Understanding & Comparing Spec

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Understanding andcomparing instrument

specifcations Application Note

The initial selection o pro-spective equipment is usuallybased on its written specifca-tions. They provide a meansor determining the equip-ment’s suitability or a particularapplication. Specifcations area written description o theinstrument’s perormance inquantifable terms and apply tothe population o instrumentshaving the same model number.

Since specifcations are basedon the perormance statisticso a sufciently large sample o instruments, they describe groupbehavior rather than that or asingle, specifc instrument.

Good specifcations have theollowing characteristics:• complete or anticipated appli-

cations easy to interpret anduse

• defne the eects o environ-ment and loading

Completeness requires thatenough inormation be providedto permit the user to determinethe bounds o perormance

or all anticipated outputs (orinputs), or all possible envi-ronmental conditions withinthe listed bounds, and or allpermissible loads. It is not atrivial task even or a simplestandard. Designing completespecifcations or a complexinstrument like a multiunctionor multi-product calibrator is abig challenge.

Ease o use is also important.

A large array o specifcations,including modiying qualif-cations and adders, can beconusing and difcult to inter-pret. Mistakes in interpretationscan lead to application errors oraulty calibrations.

The requirement or com-pleteness conicts somewhatwith that or ease o use; onecan be traded or the other.The challenge o specifcationdesign is to mutually satisy bothrequirements.

Specifications for precision instruments are often complex and difficult to interpret. Users should consider easily overlooked details in order toproperly compare these instruments or apply them to their workload.

An instrument’s specifcations must match the workload requirements. For example, the 5522A Multi-Product Calibrator was designed to calibrate a broad range o electrical test tools, rom handhelds to RF voltmeters to system DMMs.

This is sometimes satis-fed by bundling the eects o many error contributions withina window o operation. Forexample, the listed perormancemay be valid or six months in atemperature range o 23 ± 5 °C,or rh up to 80 %, or power linevoltage o nominal ± 10 %, andor all loads up to the maximumrated current. This is a greatsimplifcation or the user since

the error contributions o time,temperature, humidity, powerline uctuations and loads canbe ignored as long as operationis maintained within the listedbounds.

Since specifcations are com-plex or complex instruments,they can be misused and abused both by users and manuactur-ers. Reputable manuacturerswill attempt to describe theperormance o the product asaccurately and simply as pos-sible without hiding areas o poor perormance by omission o relevant specifcations.



2 Fluke Corporation Understanding and Comparing Instrument Speciications

Much o the ollowing mate-rial describes the eects o incomplete specifcations, andhow specifcations can be misin-

terpreted or abused.The various specifcations allinto three distinct categories;these will be defned later. Eachspecifcation must be careullyconsidered when comparinginstruments rom dierent ven-dors. All o these componentscombine to describe the peror-mance o a given instrument.Purchasers need to be profcientat adding the contribution o each component to arrive at theactual instrument perormanceor any given output. This skillwill lead to aster and moreintelligent purchasing decisions.

Analyzing specications

The analysis o specifcationsas part o the decision makingprocess can be complex. To havea clear picture o the true speci-fcations, laboratory managers,metrologists, and techniciansshould all be aware o all thecomponents o a specifcation,

and how to extract them rom allthe ootnotes, rom the fne print,and rom the specifcation itsel.

Advertisements are notori-ous or their use o ootnotes,asterisks, and superscripts. Thefne print can turn a data sheetinto a reader’s nightmare. Ingeneral, there are two types o ootnotes: those that inorm andthose that qualiy. Always lookover all ootnotes careully anddetermine which have a direct

eect on the specifcation.Once the main componentso the specifcation are identi-fed, one must interpret andcalculate the true specifcations.Only ater the total specifca-tion has been described will theuser have a clear understandingo the real instrument specif-cation. This will help to avoidconusing purchasing decisions

with extraneous actors. As willbe shown, the listed specifca-tions ound in advertisementsand brochures are oten not

even hal o the total usablespecifcation.

Interpreting specicationsConsider how a company goesabout evaluating electronicinstruments or potential pur-chase. Many companies havecomplex procedures and teststhat an instrument must passprior to purchase and accep-tance. But beore that evaluationcan begin, one must decidewhich o the many instru-

ments on the market should beevaluated. The instrument speci-fcations are usually the frst stepin the process. The specifcationsmust meet the workload require-ments i the instrument is to beconsidered.

