Instr 12205 Elements, Transmitters, Transducers, Displacers

8/13/2019 Instr 12205 Elements, Transmitters, Transducers, Displacers

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Instrumentation Trainee Task Module 12205

Objectives

Upon completion of this module, the trainee will be able to:

1. Given the basic instrument channel, describe the major functions ofthe detectors, transducers, and transmitters.

2. Define the following commonly encountered measurement terms:

a. Accuracyb. esolutionc. e!roducibilityd. "ensitivitye. es!onsiveness

#. $ist four classifications of errors associated with instrumentation.%. Given a ty!ical measurement system a!!lication, determine

whether the !rocess variable is being measured directly or whetherit is being inferred.

&. "tate the significance of a calibration stic'er on a device.(. )e able to e*!lain the !rinci!le of o!eration of an orifice !late.

+. Describe the relationshi! between flow and differential !ressure in afluid system.. -*!lain at least four methods of measuring !ressure or differential

!ressure.. Discuss three common methods for measuring tem!erature.1/.$ist the advantages and disadvantages of thermocou!les and 0Ds.11.Given a diagram, describe the o!eration of a current to !ressure or

a !ressure to current transducer.12.12. Describe the basic function of a transducer.1#.$ist the standard in!ut and out!ut voltages and currents for most

transmitters.1%.Given a diagram, e*!lain the o!eration of a transmitter in a system.

rere3uisites

"uccessful com!letion of the following 0as' 4odule5s6 is re3uired beforebeginning study of this 0as' 4odule: 788- 8ore 8urricula9 788- 0as'4odule 122/1, Craft-Related Mathematics; 788- 0as' 4odule 122/2,Instrumentation Drawings and Documents II; 788- 0as' 4odule 122/#,Principles of Welding; 788- 0as' 4odule 122/%, Process Control Theory.

e3uired "tudent 4aterials

1. "tudent 4odule2. e3uired "afety -3ui!ment

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ELEMENTS,TRANSMITTERS,


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8ourse 4a! nformation

0his course ma! shows all of the Wheels of earning tas' modules in the

second level of the nstrument curricula. 0he suggested training orderbegins at the bottom and !roceeds u!. "'ill levels increase as a traineeadvances on the course ma!. 0he training order may be adjusted by thelocal 0raining rogram "!onsor.

8ourse 4a!: nstrument, $evel 2

$-;-$ 2 8O4$-0-

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Section Topic ..

Page

1././ ntroduction>>>>>>>>>>>>>>>>>>>>>>>>>>>>...(1.1./ eview of )asic nstrument and 8onstrol 8hannels >>>>>>>>

+1.1.1 Detector 5"ensor6>>>>>>>>>>>>>>>>>>>>>>>>>.. 1.1.2 0ransducer?8onvertor

>>>>>>>>>>>>>>>>>>>>>>>.. 1.1.# Am!lifier 5"ignal 8onditioner6>>>>>>>>>>>>>>>>>>>.. 1/

1.1.% 0ransmitter>>>>>>>>>>>>>>>>>>>>>>>>>>>>>.1/1.2./ eview of 4easurement 0erminology >>>>>>>>>>>>>>>.

111.2.1 Accuracy>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>.. 111.2.2 recision vs. Accuracy>>>>>>>>>>>>>>>>>>>>>>>.. 121.2.# 4easurement -rrors>>>>>>>>>>>>>>>>>>>>>>>>. 1%1.2.% e!roducibility and Drift

>>>>>>>>>>>>>>>>>>>>>>. 1+1.2.& "ensitivity and es!onsiveness>>>>>>>>>>>>>>>>>>.. 1+1.#./ 4easurement "tandards and -lements >>>>>>>>>>>>>>..

11.#.1 Direct vs. nferred 4easurements >>>>>>>>>>>>>>>>>.

11.#.2 4easurement "tandards>>>>>>>>>>>>>>>>>>>>>>. 11.#.# rimary "tandards>>>>>>>>>>>>>>>>>>>>>>>>>. 11.#.% "econdary "tandards

>>>>>>>>>>>>>>>>>>>>>>>> 11.#.& @or'ing "tandards>>>>>>>>>>>>>>>>>>>>>>>>>. 2/1.#.( rimary and "econdary -lements>>>>>>>>>>>>>>>>>.. 2/1.#.+ 8alibration>>>>>>>>>>>>>>>>>>>>>>>>>>>>>.. 211.#. "ignificant =igures>>>>>>>>>>>>>>>>>>>>>>>>>.. 222././ Detectors>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 222.1./ Orifice lates>>>>>>>>>>>>>>>>>>>>>>>>>>>> 22

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2.2./ ;enturi 0ube>>>>>>>>>>>>>>>>>>>>>>>>>>>>. 2+2.#./ 0he ilot 0ube>>>>>>>>>>>>>>>>>>>>>>>>>>>. 22.%./ Annubar 0ubes

>>>>>>>>>>>>>>>>>>>>>>>>>>>. #/2.&./ 4agnetic =lowmeters>>>>>>>>>>>>>>>>>>>>>>>> #/2.(./ ltrasonic =lowmeters>>>>>>>>>>>>>>>>>>>>>>>.. ##2.+./ 8a!acitance 0y!e $evel Detectors >>>>>>>>>>>>>>>>>

#2../ ltrasonic $evel 4easurement>>>>>>>>>>>>>>>>>>>. %22../ )imetallic "tri! 0hermometers>>>>>>>>>>>>>>>>>>> %%2.1/./0hermocou!les

>>>>>>>>>>>>>>>>>>>>>>>>>>> %2.1/.10hermoelectric ower>>>>>>>>>>>>>>>>>>>>>>>.. &/2.1/.20hermocou!le4etals>>>>>>>>>>>>>>>>>>>>>>>>. &22.1/.#0hermocou!le $aws>>>>>>>>>>>>>>>>>>>>>>>>.. &22.1/.%0hermocou!le

0ables>>>>>>>>>>>>>>>>>>>>>>>>. &%2.1/.&Designations for 0hermocou!le @ire >>>>>>>>>>>>>>>..

&(2.1/.(0hermocou!le 8onstruction >>>>>>>>>>>>>>>>>>>>.

././ "econdary -lements>>>>>>>>>>>>>>>>>>>>>>>>. (/#.1./ )ourdon 0ube>>>>>>>>>>>>>>>>>>>>>>>>>>>> (/#.2./ Dia!hragm ressure Devices >>>>>>>>>>>>>>>>>>>..

(.#./ ressure 8a!sules>>>>>>>>>>>>>>>>>>>>>>>>>. ((#.%./ )ellows ressure Devices>>>>>>>>>>>>>>>>>>>>>> (+

#.&./ 8a!acitance 0y!e ressure "ensor >>>>>>>>>>>>>>>>..(#.(./ Dia!hragm "eals>>>>>>>>>>>>>>>>>>>>>>>>>>. +1%././ 0ransducers>>>>>>>>>>>>>>>>>>>>>>>>>>>>> +2%.1./ 0ransducer =unctions>>>>>>>>>>>>>>>>>>>>>>>.. +2

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Section Topic .

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%.2./ 0ransducer 0y!es>>>>>>>>>>>>>>>>>>>>>>>>>.. +2%.#./ ? 0ransducers>>>>>>>>>>>>>>>>>>>>>>>>>>.. +#%.%./ ? 0ransducers

>>>>>>>>>>>>>>>>>>>>>>>>>>.. +#%.&./ 0ransducer O!eration>>>>>>>>>>>>>>>>>>>>>>>.. +%%.(./ ? 0ransducer O!eration >>>>>>>>>>>>>>>>>>>>>..

+%%.+./ ? 0ransducer O!eration >>>>>>>>>>>>>>>>>>>>>..

+&%../ 4etallic "train Gauge>>>>>>>>>>>>>>>>>>>>>>>> ++%..1 "emiconductor "train Gauges>>>>>>>>>>>>>>>>>>>. +%../ ressure "train Gauges

>>>>>>>>>>>>>>>>>>>>>>.. +%..1 ;oltageBdivider ressure 0ransducer>>>>>>>>>>>>>>>>

+%.1/./;oltageBGenerating 0ransducers>>>>>>>>>>>>>>>>>> +%.1/.1ieCoelectric 0ransducers>>>>>>>>>>>>>>>>>>>>>.. +%.11./$inearB;ariable Differential 0ransformer >>>>>>>>>>>>>..

/%.11.1A!!lications>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 1%.12./Accelerometer>>>>>>>>>>>>>>>>>>>>>>>>>>>. 1&././ 0ransmitters>>>>>>>>>>>>>>>>>>>>>>>>>>>>. 2&.1./ neumatic 0ransmitters>>>>>>>>>>>>>>>>>>>>>>.. 2&.1.1 =orce )alance Differential ressure neumatic 0ransmitters >>>.

2&.1.2 rocess 4easuring "ection >>>>>>>>>>>>>>>>>>>>..

#&.1.# =orce )ar "ection>>>>>>>>>>>>>>>>>>>>>>>>>.. %

&.1.% )alancing "ection>>>>>>>>>>>>>>>>>>>>>>>>>.. &&.1.& neumatic n!ut?Out!ut "ection >>>>>>>>>>>>>>>>>..

