above lOO°F (37.8°q. Combustible liquids are subdivided as follows: (a) Class II liquids include those with flash points at or above 37.8°C (lOO°F) and below 60°C (140°F); (b) Class IlIA liquids include those with flash points at or above 60°C (140°F) and below 93°C (200°F), and (c) Class I1IB liquids include those with flash points at or above 93°C (200°F). Note: For additional informa­tion, refer to NFPA 325. Note that these classes have no relation to the hazardous location classes. [ANSI/ISA-12.01.01-1999]

liquid barometer A simple device for mea­suring atmospheric pressure. It can be con­structed by filling a glass tube having one closed end with a liquid such as mercury, then temporarily plugging the open end. The tube is then inverted into a container that is partly filled with the liquid and the open end unplugged. If the liquid is mercury the tube must be at least 30 in. (76.2 mm) long. Liq­uids of different densities require tubes of different lengths.

liquid combustible, Class II A liquid that has a flash point at or above lOO°F (37.8°q and below 140°F (60'C). See NFPA 321. [ANSI/ISA-12.01.01-1999]

liquid combustible, Class IlIA A liquid that has a flash point at or above 140

c

F (60°C) and below 200°F (93°C). See NFPA 321. [ANSI/ ISA-12.01.01-1999]

liquid combustible, Class lIIB A liquid that has a flash point at or above 200°F (93°q. See NFPA 321. [ANSI/ISA-12.01.01-1999]

liquid-cooled dissipator See cold plate. liquid crystal display (LCD) A type of digi­

tal display device. Specifically, a reflective visual readout of alphanumeric characters that can generally be divided into two types: passive-matrix displays (PMLCDs) and active-matrix displays (AMLCDs). The names of th.e~e ~.6 type~ refer to how the pixels in' the'displaY, ~(~ controlled. See pas­sive-matrix liquid crystal displays (PMLCDs) and active-rnatrix liquid crystal displays (AML­CDs).

liquid crystal light..valve A device that is used in optical processing to convert an inco­herent light image into a coherent light image.

liquid-filled thermometer Any of several designs of temperature-measurement devices that depend for their operation on the predictable change in volume \vith tem­perature of a liquid medium confined in a closed system.

liquid knockout· 'See impingement. liquid-level control A device for sensing and

regulating the position of a liquid surface within a vessel.

liquid-level manometer A differential-pres­sure gauge in which the reading is obtained by viewing the change in level of one or both of the free surfaces of a liquid column that spans both gauge legs.

liquid-metal embrittlement A decrease in the strength or ductility of a solid metal caused by contact with a liquid metal.

liquid pressure recovery factor (FL) The square root of the ratio of the actual pressure differential across a valve to the pressure dif­ferential to the vena contracta under nonva­porizing conditions and without attached fittings. It is used to predict choked flow of liquids. See ISA-75.01-1985 (R1995), "Flow Equations for Sizing Control Valves," and ANSI/ISA-75.02-1996, "Control Valve Capacity Test Procedures." [ANSI/ISA­75.05.01-2000]

liquid-pressure recovery factor The ratio (F1)

of the valve flow coefficient (Cv)'~"'hich is based on the pressure drop at th~..vena con­tracta, to the usual valve flow coefficient (C1,),

which is based on the overall pressure drop across the valve in nonvaporizing liquid ser­vice. These coefficients compare with the ori­fice-metering coefficients of discharge for vena contracta taps and pipe taps, respec­tively. [Ar\SI/ISA-75.05.01-2000]

293

LISP / load

LISP List processing language. A computer language created by John McCarthy that uses a data type list as its basic element. Many artificial intelligence applications are written in LISP.

LISP (From "LISt Processing language.") An interpretive language that was developed to manipulate symbolic strings of recursive data. LIPS can also be used to manipulate mathematical and arithmetic logic.