Ideally, specifcations area written description o aninstru-ment’s perormance thatobjectively quantifes its capa-bilities. It should be rememberedthat specifcations do not equal

the perormance, they are per-ormance parameters. They canbe conservative or aggressive.Manuacturers are not bound byany convention as to how theypresent specifcations. Some willspeciy their products conser-vatively. Such instruments willusually outperorm their speci-fcations. Other manuacturersmay manipulate the specifca-tions to make an instrumentappear more capable than itreally is. Impressive specifca-tions touted in advertisementsor brochures may be incomplete.They oten reect only a smallpart o the total usable instru-ment perormance.

The buyer should also beaware that an instrumentspecifcation applies to an entireproduction run o a particularinstrument model. Specifcations

are probabilities or a group,not certainties or an item. Forexample, the specifcations or aFluke 5500A Multi-Product Cali-

brator apply to all the 5500Asmade. The specifcation does notdescribe the actual perormanceon any individual 5500A. Instru-ment specifcations are chosenso that a large percentage o allinstruments manuactured willperorm as specifed. Since thevariation in the perormanceo individual instruments romnominal tends to be normallydistributed, a large majority o the units o a specifc modelshould perorm well with theirspecifcation limits. Thereare two applications o thisinormation:• Most individual instruments

can be expected to perormmuch better than specifed.

• The perormance o anindividual instrument shouldnever be taken as representa-tive o the model class as awhole.

Consequently, the instrument

purchased will most likely giveexcellent perormance eventhough there is always a smallchance that its perormancewill be marginal or even out o specifcation, at some parameteror unction.

To compare products romdierent manuacturers, one willoten have to translate, interpret,and interpolate the inormationso that one can make a logicalcomparison. Look again at thedefnition o an ideal specifca-

tion: “a written description o an instrument’s perormancethat objectively quantifes itscapabilities.” But what aspectso perormance are described?Objective to whom? A manu-acturer cannot include everypossible specifcation, so what isincluded and what isn’t?




CondenceThe most critical actor in acalibrator’s perormance ishow small the deviation o its

actual output is rom its nominaloutput. On the one hand, onewill always have a measure o uncertainty as to the magnitudeo the deviation. On the otherhand, one can have confdencethat the deviation is unlikely tobe greater than a determinedamount. Moreover, confdencecan be numerically stated interms o the confdence level orthe specifcation.

For example, say that ven-dors X, Y, and Z oer calibrators.Vendor X’s specifcations statethat its calibrator can supply10 V with an uncertainty o 1.55 ppm. Vendor Y’s calibra-tor is specifed to provide 10 Vwith 3.03 ppm uncertainty, andvendor Z’s specifcation is 4 ppmuncertainty or the 10 V output.None o the data sheets or thecalibrators supply a confdencelevel or the specifcations, nordo they state how the uncer-tainty is distributed.

When questioned, vendorswill state that their specifca-tions are based on a normaldistribution o uncertainty andhave the ollowing confdencelevels (or coverage actor, k).Their responses are tabulated inTable 1.

Table 1. Specication comparison

VendorStated Spec

@ 10 VCondence Level(coverage factor)

X 1.55 ppm 1 Standard Deviation90 % (k = 1)

Y 3.03 ppm 95 % (k = 2)

Z 4 ppm 99 % (k = 2.58)

As Figure 1 shows, the threecalibrators can be expectedto have identical perormancebecause their confdenceintervals apply to dierentareas under the same curveor the normal distribution o uncertainty.

In the example above, VendorX, with a confdence level o 90 %, or 1 standard deviation,is acknowledging that approxi-

mately 10% o the calibrators itproduces may not meet the 10 Vspecifcation o 1.55 ppm.

Fluke uses a 99 % confdencelevel or its specifcations orcalibrators and standards. Asa buyer, one should be alertto the issues surrounding theconfdence level or a specifca-tion and should ask the vendorto clariy the confdence levelwhen there is doubt as to whatit is.

