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Application Note

The initial selection of a cali-brator is often based on aspecification sheet, a writtendescription of the equipmentsperformance in quantifiable

terms that applies to all cali-brators having the same modelnumber. Since specificationsare based on the statistics of alarge sample of calibrators,they describe the performanceof a group rather than a single,specific calibrator. Any singlecalibrator would meet all thespecifications, and usuallywould significantly exceedmost of the various specifica-tion details.

Good specifications have thefollowing characteristics:

They are complete. They are easy to interpretand use.

They include the effectsfound in normal usage, suchas environment and loading.

Completeness requires thatsufficient information beprovided so the user can deter-mine the bounds of performancefor all anticipated outputs (orinputs), all possible andpermissible environmentalconditions within the listedbounds, and all permissibleloads.

Ease of use is also impor-tant. Many specifications canbe confusing and difficult tointerpret, thus causing mistakesin interpretations that can leadto application errors or faultycalibrations.

The requirement forcompleteness conflicts some-what with that for ease of use;one can be traded for the other.The challenge of specification

design is to mutually satisfyboth, which is sometimesaccomplished by bundling theeffects of many error contribu-tions within a useful and

common window of operation.For example, the listedperformance may be valid for aperiod of six months whenused in a temperature range of23 + 5 C, and in humidity upto 80 percent, and for all loadsup to a specified maximumrating. This is a great simplifi-cation for the user since theerror contributions of time,temperature, humidity, andloads are included in the basicspecification and can beignored as long as operation ismaintained within the listedbounds.

The importance ofspecificationsComprehensive specificationsare essential in maintaining achain of traceability and inensuring global uniformity of products, quality and productsafety.

TraceabilityTraceability is a term thatrefers to the fact that instru-ments have been proven toconform with the official stan-dards for the parameters whichthey measure. This means themeasurements made by thisequipment are traceable tonational standards. A certifiedinstrument is one which itself has been regularly tested byeven better-performing certi-fied devices. Specific testprocedures are used, and theresults are documented and

Understanding

specificationsfor process calibrators

F r o m t h e F l u k e D i g i t a l L i b r a r y @ w w w . f l u k e . c o m / l i b r a r y

must be repeated at specificintervals of time. This chainsequence of comparing tosuperior performing certifieddevices is repeated until,finally, specific comparisons aremade with standards main-tained by national authorities,such as the National Institute of Standards and Technology(NIST) in the U.S. This unbro-

ken chain of comparisons isoften called a traceabilitychain.

For a process calibrator,traceability refers to the factthat the process calibrators testand measurement functionshave been verified to performwithin its required specifica-tions, and that this usage of thecalibrator falls within theappropriate limits of perform-ance, including signal levels,environmental conditions,

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90 day 1 year 2 year

U n c e r t a i n t y

( A c c u r a c y

)

Time

tors will usually outperformtheir specifications. Othermanufacturers may manipulatespecifications to make aninstrument appear more capa-ble than it really is. Impressivespecifications touted in adver-tisements or brochures may beincomplete and reflect only asmall part of the total usablecalibrator performance.

Advertisements and productliterature often make liberaluse of footnotes, asterisks, andsuperscripts. In general, thereare two types of footnotesthose that inform and thosethat qualify. Always read allfootnotes carefully and deter-mine which have a direct effecton the specification.

The buyer should also beaware that a calibrator specifi-cation applies to an entireproduct run of a particularinstrument model. For example,the specification for a Fluke702 applies to all 702s made; itdoes not describe the actualperformance of any individual702. Since the variation in theperformance of individual cali-brators from nominal tends tobe normally distributed, a largemajority of the units of aspecific model should perform

well within their specificationlimits. In fact, most individualcalibrators can be expected to

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perform much better than spec-ified, although the performanceof an individual calibratorshould never be taken asrepresentative of the modelclass as a whole.

The calibrator that ispurchased will most likely giveexcellent performance eventhough there is always a smallchance that its performancewill be marginal, or even out of specification, at some parame-

ter or function.Accuracy vs. uncertaintyTypically, the number on thecover of a data sheet orbrochure will read accuracy to0.02 percent. This is commonlyaccepted usage equivalent tosaying measurement uncer-tainty of 0.02 percent. Thismeans that measurementsmade with this device can beexpected to be within 0.02percent of the true value. Inexamining a specification, youneed to be aware that a speci-fication such as this(1) is often over the shortesttime interval, (2) is often overthe smallest temperature span,(3) is sometimes a relativespecification, and (4) may bederived using a non-conserva-tive confidence level. Theimpact of each of these factorsis discussed below.