Lissajous figure A pattern on an oscilloscope screen that indicates the relative phase and magnitude of sinusoidal signals. [ISA-26­1968]

list An ordered set of items that is contained within an electronic memory in such a way that only two items are readily addressable by a program. These items are the earliest appended (beginning item) and the most recently appended (ending item). Items stored into the list are "appended" to it fol­lowing the ending item. Items read from the list are "removed." A list is the same as

11� push-up list.�

listed Equipment or materials that are included in a list published by an organiza­tion such as Universal Laboratories that eval­uates products. Such organizations periodically inspect the production of listed equipment or materials and publish lists that state either that the equipment or materials meet appropriate standards or have been tested and found suitable for use in the spec­ified manner. See labeled. [ANSI/ISA­12.01.01-1999; ANSI/ISA-RPI2.6-1995]

listing The hard copy that is generated by a line printer. The term may also refer to a visual display on a CRT that is generated in lieu of hard copy.

list processing A method for processing data in the form of lists. Usually, chained lists are used so that the logical order of items can be changed without altering their physicalloca­tion.

liter Also spelled "litre." Abbreviated I. The SI unit of volume. It equals 0.001 m3 or 1.057 quarts.

literal An element of a programming lan­guage that permits character strings in expressions and command and function ele­ments to be represented explicitly. In most languages, a literal element is enclosed in either single or double quotation marks to denote that the enclosed string is to be taken "literally" and not evaluated.

litmus A blue, water-soluble powder derived from lichens and used as an acid-base indica­

tor. It is blue at pH 8.3 and above and red at pH 4.5 and below.

live front An assembly arrangement in which all moving or energized parts are exposed on the front of the panel, frame­work, or cabinet.

live loading A type of sealing system that incorporates a spring element that is capable of providing a sufficient load over a range of motion. Its purpose.is to compensate for thermal,~xp'a~sion, wear, or the consolida­tion of pctcking 6r~gaskets. [ANSI/ISA­75.05.01-2000]

live object In the context of the Internet, a term coined by Netscape to describe "plug­ins," that is, applications that enable the browser to play image files, videos, and sound files as an inline part of a Web page.

live part A part that is considered capable of rendering an electric shock.

live room An enclosed space that is charac­terized by an unusually small capacity for absorbing sound.

live steam Steam that has not performed any of the work for which it was generated.

live zone See zone, live. LIW Loss in weight. A loss that is generally

measured in processes that create change in the density of a product or the volume of a product in some vessel during operations on the product.

LLC Link Layer Control. Also called Logical Link Control and Link Level Control. A pro­tocol developed by the IEEE 802 committee for data-link level transmission control. LLC addresses distinguish between different applications within the same station. This protocol includes end-system addressing and error checking.

LLSAP Link Layer Service Access Point. Refers to an ISO model for digital communi­cation. The access point is different for par­ticular applications in specific stations.

L network An electronic network that is com­posed of two branches in series. The junction and the free end of one branch is connected to one pair of terminals, and the free ends of both branches are connected to another pair of terminals.

load 1. An electrical device thaJ-is connected to the output terminals. [ISA-RP55.1-1975 (RI983)] 2. To connect a signal-receiving device to the output terminals of a signal source. [ISA-RP55.1-1975 (RI983)]3. The rate of output. [ISA-77.41-1992]4. To store a com­puter program or data into memory. 5. To mount a magnetic tape onto a device so the read point is at the beginning of the tape. 6.

294

load-and-go / local area network (LAN)�

To place a removable disk in a disk drive and start the drive. 7. The amount of force that is applied to a structural member in service. 8. The quantity of parts placed in a furnace, oven, or other piece of process equipment. 9. The quantity or mass of bulk material placed in a hopper, railcar, or truck. 10. The power demand on an electrical distribution system. 11. The amount of power that is needed to start or maintain motion in a power-driven machine. 12. The term process load denotes the nominal values of all variables in a pro­cess that affect the controlled variable. 13. In an electric power circuit, the resistive and reactive components that comprise the device that is being powered by the circuit. 14. In a physical structure, the externally applied force, or the sum of the external forces and the weight of the structure borne by a single member or by the entire structure. 15. A device that receives power or the '. power that is delivered to such a device, as in the rate of output, lb/hr (kg/s) of steam, or megawatts (kilowatts) of electrical genera­tions. [ANSI/ISA-77.13.01-1999]

load-and-go In data processing, an automatic coding procedure that not only compiles the program, creating machine language, but also proceeds to execute the created pro­gram. Load- and-go procedures are usually part of a monitor.

load cell A transducer for measuring force or weight. Its action is based on strain gauges mounted within the cell on a force beam.