Beware of the word“accuracy”Typically, the number on thecover o a data sheet or brochurewill read “Accuracy to xx ppm.” What it really means is “xx ppmuncertainty.” This is the result o imprecise language: substituting“accuracy” when “uncertainty” isintended. One also needs to beaware that a specifcation suchas this is oten over the shortesttime interval, the smallest tem-

perature span, and is sometimesa relative specifcation.

Components of aspecication

Determining an instrument’strue specifcations requires thatall important specifcations becombined. For this discussion,the uncertainty terms compris-ing calibrator specifcations aredivided into three groups: base-line, modifers, and qualifers.• Baseline specifcations

describe the basic peror-mance o the instrument.

• Modifers change the baselinespecifcations, usually to allowor the eects o environmen-tal conditions.

• Qualifers usually defneoperating limits, operatingcharacteristics, and compli-ance with industrial or militarystandards.

P r o b a b i l i t y

D e n s i t y

-6.2 -4.65 -3.1 -1.55 0.0 1.55 3.1 4.65 6.2

-4 -3 -2 -1 0.0 1 2 3 4

-6 .06 -4 .545 -3.0 3 -1.51 5 0.0 1.51 5 3 .03 4.54 5 6.0 6

Vendor ‘X’(PPM)

Vendor ‘Y’(PPM)

Standard Deviations (Coverage Factor)

0.00

0.10

0.20

0.30

0.40

-6.20 -4.65 -3.1 -1.55 0.0 1.55 3.1 4.65 6.20

Vendor ‘Z’(PPM)

4.0-4.0

Figure 1. Same perormance, dierent specifcations.

The ollowing paragraphsdescribe each in detail.

Baseline specications

Baseline specifcations consisto three components. They arelisted here along with the nameused to represent them in thisapplication note:• output

• scale

• oor

The general orm or a baselinespecifcation can be expressedas: ± (output + scale + oor) Where:output = percent or parts per

million (ppm) o theoutput or setting

scale = percent or ppm o therange, or ull-scale

value o the range,rom which the outputis supplied (sometimes given as a fxedvalue in units or asdigits)

oor = a fxed value in units

One or more o these terms will

be present in the specifcation,though not all specifcations willinclude all the terms.




1C

1B

1A

1000

100

10

1

0.01 0.1 1 10 100 1000

XXX CALIBRATOR

DIRECT VOLTAGE

U N C E R T A I N T Y O

F

O U T P U T V O L T A G E I N P P M

Figure 2. Baseline specifcations.

The output term o the base-line specifcation is oten quotedcasually to describe the instru-ment’s capabilities. But this

output uncertainty term aloneshould not be used to technicallyevaluate the unit’s actual peror-mance. The other two terms canmake a signifcant contribution tothe total uncertainty. The ollow-ing discussion shows how thethree terms are combined. Reerto the calibrator specifcationgraph in Figure 2.

The output term

Line 1 A is output—the uncer-tainty o the basic output signal.

This 50 ppm uncertainty termshows up as a horizontal line.It is a constant percentage o output or all settings o thecalibrator.

The scale term

Line 1B in Figure 2 includesscale. The scale term is theuncertainty associated with theoutput range. Whether specifedas a percentage o ull scale, oras a percentage o range in unitsor digits, scale describes the

fxed uncertainty or a specifcrange. This means that althoughthe magnitude o the scale termis the same or all outputs acrossthe range, the eect it has at thelow end o the range is greater.At the low end it represents alarger portion o the output thanit does at ull scale.

Note that Line 1B takes ona saw-tooth orm. The peakso the sawtooth are where thenext-highest range is selected.This profle is caused by scalebeing a larger percentage o a ractional scale output. Forexample, on the 20 V range, the10 ppm uncertainty o the ull-scale 20 V value is 200 mV. Thisuncertainty applies to any outputon the 20 V range whether it is3.0 V or 19.93 V.

Two steps are perormed hereto translate the scale uncertaintyterm to a specifc percent-age o any given output value.First, scale was converted to

microvolts, using the ull-scaleoutput level. This magnitude wasthen converted to ppm at eachspecifc output value.