(&.1.( neumatic =orce )alance 0ransmitters A!!lications >>>>>>>..

&.2./ D 8ell =low 4easurement>>>>>>>>>>>>>>>>>>>>> &.2.1 D 8ell $i3uid $evel 4easurement>>>>>>>>>>>>>>>>>. &.2.2 D 8ell ressure 4easurement >>>>>>>>>>>>>>>>>>..

/

&.#./ neumatic =orce )alance 0em!erature 4easurement >>>>>>../

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&.%./ 4otion )alance neumatic 0ransmitters >>>>>>>>>>>>>>1

&.%.1 4easuring "ection>>>>>>>>>>>>>>>>>>>>>>>>>. 2&.%.2 $in' and =la!!erB7oCCle "ection >>>>>>>>>>>>>>>>>>

#&.%.# )ellows and elay "ection >>>>>>>>>>>>>>>>>>>>>#

&.%.% A!!lications of 4otion )alance 0ransmitters>>>>>>>>>>>.. %

&.%.& neumatic 4otion )alance 0em!erature 4easurement >>>>>>.%

&.%.( neumatic 4otion )alance ressure 4easurement >>>>>>>>..&

(././ -lectronic 0ransmitters>>>>>>>>>>>>>>>>>>>>>>> &(.1.1 =orce )alance Differential ressure -lectronic 0ransmitters>>>>

((.1.2 "ensor Assembly>>>>>>>>>>>>>>>>>>>>>>>>>>((.1.# Out!ut "ection>>>>>>>>>>>>>>>>>>>>>>>>>>>. +(.1.% Out!ut Device>>>>>>>>>>>>>>>>>>>>>>>>>>>.. +././ =iberBO!tics>>>>>>>>>>>>>>>>>>>>>>>>>>>>> +.1./ O!tical 0ransmitters>>>>>>>>>>>>>>>>>>>>>>>>. +.1.1 hotodectors>>>>>>>>>>>>>>>>>>>>>>>>>>>>.

0rade 0erms ntroduced in this 4odule

Calibrate:0he action of chec'ing the readings of an instrument against a'nown standard and adjusting the instrument to correct any errors.

Calibration: 0he !rocedure laid down for determining, correcting, orchec'ing the absolute values corres!onding to graduations on ameasuring instrument.

Drift: 0he gradual change of an instrument out!ut from the correctvalue.

Mea!re"ent error:0he difference between a measured value and anactual value.

Noble "etal: A metal that is so inert that it is usually found as

uncombined metal in nature. latinum, gold, and silver are noble metals.Also, the metal which does not corrode.

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Preciion:0he degree of re!roducibility of measurement by aninstrument.

Pri"ar# ele"ent:0he element in a measurement device which is acted

on directly by the !rocess.

Relati$e error:0he e*!ression of an error as a !ercent of the value beingmeasured.

Reponi$ene:0he ability of an instrument to follow changes.

Secon%ar# ele"ent:0he element of a measurement device which ta'esthe out!ut from the !rimary element and sends a signal !ro!ortional to itto the controller.

Tran%!cer: A device that !rimarily functions to convert its in!ut signal

to an out!ut signal of a different form.

Tran"itter: A device that !rimarily functions to !re!are and send in!utinformation to a remote location.

1././ 70OD80O7 0O O8-"" 4-A"-4-70B

0he industrial !lants of today re3uire the measurement and control ofnumerous !arameters to o!erate in the most efficient, reliable way!ossible. 8onditions must be constantly monitored to !rovide a safe andcomfortable atmos!here for the wor'ers in the !lant, ensure the!roduction of high 3uality !roducts or services, and to limit emissions fromthe !lant that could have adverse effects on the environment. )ecausethere are such a wide range of !arameters monitored in a ty!ical !lant,the instrumentation industry has become a highly diversified field. ualitytechnical !ersonnel are re3uired to install, o!erate, calibrate, and maintainthe devices used to measure and control these various !rocess!arameters. 0o !erform these tas's you must understand the meaning of!rocess measurement and be familiar with the devices that !rovidemeasurements. Eou must also be familiar with the basic !rinci!lesinvolved in detecting and sensing these !arameters. 0he more commonlyused devices and the !rinci!les involved in sensing and measuring a!arameter are covered in this module.

1.1./ -;-@ O= )A"8 7"04-70 A7D 8O70O$ 8FA77-$"

n 4odule 122/%, rocess 8ontrol 0heory, the basic instrument and!rocess control channels were introduced and described in general terms.

0hey are each shown here in !igure ".

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=igure 1. )asic nstrument and rocess 8ontrol 8hannels

n this module, you will be focusing on the first four 5%6 bloc's in each ofthese channels. 0hese bloc's are common to almost all instrumentationa!!lications. t is very im!ortant that you have a good understanding ofthem and the variety of ways they can be a!!lied.

$ets review and elaborate the functions of each bloc'. 0hey are:

Detector (or Sensor):senses the !arameter being monitored 5the !rocessvariable or the controlled variable6 and changes that !arameter to amechanical or electrical signal which is !ro!ortionally related to themeasured variable. 0hese devices are often referred to as the !rimary,or measuring element of the instrument channel.

N&TE: ro!er handling storage and !rotection is critical.

Transducer:converts the out!ut signal of the detector to a signal that can

be used easily. f the detector signal can be used directly, thisconversion setu! is not needed. @hen it is used, it is most often found inthe field in close !ro*imity to the measuring element. n many cases it isin the same housing or case as the measuring element and a!!ears to beone device. De!ending u!on the a!!lication, a transducer can be !art ofthe !rimary element or the transmitter or it can stand alone.

Amplifier: increases the !rocess signal to a usable magnitude. n manycases, signal conditioning also occurs along with am!lification. t is verycommon for instrument manufacturers to combine both this am!lificationand signal conditioning function with one of the other bloc's !hysically.

0his is more cost effective. t is ty!ically found as a !art of either thetransducer or the transmitter.

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Transmitter: transmits data from one instrument com!onent to anotherwhen com!onents are !hysically se!arated. t may contain the detector,transducer and the am!lifier 5signal conditioning6 functions.

A s!ecific instrument channel may involve these basic com!onents in any

number and any combination. 0hey need not a!!ear in the order of !igure" and not all of the com!onents described may be re3uired. 0he reason forsuch a variety of !otential variations is that the manufacturers ofinstrumentation are !roducing 5and naming6 devices in inconsistent wayswith regard to these four basic functions. 4ost of the time all fourfunctions are !erformed but it is !ossible with modem instruments to haveone or more devices do them all. As you go through the e*am!les in thismodule you will see these variations. t is im!ortant for you to 'now whatto e*!ect when reading the s!ecification or data sheets in theseinstruments vendor manuals.

)efore going on, lets discuss these four bloc's individually.

1.1.1 Detector 5"ensor6

0he first contact that a measurement channel of instrumentation has withthe !rocess !arameter to be measured is through the action of thedetector or sensing device.

0o sense the !rocess !arameter, the detector receives energy from the!rocess and !roduces an out!ut that is de!endent on the measured3uantity. t is im!ortant to realiCe that the sensing element alwayse*tracts some energy from the !rocess: the measured 3uantity is alwaysdisturbed by the act of measurement. 0his effect is referred to as loading.A good instrument is designed to minimiCe this loading or disturbance ofthe !arameter being measured. n measuring systems made u! of mostlyelectric or electronic com!onents, the loading of the signal source5!rocess variable6 is almost e*clusively a function of the detector. Othercom!onents in the electronic instrument channel receive most of theenergy or !ower they need from !ower su!!lies inde!endent of the!rocess itself. 0his is one major advantage of electronic measurement andcontrol channels.

Detectors are also selected for measurement systems on the basis of the!arameter being sensed, the desired accuracy, range of measurement,

and the !articular ty!e of out!ut it su!!lies. !igure # lists some ty!icaldetectors, !arameters monitored, and detection !rinci!les. 0hesedetection !rinci!les and associated detectors are discussed in greaterdetail in the following sections of this module.

arameter Detector Detection rinci!le

0em!erature esistance 0em!eratureDetector 50D60hermocou!le

-*!ansion of a 4etal

esistance of certain metalsvaries linearly with tem!erature.0wo dissimilar metals, whenjoined, !roduce a voltage!ro!ortional to their tem!erature.

"ome metals, when heated, wille*!and or distort in !ro!ortion to

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ressure

$evel

=low

-*!ansion of a $i3uidDifferential ressure 8ell

)ourdon 0ube

Differential ressure8ell =loat

=low estrictor 8ombinedwith a Differential ressure8ell

the amount of heat absorbed.

$i3uids will also e*!and whenheated or contract when cooled. Abellows will e*!and when theinternal !ressure is greater.

"ystem !ressure can be a!!lied tothe internal volume of the bellowswith a fi*ed !ressure 5normallyatmos!heric6 bellows.

A curved, oval tube will attem!t toachieve a straight cylindricalsha!e when internal !ressure isa!!lied.

"ame as for measuring !ressure.A material less dense than thefluid being monitored will float on

the fluidHs surface.

0he !ressure dro! across the flowrestriction is !ro!ortional to thes3uare of the flow. 0he differential!ressure cell is used to measurethe !ressure dro!.