OUTPUT FOR INDICATION OR 0------'" TRANSMISSION

INPUT OR EXCITATION

R c = STRAIN GAGES

R s = COMPENSATING AND CALIBRATION RESISTOR:

Load Cell Strain Gage Circuit

load circuit A circuit or a branch of a network that carries the main portion of current flow.

loaded line A telephone line that is equipped .' with loading coils that minimizes amplitude

distortion by adding inductance. load factor The ratio of the average load in a

given period to the maximum load carried during that period.

load impedance The impedance that is pre­sented to the output terminals of a trans­ducer by the associated external circuitry. [ISA-37.1-1975 (R1982)] See also impedance, load.

loading 1. The system that is connected to the output of a device, including the transmis­sion network. [ISA-26-1968]2. A buildup of material along the cutting edge of a bit or other tool. Similarly, the buildup of grinding debris on the ~orki:0g face of a grinding wheel or abrasive disk.

loading error An error that is caused by the effect of the load impedance on the trans­ducer output. Note: In the case of force trans­ducers, the term loa~ing has been applied to the application of force. [ISA-37.1-1975 (R1982)]

loading point The location at which material to be conveyed is applieg to the conveyor. [ISA-RP74.01-1984]

load module A prOgram that has been pre­pared in a format and is ready for loading and executing.

load point The point near the beginning of magnetic tape, at which the computer can start to record data.

load reactor A device that generates a signal that is proportional to the force imposed upon it by the load sensor. [ISA-RP74.01­1984]

load regulation The change in output (usu­ally speed or voltage) from no load to full load (or other specified load limits). Note: Load regulation may be expressed as the per­centage ratio of the change from no load to full load divided by the no-load value. [ANSI/ISA-51.1-1979 (R1993)] See offset.

load resistance The load resistance is the sum of the resistances of all connected receivers and the connection lines. [ANSI/ISA-50.1­19982 (R1992)]

load sensor See weigh carriage. Also called a "load-receiving element."

load sharing In communication systems, the distribution of a given load among several computers on a network.

LOC Lines of code. Lines of executable com­puter programming. ,

local The location of an instrumeri that is neither in or on a panel or console nor mounted in a control room. Local instru­ments are commonly in the vicinity of a pri­mary element or a final control element. The term field is often used synonymously with local. [ANSI/ISA-5.1-1984 (R1992)]

local area network (LAN) 1. A communica­tions mechanism by which computers and

I�

295�

local control unit (LCU) / logarithm

peripherals in a limited geographical area can be connected. LANs provide a physical channel of a moderate to high data rate (1 to 20 Mbit) that has a consistently low error rate (typically 10-9). 2. The connecting of sewral data processing machines that can share pro­grams, data files, and printers.

PROCESS CENTRAL CONTROL COMPUTER

~

Typical Industrial Plant LAN

local control unit (LCU) A control device that performs closed-loop control and inter­faces directly with the process.

local line, local loop A channel that connects the subscriber's equipment to the central office line-terminating equipment.

local oscillator An oscillator whose output is combined with another frequency in order to generate a sum or difference frequency. Either the sum or difference frequency may be easier to amplify and use, as in a superhet­erodyne receiver.

local panel A panel that is not a central or main panel. Local panels are commonly in the vicinity of plant subsystems or subareas. The term local panel instrument should not be confused with local instrument. [ANSl/ISA­5.1-1984 (R1992)]

local processing unit A field station that con­sists of input/output circuitry and the main processor. These devices measure analog and discrete inputs, convert these inputs into engineering units, perform analog and logi­cal calculations (including control calcula­tions) on these inputs, and provide both analog and discrete (digital) outputs.

local reference A copper bar, mounted on the cabinets of a subsystem, that becomes the signal reference point for the entire sub­system. All power commons and signal com­mons of a subsystem are tied to the local reference. Each local reference is tied to the master reference, by a separate wire.

location An address in computer storage or memory where a unit of data or an instruc­tion can be stored.