The oor termIn addition to a calibrator’suncertainty relative to its outputand scale, there can be addi-tional uncertainty o a fxedmagnitude. This is typicallyan oset, but can also includenoise. This added uncertaintyis called the oor, and, or anygiven range, it is constant acrossthat entire range. For example,oten a calibrator specifcationwill include a description such

as, “with a oor o 5 mV.” This isthe oor specifcation. Becausethis constant error is a greaterpercentage o small outputsthan o large outputs, it becomesmore important as the magni-tude o the output decreases.

Line 1C in Figure 2 includesoor. This term can vary byrange or can be a constantacross all ranges. Floor uncer-tainty is similar to scale and maybe combined with the scale termor simplicity. Adding this termto the baseline uncertainty plotshown in Figure 2 raises thecurve even arther, particularlyon the lower end: the constantvoltage uncertainty is a largepercentage o the total outputor smaller voltages. In a moreextreme example, a 5 µV ooramounts to an additional 5 ppmuncertainty at 1 V, whereas 5 µVis only an additional 0.025 ppmuncertainty at 200 V output.

The true baseline calculation

Comparing Line 1C with Line1A shows that the total baselineuncertainty specifcation is quitedierent rom the basic outputuncertainty specifcation. Yet thetotal uncertainty specifcationmust be used in determin-ing whether the calibrator willactually fll your needs. Thistotal uncertainty specifcationmust support your application orworkload even at the low end o each range.

The comparison o Line 1Ato 1C is not as bad as a quickreading o this discussion mayfrst cause one to believe. This isbecause we are using voltmetersand calibrators as examples. Thesawtooth calibration specifcationin Line 1C is dierent rom 1A,but a voltmeter behaves in thesame way and its specifcationswould reveal much the samepattern so the calibrator could beused to support it at nearly thesame Test Uncertainty Ratio (TUR)or all values.

The next questions to beanswered are: What do the

numbers in the baseline speci-fcation include? Under whatconditions do these specifcationsapply? This is where the modiferand qualifer specifcations arebrought in.

Modier specicationsModifer specifcations changethe baseline specifcations. Mostmanuacturers include modifersas part o the baseline specifca-tion so that one doesn’t need tocalculate these, though not all do

this. The most important modi-fers include instrument stabilityover time, the time interval orwhich the specifcations are valid,and changes in specifcationswith changes in temperature(the ambient temperature o theinstrument’s operating environ-ment rather than its internaltemperature). Other secondaryspecifcations include the eect o inductive and capacitive loading,no load versus ull load operation,

and any changes due to changesin line power.




Modifer specifcations areimportant because the appli-cation may require particularaspects o perormance to be

detailed. Figure 3 shows the ploto the secondary specifcationsor the direct voltage output o acalibrator.

Line 3A is a plot o the basicoutput uncertainty stated in themanuacturer’s data sheet. Aspreviously mentioned, this is thenumber that is oten casuallyquoted to describe the calibra-tor’s perormance. Line 3B is thetotal baseline uncertainty (output+ scale + oor) or 24 hours,at an operating temperature o 23 ± 2 °C at no load. Now theeects o the modifer specifca-tions will be added.

The time term

Specifcations usually includea specifc time period duringwhich the instrument can beexpected to perorm as specifed.This is the calibration interval, orthe measure o an instru-ment’sability to remain within its statedspecifcation or a certain lengtho time.

Limiting this time period orcalibration interval is neces-sary to account or the drit rateinherent in an instrument’sanalog circuitry. Time periodso 30, 90, 180, and 360 daysare common and practical. Anycalibrator can be specifed tosuper-high perormance levels atthe time o calibration. Unor-tunately, such levels are goodor only the frst ew minutesollowing calibration. Any instru-

ment may drit beyond realisticperormance levels. I the speci-fcations or an instrument donot state the time interval overwhich they are valid, a stabil-ity actor corresponding to thedesired time interval will have tobe added.

Some vendors speciy thetime term as a unction o thesquare root o the years. Recentwork at Fluke indicates that the

square root o the years methodo speciying an instrumentmay not be appropriate. Flukeexperiments indicate that the

long term drit o well-aged volt-age and resistance standards isnearly linear with time. However there can be short term excur-sions rom a linear regressiondue to the superposition o lowrequency noise on the basicdrit. For short periods o time,the accumulated linear drit issmall in comparison to the noiseexcursions, so a short term, e.g.90 days, uncertainty specifca-tion predominantly reects theuncertainty due to low requencynoise. However, the long term,e.g. 1 year specifcation, includes the eects o accumulated lineardrit.