1. TimeSpecifications usually include aspecific time period duringwhich the calibrator can beexpected to perform as speci-fied. Setting this time period orcalibration interval is necessaryto account for the drift rateinherent in a calibratorsanalog circuitry. This is thecalibration interval, or themeasure of a calibrators abilityto remain within its statedspecification for a certainlength of time. Time periods of 30, 90, 180, and 360 days arecommon and practical. ForFluke 700 Series calibrators,this time period may be eitherone or two years. While thespecifications for Fluke calibra-tors include variations inperformance due to thepassage of time, other manu-facturers calibrators may notbe specified in the samemanner. Figure 1 shows how acalibrators uncertaintyincreases over time. Whenevaluating specifications, makesure youre comparing thesame time intervals.

Any calibrator can be speci-fied to super-high performancelevels at the time of calibration.Unfortunately, such levels aregood for only the first fewminutes following calibration. If the specifications for a calibra-tor do not state the timeinterval over which they arevalid, the manufacturer shouldbe contacted for a clarification.

Understanding specifications Fluke Corporation 3

Key components of aspecificationThe analysis of specificationscan be complicated. To havea clear picture of the truespecifications, you should beaware of the key componentsof a specification, and how to

extract them from all the foot-notes, from the fine print, andfrom the specification itself.Each specification must becarefully considered whencomparing calibrators fromdifferent vendors. The fourmost important componentsof a documenting process cali-brator specification are: time temperature allowance for traceability

to standards

confidence level

Figure 1. Uncertainty as a function of time.

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4 Fluke Corporation Understanding specifications

2. TemperaturePerformance over the specifiedtemperature range is also criti-cal. Make sure the temperatureintervals specified will meetyour workload requirements.

The specified temperaturerange is necessary to account

for the thermal coefficients inthe calibrators analog circuitry.The most common ranges arecentered about room tempera-ture, 23 + 5 C. This rangereflects realistic operatingconditions. It should be remem-bered that temperature boundsmust apply for the entire cali-bration interval. Thus, atemperature range specificationof 23 + 1 C presumes verystrict long-term control of theoperating environment. Such atemperature range would not berepresentative of normal opera-tion for a process calibrator.

Outside the specified range,a temperature coefficient(TC) is used to describe thedegradation of the accuracyspecification. The TC representsan error component that mustbe added to a calibrators spec-ification if it is being usedoutside of its nominal tempera-ture range.

For example, in Figure 2 weare looking at the uncertaintyas a function of temperature atfull scale on the 11 volt dcrange of a Fluke 702 calibrator.The dashed line shows thespecified accuracy for a 23 5 C temperature rangecommon on most Fluke calibra-tors. Within the span of thedashed line, the accuracy iswithin the specifications of 0.030 percent of full scale. Thisis in line with a specificationof 0.025 percent of reading+ 0.005 percent of full scalewhen used at 23 5 C. Thisapplies for a range of 18 C to28 C. Beyond this range, theinstruments performancedegrades as shown by thesolid line. TC will usually begiven in a specification foot-note, and will take the form:

TC = x % / Cwhere x is the amount that theperformance degrades perchange in degree beyond thebase range specification. Tocalculate the accuracy due to

Figure 2. Uncertainty vs temperature, at full scale on the 11 volt dc range.

temperatures outside of thegiven specification, the temper-ature modifier, t mod , is needed.The formula is:

t mod = |TC x t|Where:

t = operatingtemperatureminus the tem-perature rangelimit

t = the proposedoperatingtemperature

range limit = the range limitthat t is beyond

If one wishes to use acalibrator in an ambienttemperature outside of its spec-ified range, the effects of TCmust be added to the baselineaccuracy specification whencalculating the total accuracy.The t mod term is used to calcu-late the total specification usingthe general formula:total spec = (basic accuracy at

a specific temp.range)+ t mod

For example, suppose wehave a calibrator whose ratedaccuracy is 0.030 % @ 23 5 C. Its TC is 0.0025 % / C.To calculate the accuracy of thecalibrator for operation at 32 Cor 90 F:

As can be seen, the specifi-cation may change dramaticallywhen the effects of perform-ance due to temperature areconsidered.