location, hazardous (classified) That portion of a plant where flammable or combustible liquids, vapors, gases, or dusts may be present in the air in large enough quantities to produce explosive or ignitible mixtures. [ANSI/ISA-51.1-1979 (R1993)]

location counter 1. In data processing, the control-section register that contains the address of the instruction currently being executed. 2. A register in which the address of the ~un:ent instrydion is recorded. Synon­ymous with'''insfnidion counter" and "pro­gram address counter."

lock 1. In a forging, lock means having the flash line in more than one plane. 2. A device for securing a door, drawer, or hatch that fea­tures a.. movable bolt, which is operated by a key. 3. To prevent a movable part from mov­ing; to seize.

locked-in liner In a butterfly valve body, a liner that is retained in the body bore by a key ring or other means.

lock-in A sequence feature that retains the alarm state until acknow1edged, when the abnormal process condition is momentary. [ANSI/ISA-18.1-1979 (R1992)]

lock-in amplifier' An amplifier that selects signals at one prespecified frequency and amplifies them, while discriminating against signals at other frequencies.

locking Pertaining to code extension charac­ters that change the interpretation of an unspecified number of the characters that follow. Contrast with /lon-locking.

lockout Any condition that prevents any or all senders or receivers from communicating.

lock-step A method for synchronizing a mixed-signal simulation system in which each simulator progresses one time step and passes all interacting signals to the other sim­ulator.

locomotive boiler A horizontal fire-tube boiler that has an internal furnace. The rear of this furnace is a tube sheet that is directly attached to a shell that contains tubes through which the products of combustion leave the furnace.

log 1. A record of everything that is pertinent to a machine run. These include the identifi­cation of the machine run, aJ~cord of alter­ation switch settings, the identification of input and output tapes, a copy of manual key-ins, the identification of all stops, and a record of the action taken on all stops. 2. To record occurrences in a chronological sequence.

logarithm The real-valued function log u, which is defined by log u =v, if e" =tI, with eV

296

logarithmic amplifier / logical product

·f• .

denoting the exponential function. Also known as "hyperbolic logarithm," "Nape­rian logarithm," and "natural logarithm."

logarithmic amplifier An amplifier whose output is a logarithmic function of its input.

logarithmic decrement In an exponentially damped oscillation, the natural logarithm of the ratio of one peak value to the next succes­sive peak vq.lue in the same direction.

logger A device that automatically records physical processes and events, usually chro­nologically. .

logic 1. A means for ~.olving complex prob­lems through the repeated use of Simple functions, which define basic concepts. Basic logic functions include"AND," "OR," "NOT," and the like. 2. The science that deals with the criteria or formal principles of rea­soning and thought. 3. Th~ systematic scheme that defines the interactions of sig­nals in the design of an automatic data pro­cessing system. 4. The basic principles and application of truth tables as well as the interconnection between logical elements that are required for arithmetic computation in an automatic data processing system. Related to symbolic logic.

logical block An arbitrarily defined, fixed number of contiguous bytes. A logical block is used as the standard I/O transfer unit throughout a computer operating system. For example, the commonly used logical block in PDP-ll systems is 512 bytes long. An I/O device is treated as if its block length is 512 bytes, although a device's actual (phys­ical) block length may be different. Logical blocks on a device are numbered from block oconsecutively up to the last block on the volume.

logical circuit See virtual circuit. logical connectives The computer operators

or words, such as and, or, or else, if then, nei­ther, nor, and except, that make new expres­sions from given expressions. These connectives have the property that the truth or falsity of the new expressions can be cal­culated both from the truth or falsity of the given expressions and from the logical mean­ing of the operator.

logical decision 1. The choice or the ability to choose between alternatives. Basically, a logi­cal decision amounts to an ability to answer yes or no with respect to certain fundamental questions involving equality and relative magnitude. 2. The utilization of a logic inst;-uction.

logical device name An alphanumeric name that is assigned by a user to represent a phys­

ical device. The name can be used synony­mOLlsly with the physical device name in all references to the device. Logical device names are used in device-independent sys­tems to enable a program to refer to a logical device name that can be assigned to a physi­cal device at run time.

logical difference All elements that belong to Class A but not to Class B, when two classes of elements, Class A and Class B, are given.