=igure 2. $ist of arameters 4onitored and 0y!ical Detectors

Detectors measure !rocess variables such as !ressure, tem!erature, fluidlevel and li3uid flow. As !igure # suggests, the most common detector

out!ut is a very small dis!lacement or distance moved that is !ro!ortionalto the measure of the !rocess variable. 0he detectors out!ut is usuallynot directly usable in the control or instrument channel. Often it must beconverted, am!lified or conditioned in some way before it can be used toindicate or control the !rocess !arameter.

1.1.2 0ransducer?8onvertor

Almost immediately after being sensed by a detector the out!ut of thedetector must be changed or converted to a more easily used form. 0his isthe function of the transducer. t is almost always !hysically connected

to the !rimary detecting element. Once the measured variable out!ut ofthe transducer is converted to some usable form it can be mani!ulated bythe instrument channel com!onents as necessary without loading the!rocess variable which !roduced it.

0he !ur!ose of the transducer is to convert any in!ut it receives toanother ty!e of out!ut signal more readily usable by the ne*t com!onentor !ortion of the instrument or control channel. @hile this conversion iseasily seen as necessary at the out!ut of the detector 5few detectors!rovide out!uts directly usable6, it is also !ossible that a second or a thirdtransducer may be found in some instrument and control channels.@herever a signal conversion must occur, you will find a transducer.

t is !ossible to have one transducer convert the detector out!ut to a formthat can be am!lified and also !ossible to have another transducer

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convert the am!lified signal to a form where it can easily be transmitted toan indicator or a controller elsewhere in the !lant 5i.e. in the control roomfor e*am!le6. t is just as li'ely that the controllers out!ut might needconversion or transducing again for transmission bac' into the !lant too!erate a control element 5i.e. valve or heater, etc.6.

t is im!ortant that you understand that a transducer functions to convertsignals 5i.e. mechanical, electrical, !neumatic, etc.6 from one form toanother that is needed at that !oint in the channel, and that a transducercan be found anywhere in a channel, even as !art of another bloc' li'e thetransmitter. 4ost detector out!uts must be converted for use by thechannel, so in most diagrams you will find a transducing element alsodirectly connected to the !rimary sensing element, or detector.

1.1.# Am!lifier 5"ignal 8onditioner6

4ost measured variable signals must be increased in either am!litude or

!ower so they can be used by indicators or controllers directly or so theycan be transmitted to them. 0he am!lifier bloc' indicates this usuallyha!!ens somewhere in the channel. Actually, it may occur several timesbefore the signal is able to be used.

"e!arate !hysical instruments !erformed this function in the !ast9however, more often now instrument manufacturers are including theam!lification device or stage as !art of other !hysical elements in thechannel. 0he am!lification can ta'e !lace in the transducer, thetransmitter or wherever the instrument manufacturer finds it is mosteconomical. Often, if other conditioning or modification of the signal isre3uired, am!lification is !erformed also.

0herefore, an am!lifier bloc' may not a!!ear on your instrument channeldiagrams, but rest assured am!lification is most li'ely being !erformedsomewhere, inside another element of the channel.

1.1.% 0ransmitter

0he transmitter is a device that originally was very s!ecific in function. t!re!ared a signal to be sent from one location to another involving somedistance. 0he term transmitter has now grown to the !oint where itcould embrace that function and all of the others above. t usually does

not include the detector 5but it could6.

Again, this has evolved due to instrument manufacturers becoming moreinnovative. 4uch of the time now in industry you will find that a detector isconnected to the in!ut side of a transmitter directly and the out!ut sidecan be connected to the final indicator or controller.

0he transmitter very often will contain the transducing element, am!lifiersand signal conditioning element, and it will !rovide the out!ut in a formready for direct transmission to a remote location.

0here are advantages to this. Obviously, it is easier to wor' with or install.t has less !arts so it may be easier to maintain. t will cost less tomanufacture ultimately.



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0here are also disadvantages for the user. @hen it brea's, it usually mustbe re!laced. @hile relatively easy to do, this can be e*!ensive. t can bedifficult to maintain if you are not s!ecially trained to wor' on such acom!le* device.

Eou will see e*am!les of all of the above in this module.

1.2./ -;-@ O= 4-A"-4-70 0-47O$OGE

0he following terms a!!ly to any and all of the bloc's in the basicinstrument channel. "ome have been !resented in !ast modules. "omehave only been mentioned in !assing. $ets loo' more closely at severalterms and how they a!!ly to the instrumentation in this module. 0heseterms have very s!ecific meanings but, since the areas they deal withoverla!, often their usage is not !recise.

1.2.1 Accuracy

Accuracy is the degree to which the out!ut of an instrument a!!roachesan acce!ted standard or true value. 0here is no absolute accuracy9however, often you will hear someone say that a detector is accurate towithin I.1J ... I/.1J of whatK As the definition states, the out!ut of adevice is com!ared or referenced to some value or standard todetermine whether the instrument is !erforming as re3uired. 0herefore,when used as a !erformance s!ecification for an instrument, accuracymeans reference accuracy.

eference accuracy is a number or 3uantity that defines the limits thaterrors will not e*ceed when the device is used under referencedconditions. eference accuracy can be e*!ressed in a number of ways:

1. t can be e*!ressed in terms of the measured variable. =or atem!erature measuring device, the reference accuracy would bee*!ressed sim!ly as L1M=.

2. eference accuracy can be e*!ressed in !ercent of s!an. 0his canbe e*!lained by using the following e*am!le: A meter is used toindicate the water level in a tan' between &/ inches and 1&/inches. 0he reference accuracy of the indicator is LNJ of s!an.

0herefore, the reference accuracy of the indicator is N inch of level.

#. eference accuracy can be e*!ressed in !ercent of the u!!er rangevalue. f the u!!er range value of a !ressure gauge is 1// !si, andthe reference accuracy is L/.1J of u!!er range value, thereference accuracy of the gauge would be /.1 !si.

%. t can be e*!ressed in !ercent of scale length. =or an indicatingmeter with a (Binch scale length and a reference accuracy of I1.2Jof scale length, the reference accuracy would be /.# inches 5about&?1( inch6.

&. =inally, reference accuracy can be e*!ressed in !ercent of actualout!ut reading. f the &/Binch to 1&/Binch level indicator discussed!reviously has a reference accuracy of L1J of actual reading andthe tan' !resently has 12& inches of water the indicator should bereading 12& inches L1.2& inches.

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@hen stating the accuracy of an instrument, it is very im!ortant toe*!ress the 3uantity to which the accuracy is referenced. 0o say that acom!onent is accurate to within /.1 J is meaningless. 0he !ercents!ecification must be related to some s!ecific magnitude. "ee !igure $foran e*am!le of e*!ressing accuracy in various ways.

1.2.2 recision vs. Accuracy

0he word !recision means shar!ly or clearly defined. f an instrument isused to !erform a re!eated set of measurements on a !rocess, the!recision of the measurement de!ends u!on how closely the individualresults agree among themselves.

t should be understood that in instrumentation, accuracy and !recisionhave two distinct meanings. =or e*am!le, if two detectors of the samema'e, date and model are com!ared, it may be found that they eachhave the same internal !arts arranged in e*actly the same manner.

@hen they are used, each re!eatedly senses the !rocess variable and!rovides an out!ut measured variable signal that is in e*act agreementwith their !revious measures of the !rocess variable. -ach detector canbe considered to have, therefore, a high degree of !recision.

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=igure #. -*am!les of Accuracy

Fowever, it is !ossible that each may !rovide a different value for themeasured variable or out!ut. 0here are a number of reasons why this can

ha!!en. 0he two detectors may have been installed in a slightly differentmanner9 they may have been adjusted or calibrated in different ways 5orto different reference standards69 or one may be e*hibiting more wear orfriction than the other internally. )oth can be !recise instruments but,obviously, one is more accurate than the other.

0he !recision of a device may be high, but !recision is no guarantee ofaccuracy. t is often u! to the instrument installer to ta'e the necessary!recautions to ensure that instruments are functioning !ro!erly and thatno controllable outside !henomenon is influencing the accuracy of themeasurements ta'en. 0his is done by reducing the !otential for errorwhere !ractical.

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4easurement is the combined result of a human o!eration oninstrumentation as well as the functioning of that instrumentation. Eour

judgement is a !art of the nature of measurement.

@hen !erforming !recision measurements, it is normal !ractice to ma'e

and record a series of observations rather than be content with only onevalue or reading. 0his is one reason you !erform instrument chec's sofre3uently. t is one reason why many adjustment forms and !roceduresoften re3uire you to chec' and record the !erformance values of a devicefirst when you start wor' on it and, again, when youve finished wor' on it.n these instances you are using the fact that a change in the accuracy ofan instrument, due to something you may have done to it, will be noticedby a change in the out!ut of the !recision instrument by your com!aringthe readings before and after wor' has been done. f a significant changeis noticed, you will be re3uired to verify or recalibrate the device to a'nown standard. 0his is the only way you can ensure the accuracy of thedevice is acce!table.