Knowing how to calculatet mod will be necessary whencomparing two instrumentsthat are specified for differenttemperature ranges. For exam-ple, Fluke specifies most of itscalibrators with a range of23 5 C. However, anothermanufacturer may specify acalibrator at 23 1 C. To trulycompare the two calibrators,

one needs to put them in thesame terms (23 5 C) usingthe preceding calculation.

The most modern calibratorsand instruments are specifiedto operate in wider tempera-ture ranges because calibrationinstruments are no longer usedoutside of the closely-controlledlaboratory. Calibration at theprocess plant demands greatertemperature flexibility. Thepreceding equation was usedto characterize the degradationin performance that occurs

when the calibrator is operatedoutside the 23 5 C tempera-ture range restriction on thedata sheet.

Uncertainty(accuracy)

%

.10.10

.075

.05 .05

0 2018 3028 40 50

0

.10

Temperature C

.085

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t = 32range limit = 23 + 5 = 28

t mod = 0.0025 % |32-28|= 0.0025 %|4| = 0.01%

total spec = 0.030 % + 0.010 %= 0.040 %

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Understanding specifications Fluke Corporation 5

3. Allowance for trace-ability to standards

Uncertainty specifications mustalso be evaluated as relative ortotal. Relative uncertainty doesnot include the additionaluncertainty of the referencestandards used to calibrate theinstrument. For example, whena calibrators uncertainty isspecified as relative to calibra-tion standards, this covers onlythe uncertainty in the calibrator.This is an incomplete statementregarding the instruments totaluncertainty. Total uncertaintyincludes all uncertainties in thetraceability chain: the relativeuncertainty of the unit, plus theuncertainty of the equipmentused to calibrate it.

4. Confidence levelThe most critical factor in a cali-brators performance is whatpercentage of the calibrators willbe out of calibration at the end of its calibration interval. Specifica-tions must be conservative toensure the calibrator is in toler-ancewith a high degree of confidenceat the end of its cali-bration interval.

For example, say thatvendors X and Y offer calibra-tors. Vendor Xs specificationsstate that its calibrator cansupply 10 V with an accuracyof 0.019 percent, and vendor

Ys specification is 0.025percent accuracy for the 10 Voutput. Neither of the datasheets for the calibratorssupply a confidence level forthe specifications, nor do theystate how the accuracy isdistributed.

When questioned, thevendors will state that theirspecifications are based on anormal distribution of accuracyand have the following confi-dence levels. Their responsesare tabulated in Table 1.

In this example, as shown inFigure 3, the actual perform-ance of the calibrators isidentical! Vendor X, choosing aconfidence level of 95 percent,

is willing to risk 5 percent of their calibrators being foundout of spec at the end of thestated time interval, and statesa spec of 0.019 percent. Theshaded and solid areas underthe normal distribution curverepresent the fraction of thecalibrator population at risk.Vendor Y, choosing a confi-dence level of 99 percent, iswilling to risk only 1 percent of their calibrators being found out

of spec, and states a spec of 0.025 percent. The solid areasunder the curve represent thefraction of the calibrator popu-lation at risk. So you see,identical calibrator performancecan yield different specifica-tions, depending on howaggressive the calibrator manu-facturer chooses to be with thespecifications of your calibrator.

Before making a purchase, itis critical to gain an understand-ing of a vendors philosophy withrespect to confidence level and

ask the vendor to clarify theconfidence level when there is

doubt as to what it is. Flukeuses a very conservative 99percent confidence level for itsspecifications for calibratorsand standards.

Other considerationsAccuracy specifications are animportant part of determiningwhether or not a particularcalibrator will satisfy a need.There are, however, manyother factors that determinewhich calibrator is best suitedfor an application, some of which are described below.

The workloadRemember that the calibratorsspecification must match yourworkload requirements. Thereis a tendency for manufacturersto engage in a numbers race,with each new calibratorhaving more and more impres-sive specifications, althoughoften this has little bearing on

true workload coverage.

0.00

0.10

0.20

0.30

0.40

0.03 0.02 0.01 0.00 0.01 0.02 0.03

Vendor X95 %

0.019 %

Accuracy(%)

Vendor Y 99 %

0.025 %

Figure 3. Same performance, different specifications.

Vendor Stated spec Confidence level@10 V (coverage factor)

X 0.019 % 95 % Y 0.025 % 99 %

Table 1. Specification comparison

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Fluke. Keeping your world up and running.