logical element Th~ ~ma.llest building block in a computer or dara processing system that can be represented by logical operators in an appropriate system of symbolic logic. Typical logical elements ar.e the AND gate and the OR gate, which can be represented as opera­tors in a suitable symbolic logic.

logical expression A logical expression con­sists of logical constants, variables, array ele­ments, function references, and combinations of those operands separated by logical operators and parentheses.

logical link control (LLC) The upper sub­layer of the data link layer (Layer 2), which is used by all types of IEEE 802 local area net­works (LANs). LLC provides a common set of services and interfaces to higher-layer pro­tocols. There are three types of services: Type 1, Connectionless; Type 2, Connection Ori­ented; and Type 3, Acknowledged Connec­tionless. Type 1, Connectionless, is a set of services that permit peer entities to transmit data to each other without establishing con­nections. Type 1 service is used by both Man­ufacturing Automation Protocol (MAP) and Technical Office Protocol (TOP). Type 2, Con­nection Oriented, is a set of services that per­mit peer entities to establish, use, and terminate connections with each other so as to transmit data. Type 3, Acknowledged Connectionless, is a set of services tha t per­m.it a peer entity to send messages that require immediate response to another peer entity. This class of service can also be used for polled (master-slave) operation.

logical operation 1. An operation in which logical (yes or no) quantities form the ele­ments being operated on, for example, A D, OR. 2. The operations of logical shifting, masking, and other nonarithmetic~~erations of a computer. Contrast with arithmetic opera­tion.

logical operator See logical connectives. logical port A port that is specifically defined

by a name (such as COM}, COM2, etc.) with­out a strictly defined physical port.

logical product Same as"AND." See AND.

I

297

logical record / loop

logical record A logical unit of data within a file whose length is defined by the user and whose contents have significance to the user. A group of related fields that is treated as a unit.

logical sum A result, similar to an arithmetic sum, that is obtuined through the process of ordinary addition. However, the rules are such that a result of one (1) is obtained when either one or both input variables is a one (1), and an output of zero (0) is obtained when the input variables are both zero (0). The log­ical sum is the name given the result that is produced by the inclusive OR operator.

logical unit number A number that is associ­ated with a physical device unit during a task's I/O operations. Each task in the sys­tem can establish its own correspondence between logical unit ryumbers and physical device units.

logical variable A variable that may have only the value "true" or "false." Also called a "Boolean variable."

logic analyzer 1. An instrument that stimu­lates digital circuits with input signals and displays the timing of the circuit's response. 2. A device that is used to analyze the logical operation of a microcomputer. 3. A test device that is used to debug digital systems.

logic cabinet A cabinet that contains logic circuits and no visual displays. [ANSI/ISA­18.1-1979 (R1992)]

logic circuit An electronic circuit that exclu­sively handles discrete-level (onl off) signals. The term is usually applied to circuits that perform combination logic functions.

logic design The specification of the working relations between the parts of a computer system in terms of symbolic logic and with­out primary regard for hardware implemen­tation.

logic diagram 1. In data processing, a dia­gram that represents a logic design and sometimes the hardware implementation. 2. A method for graphically representing a logic operation or set of operations.

logic gate A device that takes binary bits as input and produces an output bit to some specification.

log in See log on. logic instruction A computer instruction that

executes an operation that is defined in sym­bolic logic, such as AND, OR, and NOR.

logic integrity test (LIT) A specified set of test vectors that consists of a logic stimulus and the expected response over an interval of time.

logic levels An electrical convention for rep­resenting logic states. For transistorI transis­tor logic (Title) systems, the logic levels are nominally 5 V for logic 1 and 0 V for logic O.

logic network In data processing, an arrange­ment of logic gates designed to achieve spe­cific outputs.

logic probe An instrument that is used to probe logic circuitry in order to evaluate dig­ital signal patterns.

log~c solver EIE/PES components or sub­systems that exeCllfe'the application logic. Electronic and programmable electronics include input/output modules. See electrical/ electronic/programmable electronic systems (EI E/PES). [ANSI/ISA-84.01-1996]

logic system Decision-making logic equip­ment, including its associated power sup­plies, I/O hardware, and sensing devices. [ANSI/ISA-77.13.01-1999]

log on In data processing, to enter into or sign onto a system or network.

log out, log off In data processing, to exit from a computer system or network.