1.2.# 4easurement -rrors

0he error of a measurement is the numerical difference between themeasured value and the true value. n instrumentation terminology theerror of a detector or sensor then would be the difference between themeasured variable and the actual !rocess variable. =or e*am!le, the errorof a barometric !ressure measurement might be B1 mm if the barometerread +& mm when a more accurate measure indicated the true !ressurewas re!resented by +& mm. 0he actual error is given by:

-rror 5-6 4easured ;alue B 0rue ;alue +& mm B +& mm B1mm

@hile is it necessary to 'now the actual error, it is often more useful toconvert it into an indication of how accurate the measurement is, ingeneral. 0o do this, we convert the actual numerical error value into arelative error value which more closely e*!resses the accuracy 5or lac' ofit6 of the instrument. elative error is defined as the ratio of the actualerror value to the true value. n this instance it would be 1?+&. f errormulti!lied this by 1//J, it would e*!ress the relative error as a!ercentage of the true value.

%100*P

;alue0rue

;alue0rueB;alue4easuredO-rrorelative

1//JP;alue0rue

-rrorO-rrorelative *

0he relative error is always e*!ressed as a !ositive number, as theQabsoluteBvalue notation above signifies. Accuracy, then, is determined bythe magnitude of the relative error. Of course, to obtain the variousreference forms of the e*!ression the true value must be e*!ressed inthose reference values.

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-rrors in general measurement wor' are classified as:ersonal -rrorsandom -rror"ystematic -rrors

A!!lication -rrors

ersonal errors are those caused either by carelessness, lac' ofe*!erience, or bias on the !art of the wor'er. 8arelessness is a commonfactor in many errors. 4isreading a measurement can be due tocarelessness or the lac' of e*!erience of the wor'er. An e*am!le ofine*!erience can be an error due to !aralla*. aralla* occurs when awor'er is not e*!erienced 5or careful6 enough to 'now that the measuredvalue will change with the relative !osition of the eye reading it. aralla* isdemonstrated in !igure %.

=igure %. -rror Due to aralla*

Other errors due to carelessness or ine*!erience might be mathematicalerrors or not 'nowing where to find or verify something in a !rocedure,technical manual or s!ecification sheet.

)ias results in a more subjective ty!e of error. t usually is caused byhaving some !reconceived notion or e*!ectation of the magnitude or3uality of the variable under measurement or the device measuring it.Data are selected to substantiate the results or to ma'e your job easier,rather than being acce!ted with e3ual confidence or objectivity. Acommon e*am!le of bias occurs when you carefully evaluate a borderline

measurement value on a calibration chec' to be within s!ecificationsbecause you 'now someone will ma'e you go bac' and reBcalibrate thedevice if one measurement !oint is out of line. n this case, you are justfooling yourself. "oon the instrument will drift further out of tolerance andyou will need to reBcalibrate it anyway. ts also !ossible a lot of wasted!roduct could have been !roduced in the mean time, or a !lant could beo!erating a little less efficiently for the ne*t year, fouling u! theenvironment. )ias must be controlled by the individual wor'er.

A common method of overcoming !ersonal errors is to have readingsmade by more than one wor'er, but this can be time consuming and

e*!ensive.

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andom errors occur when re!eated measurements of the same 3uantityresult in differing values. 0he errors !robably e*ist, but they areconsidered indeterminate.

"ystematic errors are what can be considered builtBin errors which result

from the characteristics of the materials used in construction of theinstrumentation systems. "ystematic errors are caused by such things asthe natural inertia of moving !arts, hysteresis, friction and bac'lash ingearing. naccuracies arising from such causes are more or less regular incharacter. 0hese errors are re!eatable and result in the ty!ical hysteresisloo! of an instrument, which can be demonstrated by !lotting incrementalincreases in first ascending ste!s and descending ste!s. !igure & shows aty!ical hysteresis loo!. !igure ' shows this same loo! !lotted as astatement of continuous !ercent accuracy.

=igure &. 0y!ical Fysteresis $oo!

=igure (. 0y!ical Fysteresis $oo! lot

0hese errors are normally small enough to live with and actually ma'eu! the majority of the tolerance s!ecified by the manufacturer as the!ercent accuracy of the instrument.

A!!lication errors occur from im!ro!er use or faulty installation of aninstrument. A!!lication errors can be minimiCed by following themanufacturers s!ecifications for use and installation, as well as the design



19/106

s!ecifications for use and installation, as well as the design s!ecificationsand general rules of industrial good !ractice standard 5such as those ofthe nstrument "ociety of America.6.1.2.% e!roducibility and Drift

e!roducibility is the degree of closeness with which the same value canbe measured at different times. t is usually e*!ressed as a !ercentage ofs!an of the instrument. erfect re!roducibility indicates that an instrumentor instrument channel has no drift.

Drift is a gradual se!aration of the measured value from the calibratedvalue. 0his usually occurs over a long !eriod of time during which thevalue of the variable is assumed not to change. Drift can be caused by!ermanent setting of the mechanical or !hysical com!onents of thedetector or instrument, stress on the e3ui!ment !arts, or fatigue in themetals or other materials of construction. Alternatively, drift can be due towear, erosion, or general deterioration as a function of time.

Drift is the !rimary cause for the need to reBcalibrate instruments. ndeed,drift is defined above in terms of the variation of the measured value fromthe calibrated value. t could have been defined in terms of the changein the measured value from the true value, but it was desired to 'nowwhat change in the calibration of device was occurring. f you are involvedin maintaining instrumentation, a good way to !lan your normal calibrationcycles of instrument channels is to 'ee! a record of the drift accruingwith res!ect to time. 0hat way you can antici!ate when the instrumentchannel is most li'ely to need reBcalibration.

1.2.& "ensitivity and es!onsiveness

0he sensitivity of a device is the ratio of a change in out!ut magnitude tothe change of in!ut that causes it, after steadyBstate has been reached. tis a ratio that describes how much the in!ut variable must change to!roduce some change in out!ut magnitude. 0he sensitivity of a device isan im!ortant !ro!erty which is determined or set by the designer basedon the re3uirements of the a!!lication. 0he re3uired sensitivity will bedecided by the design engineer based u!on the smallest change neededto be measured in the given !rocess variable.

"ensitivity and res!onsiveness are fre3uently confused. 0he term

res!onsiveness denotes the amount of change in the !rocess variableneeded to cause a !erce!tible change or movement in the measuredvariable, or the amount of in!ut change that can cause the out!ut to startto change in any device.

n s!ea'ing of a thermometer, someone might say it is sensitive to/.1M8, when it is correct to say that the thermometer will res!ond to achange of L/.1M8.

-*am!le: A !ressure detector at 2// !si re3uires a change of L2 !si tocause a change to be !erceived at its out!ut.

An evaluation of the !ercent res!onsiveness of the !ressuredetector would be:



20/106

t is 3uite !ossible that the value of res!onsiveness may vary throughoutthe range of the detector, just as accuracy can. es!onsiveness may beim!roved by !ro!er lubrication and adjustment of the instrument.

1//JP5ressure6Out!utof;alue

5ressure6:n!utin8hangenesses!onsive

1//JP!si2//

!si2Onesses!onsive

1./J at 2// !si

1.#./ 4-A"-4-70 "0A7DAD" A7D -$-4-70"

0he !ur!ose of measurement is to determine the value of a 3uantity,

condition, !hysical !arameter, or !henomenon. A measuring instrument issim!ly a device used to sense and relay that value to us or another devicefor !rocessing the information. 0he value determined by the instrument isgenerally, but not necessarily, 3uantitative. =or the measurement to bereally useful it must be reliable and accurate. 0he way this is assured is toeffectively see that measuring instruments are always functioning tocom!are the !rocess variable being sensed to the e3uivalent of a 'nownmeasure or standard.

1.#.1 Direct vs. nferred 4easurements

0he wide variety of measurements made in industrial !lants and the

varying environmental conditions under which these measurements mustbe made re3uire mention of the basic nature of measurement.4easurements for this discussion fall into two general categories: thosemeasurements made directly and those that are inferred.

Fow would you measure the length of this !ageK Eou would !robably besatisfied to use a ruler or metric rule. Eou would com!are the length of the!age to the measurement mar's or increments on the rule. n the case ofthe -nglish "ystem ruler, your measurement would !robably be accurateto within 1? of an inch9 with the 4etric "ystem rule 5or meter stic'6 itwould !robably be accurate to within 1B# mm. 0his is a directmeasurement. Eou have determined the length of the !a!er by directcom!arison of that !arameter to a 'nown or acce!table standard.

Although to measure by direct com!arison is the sim!lest method, directmeasurement is not always ade3uate or !ossible. =or one thing the humansenses are not !re!ared to ma'e direct com!arisons of all 3uantities withe3ual facility. n many cases they are not sensitive enough. @e can ma'edirect com!arisons of small distances using a rule, with a !reciseness ofabout 1 mm 5a!!ro*imately /./% in.6. Often we re3uire greater accuracy.Often our senses just dont detect in a 3uantitative way what we want tomeasure. -*am!les of this would be !ressure, tem!erature or flow. nthese instances we rely u!on some more com!le* form of measurement

system. Direct measurement is much less common than you thin'.