Fluke CorporationPO Box 9090, Everett, WA USA 98206Fluke Europe B .V.PO Box 1186, 5602 BDEindhoven, The NetherlandsFor more information call:In the U.S.A. (800) 443-5853 orFax (425) 446-5116In Europe/M-East/Africa (31 40) 2 675 200 orFax (31 40) 2 675 222In Canada (800)-36-FLUKE orFax (905) 890-6866From other countries +1 (425) 446-5500 orFax +1 (425) 446-5116

Web access: http://www.fluke.com/2002 Fluke Corporation. All rights reserved.Printed in U.S .A. 5/2002 1261947 A-ENG-N Rev C

Resolution the smallestdiscernible amount that can beeither measured or generatedwithin a specific functionsrange. In a calibrator, it oftenrefers to either the smallestincremental change that can be

made in a signal sourcing func-tion, or the smallest measurablechange in a signal measure-ment function.Test uncertainty ratio (TUR) The test uncertainty ratio fora calibration point is the speci-fied uncertainty of theinstrument under test dividedby the specified uncertainty of the calibrator or standard usedto test it. The specifications forthe instruments must reflect thesame confidence level.

Traceability a characteristicof a calibration, analogous to apedigree. A traceable calibra-tion is achieved when each testinstrument, calibrator, andstandard, in a hierarchystretching back to the nationalstandard, is itself properly cali-brated, and the results properlydocumented. The documenta-tion provides the informationneeded to show that all thecalibrations in the chain of cali-brations were properlyperformed.

This application note hasbeen adapted from Chapter31: Instrument Specifications,in Calibration: Philosophy in

Practice, Second Edition , FlukeCorporation 1994.

For more information aboutthis useful, comprehensive text-book, call 1-800-FLUKE in theU.S. or contact your local Flukerepresentative.

Support standardsThe support standard will typi-cally be three to ten timesmore accurate than the calibra-tors supported. This is knownas the test uncertainty ratio(TUR). Specialized calibrators orthose that require exotic supportequipment on an infrequentbasis may best be served by anoutside service bureau.

Manufacturer supportManufacturer support is alsoimportant. Can the manufacturerprovide support as calibrationneeds grow and vary? Are in-house experts available to assistwith technical issues? Are train-ing programs available? Areservice facilities convenientlylocated? Is there an adequateline of support products andaccessories?

ReliabilityReliability is another importantconsideration in how useful acalibrator will be. Precisionelectronic calibrators can havea seemingly high failure rate.Any condition that causes thecalibrator to fall outside of itsextremely tight toleranceconstitutes a failure. One should

ask for a Mean-Time-To-Fail(MTTF) rate to determine whenthe first failure might occur.Failures upon delivery usuallymake this interval shorter thanthe Mean-Time-Between-Fail-ure (MTBF) rate. Whichever isquoted, consider whether thenumber is based on actual fieldexperience or just calculatedprojections.

Service philosophyWhen a calibrator does fail, the

manufacturers approach toservice is critical. A responsiveservice organization is essentialto getting equipment back inaction fast. Issues to considerinclude ones proximity to serv-ice center locations, stockinglevels for spare parts andsubassemblies, availability of service manuals and servicetraining for ones own techni-cians, all of which go intodetermining how soon equip-ment can be returned toservice.

ReputationFinally, the manufacturersreputation should be assessed.Overall, how credible are itsclaims with respect to perform-ance, reliability, and service?Will the company still be inbusiness five years from now?All of these issues define thetrue cost of owning and using acalibrator.

TerminologyAccuracy how close themeasurement of a parameter isto the true value of that param-eter. Often expressed as apercentage of the reading or asa percent of full scale.Confidence level thepercentage of the area of thenormal curve that lies withinthe confidence interval. Aconfidence level of 95 percentis obtained when the range of confidence is from minus twostandard deviations to plus twostandard deviations. Flukeusually uses a confidence levelof 99 percent or greater whenspecifying test instruments.Hysteresis the maximumdifference between outputreadings for the same measur-and point, one point obtainedwhile increasing from zero andthe other while decreasing fromfull scale. The points are takenon the same continuous cycle.The deviation is expressed as apercent of full scale.NIST(National Institute of Standards and Technology):the U.S. national standardslaboratory, which is legallyresponsible for maintaining thestandards on which all U.S.measurements are based.Noise a disturbance, caused

by random changes in voltageand current, which interfereswith the measurement of asignal.Nonlinearity the deviationfrom standard line output plot-ted against linear input.Repeatability the ability of acalibrator to output the samereadings given the same condi-tions of temperature, humidity,time, atmospheric pressure, etc.

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