long flame burner A burner in which the fuel emerges with an extended flame, or one in which the air for combustion is admitted in such a way that the two do not readily mix, which produces a comparatively long flame.

longitudinal drum boiler A sectional header or box header boiler in which the axis on the horizontal drum or drums is parallel to the tubes in a vertical plane.

longitudinal interference See common mode interference.

longitudinal redundancy check (LRC) 1. A system of error control that is based on the formulation of a block check that follows preset rules. The check formation rule is applied in the same manner to each charac­ter. 2. An error-detection scheme that consists of a byte wherein each bit is calculated on the basis of the parity of all the bits in the block that have the same power of two (i.e., the same position in each byte of the block).

longitudinal wave A wave in which the medium is displaced in a direction that is perpendicular to the wave front at all points along the wave.,

look-up table The same as "taf;le." See table. Not to be confused with the v:erb form, "table look up." See table look liP.

loop [Cant] 1. A combination of one or more interconnected instruments that are arranged to measure and/or control a process vari­able. [ISA-67.02.01-1999; ANSI/ISA-5.1-1984 (R1992)] 2. All the parts of a control system: the process, sensor(s), transmitter(s), the con­

298 I

loop, closed (feedback loop) / loudness level

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troller, and the final control element. 3. Syn­onymous with control loop. See closed loop and open loop. 4. The doubled part of a cord, wire, rope, or cable. A bight or noose. 5. A com­plete hydraulic, electric, magnetic, or pneu­matic circuit. 6. A length of magnetic tape or motion picture film that has been spliced together, end to end, so it can be played repeatedly without interruption. [Camp 7. In data processing, a closed sequence of instruc­tions that are repeated. 8. A sequence of instructions that is executed repeatedly until a terminal condition prevails. 9. In a comput­ing program, a sequence of instructions that is written only once but executes many times (iterates) until some predefined condition is met. 10. In computing, instructions that actu­ally perform the primary function of a loop, as distinguished from loop initialization, modification, and testing.

loop, closed (feedback loop) A signal path that includes a forward path, a feedback path, and a summing point and that forms a closed circuit. [ANSI/ISA-51.1-1979 (R1993)]

loop, feedback See loop, closed (feedback loop). loop, open A signal path without feedback.

[ANSI/ISA-51.1-1979 (R1993)] loopback A type of diagnostic test in which

the transmitted signal is returned to the sending device after it passes through all or part of the data communications link or net­work. This enables the returned signal to be compared with the transmitted signal.

loop diagram A schematic representation of a complete hydraulic, electric, magnetic, or pneumatic circui t.

loop gain The product of the gains of all the elements in a loop. See also gain, loop.

loop-gain characteristics In process instru­mentation, of a closed loop, the characteristic curve of the ratio of the change in the return signal to the change in the corresponding error signal for all real frequencies. [ANSI/ ISA-51.1-1979 (R1993)]

loop identification 1. A first letter and a num­ber that identifies an instrument loop. Each instrument within a loop has assigned to it the same loop number and, in the case of parallel numbering, the same first letter. 2. Each instrument loop has a unique loop identifica­tion.

loop (initialization) The instructions that immediately precede the loop proper. They set addresses, counters, or data to initial val­ues.

loop modification The instructions of a loop that alter instruction addresses, counters, or data.

299

loop testing The instructions of a loop that determine whether the loop is complete.

loop transfer function Of a closed loop, the transfer function that is obtained by taking the ratio of the Laplace transform of the return signal to the Laplace transform of its corresponding error signal. [ANSI/ISA-51.1­1979 (R1993)]

loose stem A design in which the stem is not physically or mechanica!1Y attached to the

. ball, but drives the ball through intimate con­tact of surfaces. Typical loose stem drives include tang, pin, and splined. [ANSI/ISA­75.05.01-2000] .:

loran A navigation aid that consists of long-range pulsed radio waves. The posi­tions are determined by measuring the time of arrival of synchronized pulses and then finding the intersection of position lines as determined from signals transmitted by two or more fixed transmitters.