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nferred measurement occurs when there is an indirect com!arison. 0he!arameter of interest is affecting a characteristic or !ro!erty of thematerial of the measuring system or detector. 0he change in the detectoris what is actually being measured. =or e*am!le, when a difference intem!erature e*ists between the junctions of a thermocou!le, a voltage is

!roduced. 0he voltage is the actual !arameter being measured and thetem!erature is inferred or derived from the characteristic voltagemeasured. Again, it is im!ortant that inferred relationshi!s as this betraceable or com!arable to 'nown standards.

1.#.2 4easurement "tandards

A standard is an accurate 'nown 3uantity used for calibration ofmeasurement instruments. A standard can also be an instrument of highaccuracy.

"tandards e*ist for every ty!e of measurement. 0he set of ultimate

standards is maintained by the .". 7ational )ureau of "tandards 57)"6.On occasion, standards used for calibration !ur!oses are set to the 7)".

0he 7)" chec's these standards against their own standards for accuracy.0hese standards are then said to be traceable bac' to the 7)".

0hree basic levels of measurement standards are common. 0here are:

rimary or absolute standards

"econdary reference standards

@or'ing standards

1.#.# rimary "tandards

rimary or absolute standards are constructed to conform to the legaldefinitions of different fundamental units of measurement. An e*am!le ofa !rimary standard is the standard meter or a set of !recision weights forounces and !ounds. Another e*am!le is a set of containers that hold!recise amounts of li3uids for liters, !ints, 3uarts, and gallons. 0he termabsolute is used to indicate these measures are finite and are, therefore,inde!endently accurate and correct. Other ty!es of standard measures areultimately traceable to these. 0raceability is an im!ortant 3uality ofinstrumentation in highBtech and?or haCardous industrial a!!lications.

1.#.% "econdary "tandards

"econdary reference standards are devices that are co!ied from e*isting!rimary or absolute standards. 0hese standards are sometimes referred toas !rototy!e standards. Often these standards are maintained accurateand traceable to the 7)" !rimary standards by s!ecial com!anies that setu! regional standards laboratories. nstead of sending your measuringe3ui!ment to the 7)" in @ashington, D.8., you can send it !eriodically tothe regional laboratory and they will chec' the accuracy of youre3ui!ment against their secondary standards.

"ome construction and o!erating com!anies maintain their own secondarystandards laboratory onBsite.



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1.#.& @or'ing "tandards

@or'ing standards are used to calibrate the instruments installed insystems in the field. 0he detector or sensor accuracy is thereby traceablethrough the local wor'ing standards to secondary reference standards to

ultimately the !rimary standards at the 7)".

0his system of traceability is what ensure the reliability of the safefunctioning of industrial !lant instrumentation. 0he !a!er wor' associatedwith the calibration and adjustment of instrumentation in the filed is thedocumentation of !ro!er installation and setu! of !lant systems. n mayindustries, this !a!erwor' is just as im!ortant as actually installing andcalibrating the e3ui!ment, for without it you would be unable to assure thegovernment regulators, or the !lant o!erators and owners, or the !ublicthat the !lant can be o!erated safely and adherence to environmentalguidelines or regulations.

1.#.( rimary and "econdary -lements

All instrument channels contain various com!onent !arts or elementswhich !erform the !rescribed measurement, conversion 5transducing6,conditioning 5am!lification6 and transmitting functions described earlier.

0he !rimary sensing element is the !art of the instrument or channel thatfirst uses energy from the measured medium to !roduce a condition orsignal that re!resents the value of the measured medium 5!rocessvariable6. n most cases, an industrial !rimary element converts themeasured variable into a dis!lacement. Often this mechanicaldis!lacement is converted by a secondary element to an electrical orelectronic signal. !igure +. $ists common mechanical and electrical!rimary elements and the o!eration they ty!ically !erform.

0he !rimary element may be very sim!le, consisting of no more than amechanical s!indle, arm or contacting member used to !rovidemovement or force to the secondary element. t, of course, may be muchmore com!le*.

0he !rimary elements function is to, first, sense the 3uantity of interestand, then, to !rocess the sensed information into a form that is usable bythe instrument channel. t usually does not !resent an out!ut that is!owerful enough to be a!!lied directly for indication or control of

e3ui!ment.

!igure + is only a re!resentative listing. 4any of these elements will bedescribe in more detail later. t is significant, however, to note that manyof the mechanical elements !roduce an out!ut that is a !hysicaldis!lacement, which is easily converted 5or transduced6 by a secondaryelement to an electrical signal, and many of the electrical elements!rovide an electrical signal out!ut directly. -lectrical elements haveseveral im!ortant advantages:

1. Am!lification and signal conditioning is easily !erformed.2. 4assBinertia effects are minimiCed.#. =riction is minimiCed.%. An out!ut signal !ower of almost any magnitude can be !rovided.



23/106

&. -lectronic transducer?transmitter combinations can often beminiaturiCed.

Of course, dis!lacement and?or force can also be easily converted to a!neumatic signal for !rocessing and transmission. @hile a bit more

cumbersome, it has a major advantage that a loss of !lant !ower will notimmediately inca!acitate the instruments.

1.#.+ 8alibration

-very measuring system must be !rovable9 that is it must !rove its abilityto measure reliability. 0he !rocedure for establishing an instrumentsaccuracy is called calibration.

=igure +. 4echanical and -lectrical 4easuring -lements

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8alibration is the testing of the validity of the measurements by aninstrument in normal o!eration by com!arison with measurements madeby !rimary, secondary or, more commonly, wor'ing standards. t isessential for good !erformance to wor' to achieve a high degree ofaccuracy and reliability.

At some !oint during the installation of instruments and systems, 'nownmagnitudes of the !rocess variable 5or in!ut !arameter6 must be a!!liedto the detector and the detector, as well as the whole instrument channel,o!eration must be observed and recorded. 0he instrument manufacturerwill !rovide in the accom!anying manuals sam!le !rocedures forcalibrating the devices. 0his often must be combined with general goodindustrial !ractice guidelines and local?com!any !rocedures for testingand calibrating system installations.

Eour a!!lication of a tag or stic'er to an instrument or detector meansthat it has been tested and adjusted in accordance with acce!ted

!rocedures and !ractices and is certified as calibrated by you to its degreeof accuracy recorded in accom!anying documentation. 0his means that,given the rigidness of your local calibration !lan and !ractices, theaccuracy of the instrument is valid and traceable to a set standard.

1.#. "ignificant =igures

0he documentation of calibration is very im!ortant. t is im!ortant that thevalues or numbers used in the calibration records be consistent with the!ossible sensitivity of the instrument and testing e3ui!ment being used.

n writing a measured value as a series of digits, some of these digits willhave an element of doubt associated with them. 0he total number ofsignificant figures is de!endent u!on the !robable error associated withthe observation. f the reading is inter!reted by the observer as being#.(#% and the accuracy of the detector is stated to be /./& and of thetest e3ui!ment is L /.//1, then the reading should be ta'en to be theleast accurate limit involved 5i.e. #.( L /./&6.

2././ D-0-80O"B

0he first major bloc' of a !rocess instrument channel is the detector. Aswe covered earlier, the detector senses the !arameter being measured. t

is also called the !rimary or measuring element.

2.1./ O=8- $A0-"

Orifice !lates are the most common ty!e of flow measuring element. 0heorifice !late is a thin circular metal !late with a shar! edged hole. 0heorifice !late is usually mounted between two flanges. 0hree 'inds of orifice!lates are used: the concentric, the eccentric and the segmental as shownin !igure (.

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=igure . Orifice lates

0he concentric orifice !late is the most commonly used of the three ty!es.

t is usually made of stainless steel from 1? to 1?2 in. thic' de!ending!rimarily on the diameter of the !i!e for which it is manufactured. Othermaterials such as 4onel or Fastelloy are used for fluids corrosive tostainless steel. 0he !late is usually manufactured with a tab on which!ertinent orifice !late data is stam!ed such as orifice bore or hole siCe.

0he ratio of the orifice bore to the internal !i!e diameter is called the)eta.

) d?D

where:

) orifice bore to internal !i!e diameter ratiod orifice bore 5inches6D internal diameter of !i!e 5inches6

0he flow !attern and the shar! leading edge of the orifice !late that!roduces it are of major im!ortance to the accuracy of the flowmeasurement when using a concentric orifice !late. Any nic's or roundingof the shar! edge changes the flow !attern significantly and therefore,affects the accuracy of the measurement. 0he ty!e of flow as reflected bythe eynolds number, also has a considerable influence on the flow!attern. At low eynolds number, laminar flow, the velocity !rofile of thefluid reveals that the greatest flow rates occur at the center of the !i!e. Asthe fluid !asses through the orifice only a small energy conversion isre3uired to constrict the flow, therefore, a relatively small differential!ressure results. At high eynolds numbers, the velocity !rofile isrelatively flat so a large energy conversion ta'es !lace and as a result arelatively large differential !ressure is !roduced. 0he !attern obtained athigh eynolds numbers is desirable.

8oncentric orifice !lates should be used for clean va!or free li3uids orcondensate free va!ors and gases. n li3uid flow a!!lications as the fluid

converges in order to !ass through the orifice, !articles tend to dro! outand collect at the bottom of the u!stream face of the orifice !late9 gases

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and va!ors tend to collect at the to! of the u!stream face. n gas or va!orflow a!!lications condensate tends to form a !uddle at the bottom of thehoriContal line u!stream of the orifice !late. Any of these conditionschanges the area of the u!stream fluid stream and, therefore, causesinaccurate flow measurement.