The calculation o the timeterm due to calibration inter-val is based on the root/years’square rule. Simplifed, it takesthe orm: rys = y√i Where:rys = root/year specifcation

y = 1 year specifcation

i = desired calibrationinterval in years

As the equation suggests, aninstrument’s 1 year specifca-tion is published by its vendor.Specifcations or other intervalsare user computed by multiply-ing the 1 year specifcations bythe square root o the numbero years or which they areintended to apply. For example, aperiod o 90 days is about 0.25years so the square root o 0.25(0.5) is used to multiply the 1year specifcations to obtain the90 day specifcations.

While the specifcations orFluke calibrators include varia-tion in perormance due to thepassage o time, other calibratorsmay not be specifed in the samemanner. In this case the speci-fcation or their stability withtime must be added as line 3Cin Figure 3.

The temperature term

Equally important is perormanceover the specifed temperaturerange. This range is necessary toaccount or the thermal coef-cients in the instrument’s analogcircuitry. The most commonranges are centered about roomtemperature, 23 ± 5 °C Thisrange reects realistic operatingconditions. It should be remem-bered that temperature boundsmust apply or the whole calibra-tion interval. Thus a temperaturerange specifcation o 23 ± 1°C presumes very strict long-term control o the operating

environment.Outside the specifed range,

a temperature coefcient (TC) isused to describe the degradationo the uncertainty specifcation.The temperature coefcient isan error value which must beadded to an instrument’s base-line specifcation i it is beingused outside o its nominaltemperature range. Look at thetemperature coefcient graphshown in Figure 4.

1000

100

10

1

0.01 0.1 1 10 100 1000

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U N C E R T A I N T Y O

F

O U T P U T V O L T A G E I N

P P M

3F

3E

3D3C3B

3A

Figure 3. Secondary specifcations.

Figure 4. Uncertainty due to temperature.

TEMPERATURE, DEGREES C

E R R O R ,

P P M

0 10 20 30 40 50

0

5

10

15

20

4 A




The x-axis is temperatureand the y-axis is uncertainty.The dashed portion o Line 4 Ashows the specifed uncertainty

or a 23 ± 5 °C temperaturerange common on most Flukecalibrators. Within the span o the dashed line, the uncertaintyis within the specifcations o 5 ppm. This is in line with abaseline specifcation o “± 5ppm when used at 23 ± 5 °C.”This is an absolute range o 18 °C to 28 °C Beyond thisrange, the instrument’s per-ormance degrades as shownby the solid TC line. TC willusually be given in a specifca-tion ootnote, and will take theorm: TC = x ppm ⁄ °C where xis the amount that the peror-mance degrades per change indegree beyond the base rangespecifcation. To calculate theuncertainty due to temperaturesoutside o the given specifca-tion, the temperature modifer,tmod, is needed.

The ormula is: tmod = |TC x ∆t| Where:∆t = operating temperature

minus the temperaturerange limit

t = the proposedoperating temperature

range = the range limit that tlimit is beyond

I one wishes to use a calibratorin an ambient temperature out-side o its specifed range, theeects o TC must be added tothe baseline uncertainty specif-cation when calculating the total

uncertainty. The tmod term is usedto calculate the total specifca-tion using the general ormula:total spec = (basic uncertainty ata specifc temp. range)+tmod

For example, suppose wehave a calibrator whose rateduncertainty is 8 ppm @ 23± 2 °C. Its TC is 3 ppm/C°. To

calculate the uncertainty o thecalibrator or operation at 30 °C:t = 30

range = 23+2 = 25

limittmod = 3 ppm|30-25| =

3 ppm|5| = 15 ppm

total = 8 ppm + 15 ppm =spec 23 ppm

As can be seen, the specifcationchanges dramatically when theeects o perormance due totemperature are considered.

Knowing how to calculate tmod

will be necessary when compar-ing two instruments that are

specifed or dierent tempera-ture ranges. For example, Flukespecifes most o its calibratorswith a range o 23 ± 5 °C How-ever, another manuacturer mayspeciy a calibrator at 23 ± 1 °CTo truly compare the two cali-brators, one needs to put themin the same terms (23 ± 5 °C)using the preceding calculation.