LOSe Londonderry Occupational Safety Centre (United Kingdom). A British certifica­tion and testing laboratory for testing the equipment of different vendors to some com­mon standard.

loss 1. The reduction of signal level as the sig­nal passes along a signal channel. 2. The dis­sipation of power, which reduces the efficiency of a machine or system. 3. The dis­sipation of material or energy because of leakage.

lossless A digital data technique that reduces the size of the file without sacrificing any of the original data. This tool allows the expanded or restored file to be the exact rep­lica of the original file before compression.

lossy compression A digital data compres­sion technique in which some data is deliber­ately discarded so as to achieve massive reductions in the size of the compressed file.

lost cluster A group of one or more disk sec­tors that are not available for storing data.

lot A unique amount of material sharing a set of common traits. Note: Examples of com­mon traits include the source of the material, the master recipe that was used to produce the material, and distinct physical properties. [ANSI/ISA-88.01-1995]

loudness The relative auditory in'tnsity of a sound wave.

loudness level A measurement of sound intensity. It is numerically equal to the sound pressure, in decibels, relative to 0.0002 microbar, of a simple tone whose frequency is 1,000 Hz and is judged by the listeners to be equivalent in loudness. The units of mea-

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-loudspeaker / LS-TTL-compatible

sure determined in this way are called "phons."

loudspeaker An electroacoustic transducer that is usually constructed so as to effectively radiate sound of varying frequencies into the air.

low-alloy steel An iron--earbon alloy that contains up to about 1 percent C and less than 5 per~ent, by weight, of additional ele­ments. .

low braSs A binary copper-zinc alloy that contains about 20 percent zinc.

low-carbon steel An iron--earbon alloy that contains about 0.05 to 0.25 percent C and up to about 0.7 percent Mn.

low-draft switch A control for preventing the operation of a burner if the draft is too low. Used primarily with mechanical draft.

lowercase In the typographical composition of screen displays and printing, the small let­;ters of a type face, as distinguished from the

," capital letters. lower explosive limit (LEL) See flammable

(explosive) limits. [ISA-12.01.01-1999] lower flammable limit (LFL) See flammable

(explosive) limits. [ISA-12.01.01-1999] lower limit 1. The lower limit of the signal

current is the current that corresponds to the minimum value of the DC current signal. [ANSI/ISA-50.1-1982 (RI992)] 2. The pneu­matic signal that corresponds to the mini­mum value of the transmitted input. 3. The lowest value of the measured variable that a device can be adjusted to measure.

lower range limit See range limit, lower. lower range value See range value, lower. low-fire start The firing of a burner with con­

trols in a low-fire position in order to provide a safe operating condition during light-off.

low gas pressure switch A control for stop­ping the burner if gas pressure is too low.

low head boiler A bent tube boiler that has three drums with relatively short tubes in a vertical plane.

low-heat value The high heating value minus the latent heat of vaporization of the water that is formed by burning the hydrogen in the fuel.

low-level language A computer language that consists of instructions that directly cor­respond to machine-language instructions.

low limiting control See control, low li/lliting. low oil temperature switch (cold oil

switch) A control for preventing the opera­tion of a burner if the temperature of the oil is too low.

low-order Pertaining to the weight or signifi­cance that is assigned to the digits of a num­

ber. For example, in the number, 123456, the low-order digit is six. One may refer to the three low-order bits of a binary word, as another example. See order.

low-pass filter A filter that passes frequen­cies that are below its cut-off frequency with little attenuation.

low-pass output filter (LPOF) In a subcarrier discriminator, the filter that rejects subcarrier components a'nd all extraneous noise while pass.ing the frequencies that are known to contain data. . .

low-pressure hot water and low-pressure steam boiler A boner that furnishes hot water at pressures that do not exceed 160 pounds per square inch or at temperatures not more than 250'F or furnishes steam at pressures of not more than 15 pounds per square inch.

low-resolution graphics In data processing, the ability of a dot matrix printer to repro­duce simple forms or pictures.

low-temperature hygrometry The measure­ment of water vapor at low temperatures. This form of hygrometry requires special techniques because of the small amounts of moisture that are typically present and because of the unusual operating characteris­tics of instruments at such temperatures.