0he collection of !articles and condensate can be alleviated by drilling asmall drain hole nearly flush with the inside diameter of the !i!e at thebottom of the orifice !late, as indicated in !igure ). 0his small drain holealso !ermits drainage of a horiContal !i!e that contains the orifice. 0hecollection of gases or va!ors can be eliminated by drilling a small venthole nearly flush with the inside diameter of the !i!e at the to! of theorifice !late. 0his is also indicated in !igure ).

=igure . 8oncentric Orifice late with ;ent and Drain Foles

0he vent and drain holes have little effect on the flow measurementbecause, if the diameter of these holes is less than one tenth of orificebore diameter, then the ma*imum flow through these holes is less than1J of the total flow. Drain and vent holes are inade3uate for li3uid flowa!!lications where large 3uantities of solids or gases are !resent and forgas or va!or flow a!!lications where large 3uantities of condensate are!resent. =or these a!!lications, the segmental orifice or the eccentricorifice !late is normally used.

0he o!ening in a segmental orifice is a segment of a circle as shown in!igure (.0he diameter of the o!ening is J of the inside diameter of the!i!e. 0he circular section of the segment should be concentric with the!i!e. 0he segmental orifice !late is useful, because it eliminates dammingof foreign materials on the u!stream side of the orifice when the orifice!late is mounted in a horiContal !i!e.

@hen the flow of a li3uid containing solids or of a gas containing moistureis to be measured in a horiContal !i!e, the segmental orifice is used withthe circular section at the bottom of the !i!e. @hen the flow of li3uidcontaining gases is to be measured in a horiContal !i!e, the segmental

orifice is used with the circular section at the to! of the !i!e. @hen flow ismeasured in vertical runs, however, the concentric orifice should alwaysbe used.

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0he eccentric orifice has a circular hole bored tangent to the insidediameter of the !i!e. 0he diameter of the o!ening is J of the insidediameter of the !i!e. 0he eccentric orifice !late is used in the same wayas the segmental orifice !late.

0here are five commonly used ta! locations for measuring the differential!ressure across an orifice !late. 0hey are the flange ta!s, corner ta!s,vena contracts ta!s, radius ta!s, and !i!e ta!s.

=lange ta!s are the ones most often used in the .". for !i!e siCes of 2inches or greater. 0hese ta!s are drilled through the orifice flanges 1 inchfrom the surface of the orifice !late. 0his arrangement is shown in !igure"*.0hey are not recommended for !i!e siCes less than 2 inches, becausethe vena contracts may be less than 1 inch from the orifice !late.

8orner ta!s are in common use in -uro!e. 0hese ta!s are drilled through

the flange so that they sense the !ressures at the edge of the orifice !lateas shown in !igure "*.

=igure 1/. =lange and 8orner 0a! $ocations

;ena 8ontracts ta!s have an u!stream or high !ressure ta! located one!i!e diameter u!stream of the orifice !late and a downstream or low!ressure ta! located at the vena contracts, the !oint of minimum

!ressure. 0hese ta!s are indicated in !igure "". 0heoretically, this is theo!timum location for orifice !late ta!s, because the greatest differential!ressure is available between these !oints. Fowever, the !oint ofminimum !ressure varies with the d?D ratio. 0herefore, errors areintroduced if the orifice !late bore is changed due to erosion.

adius ta!s as shown in !igure ""+ are an a!!ro*imation of the venacontracts ta!s. 0he u!stream or high !ressure ta! is located one !i!ediameter u!stream of the orifice !late and the downstream or low!ressure ta! is located 1?2 !i!e diameter downstream of the orifice !late.

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=igure 11. ;ena 8ontracta and adius 0a! $ocations

i!e ta!s or full flow ta!s measure the !ermanent !ressure dro! across anorifice !late. 0he ta!s are drilled into the !i!e 2N !i!e diametersu!stream of the orifice !late and !i!e diameters downstream of theorifice !late. 0his ta! arrangement is shown in !igure "#. )ecause of thedistance from the orifice, e*act location of the ta!s is not critical.Fowever, there is some li'elihood of measurement errors created by headloss in the long length of the !i!e.

=igure 12. i!e 0a! $ocations

0he !ressure ta!s for li3uid flow are generally located along the horiContalcenterline of the !i!e. $ocation of the ta!s along the side of the !i!e!revents tra!!ed gas bubbles from interfering with the measurement and!revents sludge or !articles from fouling sensing lines. =or gas flow, theta!s are generally located at the to! vertical centerline of the !i!e. 0hislocation of the ta!s allows condensate to drain from the sensing lines. =or

best accuracy, the !ressure ta!s for segmental orifice !lates must be at1/M from the center of tangency of the o!ening, as shown in !igure "$.0he !ressure ta!s for eccentric orifice !lates must be located at 1/M or/M to the eccentric o!ening. 0his is also shown in !igure "$.



29/106

=igure 1#. Orifice 0a! $ocations

Orifice !lates are the most widely used of the !rimary flow elements,because they are ine*!ensive and easy to install. n addition, more

em!irical data has been collected on this device than has been collectedfor any of the other !rimary elements. Orifice !lates, however, have twoserious disadvantages. =irst of all, as !reviously mentioned, they cause ahigh !ermanent !ressure dro!. "econdly, they are highly susce!tible toerosion because of the shar! edges at the o!ening. -rosion of the shar!edges can cause serious inaccuracies in the flow measurement.

2.2./ ;-70 0)-

0he venturi tube is the most accurate of all !rimary elements when it is!ro!erly calibrated. =luids that contain large amounts of sus!ended solids,such as slurries, or those that are very viscous can be measured byventuri tubes with ma*imum accuracy. 0he ty!e consists of a convergingconical inlet section, a cylindrical throat and a diverging recovery cone. Aty!ical venturi tube is shown if !igure "%. 0he inlet section decreases thearea of the fluid stream causing the velocity to increase and the !ressureto decrease. 0he low !ressure is measured in the center of the cylindricalthroat. At this !oint the velocity and !ressure are neither increasing nordecreasing. n the diverging recovery cone, the velocity decreases and the!ressure is recovered. 0he recovery cone allows a relatively large !ressurerecovery, so that the !ermanent !ressure loss is only 1/J to 2&J of thedifferential !ressure develo!ed by the device. 0he differential !ressure

develo!ed by the venturi is sensed between an u!stream or high !ressureta! located N !i!e diameter u!stream of the inlet cone and a low !ressureta! located at the center of the throat.



30/106

=igure 1%. ;enturi 0ube

-ccentric venturi tubes are occasionally used in systems where the flow of

slurries is to be measured. n this ty!e of venturi, the throat is flush withthe bottom of the !i!e. 0his design, shown in !igure "&+ further assuresthat a buildu! of solids does not occur and !ermits com!lete drainage ofhoriContal !i!es.

=igure 1&. -ccentric ;enturi

2.#./ 0F- 0O0 0)-

0he itot tube measures fluid velocity at one !oint within the !i!e. t is aty!e of head flowmeter. "ince the velocity of a fluid !assing through a!i!e varies with its distance from the !i!e wall, the flow indicationobtained from a ilot tube can be highly inaccurate, !articularly in laminarflow conditions. =or this reason, the itot tube has limited industriala!!lication. 7evertheless, for velocity measurements, s!ot measurementsand laboratory measurements, the itot tube is the best and most often

used device. t is also commonly used for flow measurement in large !i!esand ducts such as in ventilation systems.

A sim!le itot tube consists of a cylindrical !robe which is inserted into theflow stream. t has two o!enings9 the first, called the im!act o!ening,faces into the stream9 the second, called the static o!ening, faces!er!endicular to the flow stream as indicated in !igure "'.

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=igure 1(. ilot 0ube

0he differential !ressure !roduced by the device is measured by aconventional differential !ressure measuring device such as a bellows. tshould be noted, however, that the high !ressure connection senses the

im!act !ressure and the low !ressure connection senses static !ressure.

0o obtain a true measurement of flow in a !i!e, it is necessary to 'now theaverage velocity of the fluid. ;elocity readings from the itot tube!ositioned at several different distances from the !i!e wall would have tobe ta'en, weighted in accordance with a factor based on distance from thewall and finally averaged in order to obtain an accurate flowmeasurement. 0his is fre3uently done in test wor', but it is hardly!ractical for industrial !rocess flow measurement.

"everal differentvariations of the itot tube have been designed in orderto !rovide a higher differential !ressure than that !roduced by im!act!ressure alone. One such variation is the itot ;enturi shown in !igure ",.

0he !ressure at the im!act o!ening, which is the sum of im!act !ressureand static !ressure, is develo!ed the same as in the conventional itottube. 0he !ressure at the im!act o!ening is com!ared to the reduced!ressure at the throat of a small venturi that is also sus!ended in the fluidstream. Eou should recall that the !ressure at the throat of a venturi dro!sbecause of a velocity increase. 0he differential !ressure created by thisdevice is measured in the same manner as with the conventional itottube.