The most modern calibratorsand instruments are specifedto operate in wider tempera-ture ranges. This is because

calibration instruments are nolonger used only in the closely-controlled laboratory. Calibrationon the production oor demandsgreater temperature exibility.The preceding equation wasused to characterize the deg-radation in perormance thatoccurs when the calibrator isoperated outside the 23 ± 2 °Ctemperature range restriction onthe data sheet. The result is Line3D in Figure 3.

Power line and output load terms

In the modifer example, anotherootnote lists an uncertaintymodifer or an assumed linevoltage variation o ± 10 % o rated uncertainty. Adding thisresults in Line 3E in Figure 3.Finally, one last ootnote coversan uncertainty modifer or loadregulation. This term must beincluded in the total uncertaintycalculation because loading the

calibrator is necessary or use.

Adding the uncertainty or 0 %to 100 % load to Line 3E yieldsLine 3F in Figure 3.

The modied baselinespecicationNotice how the total uncertaintychanges when adding terms o calibration interval, temperature,and line and load regula-tion. Comparing Line 3A withLine 3F reveals that the actualuncertainty under operating con-ditions is 10 to 100 ppm worsethan the specifed 6 ppm. Onemust careully review all o thespecifcations to assess whetheran instrument will really meet

the requirements. Again, theinstrument must satisy theapplication even at the low endo each range where the specif-cation deteriorates.

Qualier specicationsThe uncertainty terms thatcan be considered qualiferspecifcations rarely aect thegeneral specifcations or analogperormance, but they may beimportant to the application.These are perormance limits

and operating characteris-tics that include emi eects,humidity, and altitude. FCCand military standards suchas MIL-T-28800C cover emiemissions and some aspectso emi susceptibility, but theremay still be complications. Forexample, emi rom a nearbycrt could adversely aect aprecision instrument lackingadequate shielding. Compliancewith saety standards such as

UL, CSA, VDE, and IEC may alsobe important in your applica-tion. These must be kept in mindwhen comparing instruments.

Relative versus total specications

Uncertainty specifcations mustalso be evaluated as relativeor total. Relative uncertaintydoes not include the additionaluncertainty o the reerencestandards used to calibrate theinstrument. For example, when

a calibrator’s uncertainty is




specifed as relative to calibra-tion standards, this covers onlythe uncertainty in the calibrator.This is an incomplete statement

regarding the instrument’s totaluncertainty. Total uncertaintyincludes all uncertainties in thetraceability chain: the relativeuncertainty o the unit, plus theuncertainty o the equipmentused to calibrate it. This is thetrue specifcation o availableinstrument perormance.

A standards laboratory canprovide the uncertainties in theircalibration standards. Theseuncertainties must be combinedwith the specifcations relativeto calibration standards to deter-mine the perormance which isactually achieved. In general,Fluke’s practice is to state themore useul total specifcation,which assumes calibration wasin accordance with the instru-ment operator’s manual.

The units of the specication

Using dierent units also addsconusion. Output, scale, andrange can be quoted in percent-ages or parts per million. Scale,range, and oor can also appearas digits or volts. It may be nec-essary to interpolate betweenunits. The general ormulas toconvert rom % to ppm and backare:ppm = % x 104

% = ppm ⁄ 104

Table 2 shows the relation-ship o some commonly-usedppm and % values.

Table 2. The relationship of PPM to percent

Percent PPM

1 10,000

0.1 1,000

0.01 100

0.001 10

As is shown later, a good wayto compare specifcations orcalibrators and DMMs is byconverting all o the % and ppm

terms to microvolts frst. Theormulas are:specmv = specppm (output in

volts ⁄ 10-6)and

specmv = spec% (output involts ⁄ 10-4)

Comparing specications:A detailed example

Now it’s time to compare thespecifcations or two dier-ent models o calibrator. Tables

3 and 4 depict two typicalspecifcation tables ound in theliterature or modern calibrators.Baseline specifcations or Cali-brator B appear to be our timesbetter than or Calibrator A butare they really?