Low Voltage Directive (LVD) Part of regula­tions established in January 1997 for compa­nies that sell electronic systems within the European Economic Area, which is com­prised of the European Union and the Euro­pean Free Trade Association. Establishes safety guidelines for electronic products that operate at 50 volts or above so as to ensure that customers can handle such products safely (no exposed voltages or other hazards that can cause injury).

low water cutoff A device for stopping the burner when water conditions in the boiler are unsafe.

LRC Longitudinal redundancy check. An error-detecting scheme that consists of bits calculated from odd and even parity for all characters in a block.

LRDCT Linear rotary differential capacitive transducer. A device that meawres rotational movement more precisely th~ do linear dif­ferential transformers.'';'

LSP Local set point. Of a process control loop, a set point that is typically entered by the control room operator.

LS-TTL-compatible Low-power Schottky­transistor I transistor logic. For digital input circuits, a logiC 1 is obtained for inputs of 2.0 to 5.5 V which can source 20 rnA and a logic

300

ois obtained for inputs of 0 to 0.8 V which can sink 400 rnA. For digital output signals, a logic 1 is represented by 2.4 to 5.5 V with a current source capability of at least 400 rnA; and a logic 0 is represented by 0 to 0.6 V with a current sink capability of at least 16 rnA. "LS" stands for low-power Schottky.

LV Logical unit. In systems network architec­ture, (l set of protocols that provide peer-to­peer communication between applications.

LV 6.2 In systems network architecture, a set of protocols that provide peer-to-peer com­munication between applications.

lubricant ring A nonstandard valve term for "lantern ring." See lantern ring.

lubricator A device for automatically apply­ing lubricant.

lubricator isolating valve A manually oper­ated valve that is used to isolate the packing lubricator assembly from the packing box. [ANSI/ISA-75.05.01-2000]

lubricator packing box A packing arrange­ment that consists of a lantern ring that has packing rings above and below and that makes provision for lubricating the packing. [ANSI/ISA-75.05.01-2000]

lug Any projection, like an ear, that is used for supporting or grasping.

lugged body A thin annular section body that has lug protrusions on the outside diameter of the body. Its end surfaces mount between the pipeline flanges or may be attached to the end of a pipeline without any additional flange or retaining parts, by using through­bolting, tapped holes, or both. [ANSI/ISA­75.05.01-2000]

lumen A unit of measure for the flux or power of light that is visible to the human eye. The photometric equivalent of the watt.

luminance The luminous intensity of any sur­face in a given direction per unit of projected area in a plane perpendicular to that direction. Formerly known as brightness.

luminosity Emissive power with respect to visible radiation.

luminosity coefficients The constant multi­pliers for the respective tristimulus values of any color such that the sum of the three products is the luminance of the color. See tristimuilis values.

luminous Emitting radiation in the form of visible light.

luminous dial A dial or indicating scale and pointer whose scale divisions, numerals, and pointer are made of or coated with a light-emitting substance such as luminous paint so they can be seen in the dark. Com­pare with lighted dial.

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LV ILZW

luminous efficiency Luminous flux divided by radiant flux.

luminous flux The amount of light that passes a given point per unit time.

luminous intensity Luminous flux per unit solid angle.

lumped-constant wavemeter A device for determining frequency by using a tunable resonant lumped-constant (LC) circuit cou­pled to a crystal detector. The circuit gener­ally utili,.;es plug:in coils 0;1 various inductances and a continuously variable capacitor that has a di?lcalibrated in fre­quency.

lux A metric unit of illuminance. LVDT Linear variable differential transformer; . linear velocity differential transformer. A

sensor for measuring rotational movement as linear displacement.

LVHC Low volume, high concentration. Usu­ally used in reference to a pollutant measure­ment for EPA (US.) regulations.

LVIT Linear variable inductance transducer. A linear measurement sensor that is based on chemically etched planar coil technology.

LV-ROM "Laser video read-only memory." Also, "LaserVision ROM," a product name from Pioneer.

L-Z algorithm A loss less data compression technique developed by two researchers named Lempel and Ziv.

LZH Lempel-Ziv-Huffman. A method for compressing data that can reconstruct data to exactly its original form with no loss.

LZW Lempel-Ziv and Welch. Patented by Unisys, another refinement of the L-Z algo­rithm for data compression, which can recon­struct data exactly into its original form with no loss.

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