=igure 1+. itotB;enturi 0ube

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0he itot tube has two serious disadvantages. 0he major disadvantage isthat it can measure velocity at only one !oint within a !i!e or duct. 0hesecond disadvantage is that the im!act o!ening is easily bloc'ed if theitot tube is used for the measurement of dirty or stic'y fluids. t hasseveral advantages. =irst of all, it !roduces no a!!reciable !ressure dro!.

n addition, it is easy to install and it is ine*!ensive.

2.%./ A77)A 0)-"

0he Annubar tube is a variation of the conventional itot tube that nearlyeliminates the major disadvantage of conventional itot tubes. t consistsof two !robes, one that senses fluid velocity and one that senses static!ressure. 0he !robes are sus!ended in the fluid line in much the sameway as the conventional itot tube.

0he velocity sensing !robe has four o!enings or !orts that face u!streaminto the flow stream. -ach of the !orts are located at !ositions

re!resenting e3ual crossBsectional areas to the flow stream. 0hese !ortsare shown in !igure "(.

=igure 1. Annubar 0ube

A line inserted into the u!stream !robe senses the average of the im!act!ressures !resent at the four o!enings. 0his average !ressure is a resultof the average velocity in the !i!e. 0herefore, the differential betweenaverage im!act !ressure and the static !ressure sensed by thedownstream !robe gives an accurate indication of the fluid flow rate.

n addition to the fact that this device !rovides average velocitymeasurement, its other advantages are similar to the conventional itottube9 easy installation and low cost.

2.&./ 4AG7-08 =$O@4-0-"

0he magnetic flowmeter o!erates on the !rinci!le of =aradays $aw ofnduction that states: anytime a conductor is moved through a magneticfield at right angles an electrical !otential is develo!ed. 0he magneticflowmeter was develo!ed to measure the volumetric flow rate ofelectrically conductive fluids. t is !articularly useful for measuring the flowrate of fluids that !resent ery difficult handling !roblems such ascorrosive acids, sewage, slurries and !a!er !ul! stoc'.

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As was stated !reviously, when an electrical conductor moves through amagnetic field in a direction !er!endicular to the magnetic lines of flu* an-4= is induced into the conductor. 0he magnitude of the induced -4= is!ro!ortional to the magnetic flu* density, the length of the conductor inthe magnetic field and the s!eed or velocity of the conductor.

0he magnetic flu* density is a term which describes the strength of themagnetic field. t is the number of magnetic lines of force !er unit area.

0he units normally used for magnetic flu* density, ), are webers !ers3uare meter, where a weber is the measure of the number of magneticlines of force.

0o determine the -4= induced into a conductor the following e3uation isused:

-4= induced R )y

where:

) magnetic flu* density; velocity of the conductorR length of the conductor

0he direction or !olarity of the induced -4= is determined by using theleftBhand rule, as illustrated in !igure "). f the inde* finger !oints in thedirection of the magnetic lines of force, that is from the north !ole to thesouth !ole of the magnet, and the thumb !oints in the direction of themotion of the conductor with res!ect to the field, the middle finger !ointstoward the negative !otential.

0he magnetic flowmeter is com!rised of a tube, a coil, a laminated coreand the -4= sensing electrodes. 0hese com!onents are shown in !igure#*.

8urrent flow through the coil !roduces a magnetic field. 0he laminatedcore concentrates the magnetic lines of flu* of this field around the tube.)y concentrating these lines of flu*, the magnetic flu* density isincreased. 0he tube directs the !rocess fluid through the concentratedmagnetic field. 0he !rocess fluid is the moving electrical conductor. As thefluid !asses through the magnetic field, an -4= is induced into it that is

sensed by a !air of electrodes, which is the length of the conductor. f themagnetic field strength and the distance between the electrodes isconstant, then, the -4= sensed by the electrodes is !ro!ortional to thevelocity of the fluid.

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=igure 1. $eftBFand ule for nduced -4=

=igure 2/. 4agnetic =lowmeter

At the beginning of this cha!ter, the characteristics of fluid flow werediscussed. @e should remember from that discussion, that the velocity ofthe fluid close to the !i!e wall is less than the velocity of the fluid flowing

at the center of the !i!e. 0hese variations in the velocity !rofile do notaffect the flow measurement accuracy of the magnetic flowmeter. 0heelectrodes sense the average -4= of the fluid. 0herefore, the magneticflowmeter measures the average velocity of the fluid regardless ofwhether the flow is laminar or turbulent. 0his also allows biBdirectional flowmeasurement.

0he tube carrying the !rocess fluid must be made of a nonBmagneticmaterial to allow the field to !enetrate to the li3uid. n addition, it must bemade of a nonBconducting material so that the !otential induced into thefluid is not short circuited by the !i!e. 0y!ical materials used in theconstruction of the flowmeter tube are fiberglass reinforced !lastic or nonB

magnetic stainless steel. @hen stainless steel is used the interior of the

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tube is lined with a nonBconducting material such as teflon, !olyurethane,or glass.

0he electrodes are flush with the tube interior. 0hey actually come incontact with the !rocess fluid. 0he electrodes must be made of goodconducting material. 0hey are usually made of ty!e #1( stainless steel,

but for highly corrosive service, !latinum electrodes are often used. t isessential that these electrodes remain free of dirt. Dirt acts as an electricalinsulator and reduces the accuracy of measurement.

2.(./ $0A"O78 =$O@4-0-"

ltrasonic flowmeters are actually a grou! of devices based on welldocumented theory. 0heir a!!lication to field installations has awaited theavailability of costBeffective electronics ca!able of accurately measuringsmall changes in time or fre3uency. 0his section will discuss the two mostcommon a!!lications of ultrasonic flow measurement9 they are the timedifference method and the fre3uency shift method of flow measurement.

0he determination of fluid flow by the use of ultrasonics im!lies thetransmission and rece!tion of sound waves. ltrasonic sound waves areabove the range of fre3uencies audible by the human ear 52/,/// FC6. 0hety!ical fre3uencies used by ultrasonic flowmeters are in the range of 14FC to 1/ 4FC. Fowever, before the use of sound waves to determineflow can be e*!lained, some characteristics of sound and the method ofdetecting and transmitting it must be understood.

"ound is the transmission of very small !ressure variations through amedium to a receiving device. =or instrument a!!lications, the transmittermust convert an electrical signal to a mechanical motion, and the receivermust convert this motion bac' to an electrical signal. A device ca!able ofma'ing both of these conversions is the !ieCoelectric crystal. @hen acrystal is held between two flat metal !lates and the !lates are !ressedtogether, a small -4= is develo!ed between the two !lates. t is as if thecrystal became a battery for an instant. @hen the !lates are released, thecrystal s!rings bac' to its original sha!e and an o!!osite!olarity -4= isdevelo!ed between the two !lates. n this way, the crystal convertsmechanical energy into electrical energy. =urthermore, when an -4= isa!!lied across the two !lates on either side of a crystal, the normal sha!eof the crystal is distorted. @hen an -4= of o!!ositeB!olarity is a!!lied, the!hysical distortion of the crystal is reversed. n this way, the crystal

converts electric energy into mechanical energy. 0hese two reci!rocaleffects that occur in a crystal are 'nown as the !ieCoelectric effect."!ecifically, !ieCoelectricity is a !ro!erty of nonconducting solids thathave a nonBsymmetrical crystal lattice structure. 0y!ical deformations thatcan occur in a crystal are shown in !igure #".

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=igure 21. ieCoelectric 8rystal Deformation

-very crystal has a resonant fre3uency that is de!endent u!on itsstructure, its siCe and its sha!e. 0he electronic signal a!!lied to thetransmitting crystal should be of the same fre3uency at which the crystalresonates in order for the crystal to sustain its mechanical oscillations."imilarly, the receiving crystal should be matched to the transmitting

crystal.

As !reviously mentioned, sound is the transmission of small !ressurevariations through a medium. @hat actually occurs is that the moleculesof the transmission medium are alternately com!ressed and rarefied5s!read out6. 0he molecules do not travel an a!!reciable distance, onlythe variations in !ressure actually move.

n a !revious discussion, we determined that flowrate was !ro!ortional tothe density of the fluid, the crossBsectional area of the !i!e through whichthe fluid flows and the velocity of the flowing fluid. sing the followinge3uations, then flowrate can be determined:

; v * A or m * A * v

where:

; volumetric flow rate 5ft?min6m mass flow rate 5lbm?min6v fluid velocity in 5ft?min6A flow area in 5ft26S fluid density in 5lbm?ft#6

f the flow area and the fluid density were 'now constants, then, flowratecould be determined by measuring the velocity of the flowing fluid.ltrasonic flowmeters are used to measure velocity. 0he method

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em!loyed to measure the velocity of the fluid is rather sim!le. 0he rate atwhich sound is !ro!agated through a given medium at rest is constant9 forwater is &,/// ft?sec. f the fluid also has a velocity, the absolute velocityof the !ressureBdisturbance !ro!agation is the algebraic sum of the two.

0his means that if sound is transmitted in the direction of the fluid flow,

the actual velocity of the sound is the sum of the sounds velocity whenthe fluid is at rest !lus the velocity of the fluid. 8onversely, if the sound istransmitted in a direction o!!osite the fluid flow the actual velocity will bethe difference between the two velocities.