Table 3. Calibrator A

Baseline Specication1 ± 4 ppm of output± 2 µV

Uncertainty o standards ± 1 ppm o value

Calibration interval 90 days

Temperature range 18 °C to 28 °C

Line regulation n/aLoad regulation n/a

Stability n/a

Range 22 V

Full scale 21.999999 V

Confdence level 99 %

1Baseline specifcations include the uncertainty o standards, temperature coefcient or the temperaturerange, the eects o line and load regulation as wellas stability during the calibration interval.

Table 4. Calibrator B

BaselineSpecication1

± 1 ppm of output± 0.1 ppm of full scale

Uncertainty o

standards

± 1 ppm o output

Calibration interval 24 hours

Temperature range2 23 °C ± 1 °C

Line regulation 0.1 ppm o output

Load regulation 0.2 ppm o output

Stability ± (1.5 ppm o output +0.05 ppm o ull scale)or 90 days

Range 10 V

Full scale 19.999999 V

Confdence level 95 %

1Relative to calibration standards2TC = 0.4 ppm⁄°C

Comparing these two calibra-tors requires:• Identiying the specifcations

that need to be converted

• Converting applicable specif-cations to microvolts

• Adding the microvolt values

• Adjusting uncertainty orstated confdence interval

Take a look at the process o getting the results.

Identifying the items to beconvertedList all o the relevant specifca-tions or both calibrators thatapply to the situation at hand.Here, it is the comparison o thetotal uncertainty o both calibra-tors at an output o 19.99999 V.Table 5 is a result o the listing.For calibrator A, the baselinespecifcation and the rangeoor are the only items to beconverted to microvolts. All o the other baseline modifersare included in the baselinespecifcation.

Calibrator B needs severalmore conversions, because

all o its baseline componentsand modifer specifcations arelisted individually. Along withthe baseline specifcation andrange oor, the standard uncer-tainty, calibration interval, lineregulation, load regulation, andstability must all be converted tothe appropriate terms. Calibra-tor B has a maximum range o 19.99999 V. This is eectively20 V, so one would use that orthe output value.

Also, although both calibra-tors have ootnotes stating aTC o 0.4 ppm/°C, Calibrator Ais exempt rom including anytemperature modifer becauseits specifcations encompass




the widest range. Calibrator B’sspecifcation will have to bemodifed or TC. See Table 5.

Converting thespecicationsConvert the baseline specif-cations or both calibrators tomicrovolts, then add in the spec-ifcations that aren’t includedin the baseline specifcations.This will give the total specifeduncertainty or both calibra-tors at 20 V The results o theconversion and calculation areshown in Table 6.

For the beneft o the doubt,

this test assumes a high-imped-ance DMM load. Even thoughCalibrator B’s load uncertainty is0.2 ppm o output, its load modi-fer is 0.0 mV. Low impedanceloads could cause this entry torise to 40 mV at ull load current.

Applying the condenceintervalIn this adjustment, the ratio o the desired to specifed conf-dence interval is used to multiplythe total specifed uncertainty

o each calibrator. This opera-tion yields specifcations thathave the same confdence level.The ratios o the confdenceintervals are used as multipliers

because it is assumed that thespecifcations reect a normaldistribution o uncertainty. Inother instances, the t actor is

used as a multiplier.For these purposes, it isenough to know the ollowing:99 % confdence level = 2.58confdence interval95 % confdence level = 1.96confdence interval

From this, one can fndthat the multiplier ratios are2.58/2.58 = 1.00 or Calibra-tor A, and 2.58/1.96 = 1.32 orCalibrator B.

Now each total specifeduncertainty rom the preced-ing Table 6 is multiplied by themultiplier ratios. Using the ratioo desired to specifed interval,the specifcations or Calibra-tors A and B are adjusted orthe desired (higher) confdencelevel, 99 %. The results areshown in Table 7.

In the original specifcation,Calibrator B appeared to be ourtimes as accurate as Calibrator A.But with all o the additions andcorrections needed to interpret

Calibrator B’s specifcations, itturns out that Calibrator B isabout 72 % less accurate thanCalibrator A. That is, its uncer-tainty is 72 % greater underidentical conditions.