!igure ## is a sim!le s'etch of the detector !lacement for ultrasonicmeasurement of flow. 0he detectors used are !ieCoelectric crystalsidentical to those discussed in the !receding !aragra!hs. 0he o!eration ofthis instrument re3uires that the velocity of sound in the fluid, at rest, beaccurately 'nown. 0hen when flow e*ists, the sound will travel fromtransmitter to receiver at a greater velocity. 0he velocity of the fluid canbe calculated using the following e3uation:

vf vm 8

then:

8Bt

$v

*f

where:

vf velocity of fluid

vm measured velocity of sound with flow8 s!eed of sound with no flow$ distance from transmitter to receivert* time for fluid to travel from transmitter to receiver

"ince $ and 8 are constants and t can be measured, the velocity of thefluid and thus, the flow rate can be determined.

=igure 22. ltrasonic =lowmeters

A similar method of determining the flow rate is illustrated in !igure ##/0.

n this a!!lication the difference in time between transmission with theflow and transmission against the flow is measured. 7ow the increment of

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time can be measured directly and is larger than the difference betweenvmand 8 in the first e*am!le. 0he fluid velocity is calculated as shownbelow:

vB8

$tand

vI8

$t aw

and: Tt taB tw

so:vI8

$B

vB8

$Ot

or: 22 vB8

2$vt

if: v2UU c2

then 28

2$vOt

or:2$

t8Ov

2

where:

$ distance between sensors8 s!eed of sound with no flow

v velocity of fluidtw sound transit time with flowta sound transit time against flow

0his e3uation shows that fluid velocity is linearly changing with themeasured time difference. As the t increases, the fluid flow rate is alsoincreasing.

0he two devices analyCed thus far offer little resistance to fluid flow, andtherefore, cause a negligible !ermanent head loss. Fowever, both devicesre3uired that the detectors be !laced within the fluid stream. An e*am!leof an ultrasonic flow measuring device that re3uires no !i!ing

!enetrations is illustrated in !igure #$. Fere the detectors are mountede*ternal to the !i!e and function as both the transmitter and the receiver5transceiver6 for the sound. 0he e*ternal mounts re3uire the sound to betransmitted through the walls of the !i!e. 0his adds another variable forwhich an adjustment must be made. n most cases the device is calibratedfor only one schedule !i!e so the transit time of the sound through the!i!e is 'nown. Also, the sound is no longer traveling in a !ath !arallel tothe fluid. 0he !reviously develo!ed e3uation is still valid, however, if thefluid velocity is multi!lied by the cosine of the angle /.

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=igure 2#. ltrasonic =low 4easurement

0he fre3uency shift method of ultrasonic flow measurement usesdetectors !laced as shown in !igure #%.0he am!lifiers are actually selfBe*cited oscillators. =ollowing the initial !ulse, each succeeding !ulse is

triggered by the recei!t of the !revious !ulse. 0herefore, the fre3uency ofoscillation is a function of the signal transient time through the fluid. 0hetransient time, in turn, is a function of the magnitude of the fluid flow. 0headvantage of this system over the time difference method is that it can bemade to be inde!endent of the s!eed of sound in the fluid at rest.

0he o!eration of this system is based on measuring a difference infre3uency. Am!lifier A oscillates at a fre3uency that is greater than thefre3uency of am!lifier ). 0he fre3uency of either oscillator is e3ual to theinverse of its signals transient time through the fluid. 0he actualrelationshi! of the fre3uency difference to fluid velocity is develo!edbelow:

=igure 2%. =re3uency "hift ltrasonic =low 4easurement

=a 1?taand fb 1?tb

)ut:VvcosB8

$tand

VvcosI8

$t ba

0herefore:

VvcosB8

$

t1?tfBf baba /1

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so:$

2vcosf

or:2cosV

$fv

where:

fa fre3uency of am!lifier Afb fre3uency of am!lifier )ta sound transient time across fluidtb sound transient time across fluidtb s!eed of sound in fluid at restv velocity of the fluid$ distance between detectors

0his illustrates that fluid velocity is directly !ro!ortional to the differencein am!lifier fre3uencies.

2.+./ 8AA80A78- 0E- $-;-$ D-0-80O"

A ca!acitor consists of two conductors se!arated by an insulator. 0heinsulator is referred to as the dielectric and the conductors are referredto as the !lates of the ca!acitor. 8a!acitance is measured in farads. Aone farad ca!acitor is ca!able of storing one coulomb of charge for eachvolt a!!lied. !igure #& is a sim!lified s'etch of a !late and a cylindricalca!acitor.

=igure 2&. 8a!acitors

=or the !late ca!acitor, the ca!acitance is calculated using thefollowing e3uation:

d

AW/.22%O8

where:

8 ca!acitance in !icofaradsA !late area 5* X y6 in s3uare inchesd distance between !lates in inches

W dielectric constant/.22% conversion factor

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0he dielectric constant is a factor that com!ares any material to a !erfectvacuum. A ca!acitor with a vacuum dielectric has a dielectric constant ofone. e!lacing vacuum with !a!er doubles the ca!acitance9 !a!er has a Wof two. Ta/le "lists the dielectric constants of some common materials.

4aterial Dielectric 8onstant;acuumAira!eruartC

0eflon@ater 5#2Y6@ater 5(Y6@ater 5212Y6

1.//1.//(2.//%.#2./

.//./%./

0able 1. Dielectric 8onstants for 8ommon 4aterials

0he ca!acitance of the cylindrical ca!acitor in !igure #& is calculated usingthe following e3uation:

8 /.(1% W

where:

A, ), and 8 are the dimensions indicated in !igure #&/0 in inchesW dielectric constant/.(1% conversion factor

=or the measurement of level in nonBconductive materials, a bare metal!robe is used. 0he !robe serves as one !late of the ca!acitor, the tan'walls serve as the other !late. efer to !igure #' for a diagram of this ty!eof installation. 0he actual measured ca!acitance is the sum of 81, 8vand8$. 8a!acitance 81 is unaffected by tan' level, it re!resents theca!acitance of the leads and measuring system. 8a!acitor 8 vre!resentsthe ca!acitance of the !ortion of the !robe e*!osed to the va!or, its valueis largely determined by the tan' level and dielectric constant of theva!or, Wv. 8a!acitor 8$re!resents the ca!acitance of the !ortion of the!robe e*!osed to the li3uid, its value is a function of the tan' level and theli3uids

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=igure 2(. )are 8a!acitance robe

dielectric constant, W$. 0he dielectric constant of all gases is nearly unity,so, Wv is smaller than W$. efer to the !revious e3uations as necessaryduring the following discussion. Assuming the tan' is em!ty, 8$is Cero and8vis ma*imum, but still small because Wvis small. At this level, measuredca!acitance is minimum, 81 I 8v ma*. As tan' level rises, 8$ starts toincrease and 8v begins to decrease. Fowever, the rate at which 8$increases is greater than the rate at which 8vdecreases because of thedifference in their dielectric constants. @hen the tan' is full, 8vis Cero and8$is ma*imum. 4easured ca!acitance is at its ma*imum value of 81 I 8$ma*. 0he total s!an of measured ca!acitance is the difference between 81when the tan' is full and 8v, when the tan' is em!ty, as shown in thee3uation below:

"!an u!!er range value B lower value"!an 81I 8$ma* B581I 8vma* 6"!an 8$ma* B8vma*

0he change in ca!acitance is a linear function of tan' level. 8onvertingfrom a s!an given in !icofarads to one given in inches of tan' levelre3uires 'nowing only the length of the !robe. efer again to !igure #'."ome value of resistance is also !resent between ground and the !robe.

0he actual value will vary with tan' level and the ty!e of nonBconductivematerial in the tan'. 0his resistance must be high when com!ared to theim!edance of the ca!acitors to ma'e its shunt !ath for currentinsignificant. n this regard, there is a definite advantage to ma'ing themeasurement at high fre3uencies.

0he bare ca!acitance !robe can only be used on nonBconductive fluids. fthe fluid is conductive, the shunt resistance will be small, ma'ing anaccurate measurement of ca!acitance changes im!ossible.

=or use with conductive li3uids, the insulated !robe has been develo!ed.!igure #, is a diagram of an insulated ca!acitance !robe level

measurement system. 0he most commonly chosen insulating material isteflon. 0eflon has a dielectric constant of 2 and will for a ca!acitor with the

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!robe and vessel acting as the !lates. 0he ca!acitor formed by theinsulator is referred to as 8a above the li3uid and 8b below the li3uidsurface. @hen the tan' is em!ty, the measured ca!acitance will be asshown in the sim!lified s'etch in !igure #(a0. )oth 8v.and 8aare at theirma*imum values at this time. 8a!acitor 8vis small due to the low

=igure 2+. nsulated 8a!acitance robe

=igure 2. 8a!acitance $evel Detector 8ircuit

dielectric constant of the va!or, therefore, the effective ca!acitance of the

8vand 8ain series is less than 8v. As tan' level increases both 8vand 8adecrease due to a decreasing !late area at a rate largely determined by

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8v. Fowever, as s

Instr 12205 Elements, Transmitters, Transducers, Displacers

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