Table 5. Pre-conversion tabulation

Specs Calibrator A Calibrator B

Output 4 ppm o 20 V 1 ppm o 20 V

Floor 2 µV 0.1 ppm o 20 V

Standard uncertainty included 1 ppm o 20 V

90-day stability included 1.5 ppm o 20 V+.05 ppm o 20 V

TC included 4 ° x .4 ppm/°C x 20 V

Line included 0.1 ppm o 20 V

Load included 0.2 ppm o 20 V

Confdence level 99 % 95 %

Table 6. Microvolt equivalents and totals

Specs Calibrator A Calibrator B

Output 80 µV 20 µV

Floor 2 µV 2 µV

Standard uncertainty included 20 µV

90-day stability included 30 µV + 1 µVTC included 32 µV

Line included 2 µV

Load included 0.0 µV

Total 82 µV 107 µV

Table 7. Results of comparison

Calibrator A B

Uncertainty romtable 6

82 µV 107 µV

Multiplier ratio 1.00 1.32

Final uncertainty 82 µV 141 µV

ppm o output 4.1 7.0




Other considerations

Uncertainty specifcations are an important part o determining whether

or not a particular calibrator will satisy the need. There are, however,many other actors that determine which instrument is best suited or an

application. Some o these are listed as ollows.

The workload: Remember that the instrument’s specifcation must

match the workload requirements. There is a tendency or manuactur-

ers to engage in a numbers race, with each new instrument having

more and more impressive specifcations, although oten this has little

bearing on true workload coverage.

Support standards: The support standards will typically be three to

ten times more accurate than the instruments supported. This is known

as the test uncertainty ratio (TUR). Specialized instruments or those that

require exotic support equipment on an inrequent basis may best be

served by an outside service bureau.

Manufacturer support: One should also consider the level o manu-

acturer support. Can the manuacturer provide support as calibration

needs grow and vary? Do they have in-house experts who can assist

with technical issues? Are training programs available? Are service

acilities conveniently located? Do they have an adequate line o sup-

port products and accessories?

Reliability: Reliability is another important consideration in how useul

an instrument will be. Precision electronic instruments can have a

seemingly high ailure rate. Any condition that causes the instrument

Fluke CalibrationPO Box 9090,Everett, WA 98206 U.S.A.

Fluke Europe B.V.PO Box 1186, 5602 BDEindhoven, The Netherlands

For more information call:In the U.S.A. (877) 355-3225 or Fax (425) 446-5116In Europe/M-East/Arica +31 (0) 40 2675 200 or Fax +31 (0) 40 2675 222In Canada (800)-36-FLUKE or Fax (905) 890-6866From other countries +1 (425) 446-5500 or Fax +1 (425) 446-5116

Web access: http://www.luke.com

©2001-2011 Fluke Corporation. Specifcations subject to change without notice.Printed in U.S.A. 1/2011 1260494C A-EN-N

Modification of this document is not permitted without written permission from Fluke Corporation.

Fluke Calibration. Precision, performance, confidence.™

to all outside o its extremely tight tolerance constitutes a ailure. One

should ask or a Mean Time To Fail rate (MTTF) to determine when thefrst ailure might occur. Failures upon delivery usually make this inter-

val shorter than the Mean Time Between Failure rate (MTBF). Whichever

is quoted, consider whether the number is based on actual feld experi-

ence or just calculated projections.

Service philosophy: When an instrument does ail, the manuacturer’s

approach to service becomes very important. A responsive service

organization is essential to getting equipment back in action ast. Issues

to consider include service center locations and one’s proximity to them,

stocking levels or spare parts and subassemblies, availability o service

manuals, and service training or one’s own technicians, all o which go

into determining how soon equipment can be returned to service.

Reputation: Finally, the manuacturer’s reputation should be assessed.

Overall, how credible are its claims with respect to perormance, reli-ability, and service? Will the company still be around fve years rom

now? All o these issues defne the true cost o owning and using a

calibrator.

This application note has been adapted rom “Chapter 31: Instrument

Specifcations,” in Calibration: Philosophy in Practice, Second Edition.

For more inormation about this useul, comprehensive textbook, call

877-355-3225 (U.S.) or contact your local Fluke representative.

Documents

Understanding & Comparing Spec