transmille training - uncertainties.ppt

8/18/2019 Transmille Training - Uncertainties.ppt

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UncertaintiesUncertaintiesUsing & CalculatingUncertainties for Electrical

Measurement


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Expressing Uncertainty ofExpressing Uncertainty of

MeasurementMeasurementAll measurements, even the most accurate,

have an unknown inaccuracy or doubt.

The is known as the UCE!TA"T#As there is always an uncertainty with any

measurement we need to estimate this

amount. $e also need to calculate our

confidence in the estimate of uncertainty, which ishow sure we are that the true value is within the

uncertainty we have estimated.


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MeasurementMeasurementAs an e%amle we may measure ' (olt and

be )*+ confident that we are within 'u(


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MeasurementMeasurementUNCERTAINTY vs. ACCURACY

There is no connection between these terms.

Uncertainty is urely the unknown in anymeasurement.

Accuracy or Tolerance is the difference

between the desired value and the actualmeasured value.


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MeasurementMeasurementE%amle

$ith a digital watch with ' second resolution

this resolution will limit the best uncertainty

to which you can tell the time -ie. ' second,

however the watch itself may only be

accurate to a few minutes.


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MeasurementMeasurementThere are many sources of uncertainty in any

measurement which need to be combined

using statistical techni/ues to give a total.

0ifferent tyes of uncertainty need to be

treated differently to obtain an accurate

estimation.


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MeasurementMeasurementTo calculate uncertainty you must first

identify all the sources of error, estimate the

si1e of the contribution from each source and

also decide on the tye of uncertainty for

each source.

There are two tyes of uncertainty Tye A 2 3ased on using statistics

e.g. reeated readings

Tye 3 2 3ased on other factors

e.g. manufacturers secifications


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Sources of Uncertainty inSources of Uncertainty in

Electrical Measurements.Electrical Measurements. "morted Uncertainty0rift of reference instrument

Temerature effects 4ead and thermal errors -0C volts !ounding errors due to resolution !eeatability

oise 5elf heating of high current shunts


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Imported UncertaintyImported Uncertainty

"morted uncertainty is taken directly from the

certificate issued by the laboratory which

calibrated the reference instrument. The robability distribution is 67!MA48


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Drift of reference instrumentDrift of reference instrument

0rift of reference instrument can be either taken form

historical data on the instrument or from the manufacture

secification for stability. "f the drift with time can be

redicted it is ossible to use a corrected figure for the

actual value of the reference with a reduced figure for

drift. 9owever it is more normal to use an 6un corrected8

figure.

The robability distribution is 6!ectangular8.


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Temperature EffectsTemperature Effects

The effect of temerature on many modern instruments is

often very small, and in many cases the instruments

secification covers a band of temerature without any

further addition. 5ome reference standards for e%amle resistors the

Temerature coefficient may be /uite imortant. The

figure for TC can be taken from the manufactures sec or

measured.



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Lead and thermal errorsLead and thermal errors

(DC olts!(DC olts! Thermal emf can be difficult to evaluate. $ith a

little care and correct leads it is normal for

thermal EM: to be less than 'u(, or even .*u(

which is a figure often used in calculations.



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"esolution of Measurement"esolution of Measurement "t is firstly imortant to understand that there is a big difference

between the resolution of a measuring instrument and that of areference source.

A source, such as a standard resistor may have no resolution at

all, but can still be very accurate, while for a measuringinstrument resolution is essential to achieve accuracy.

"t is only necessary to include measurement resolution in the

uncertainty calculation. ote if a 0MM is used to comare say

two resistors then the resolution must be entered twice.

The robability distribution is 6!ectangular8


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Com#ining UncertaintiesCom#ining Uncertainties "t is normal these days to use a sread sheet, in E%cel, taking a

temlate from M;;.

Use column ' for a descrition of source of uncertainty Use column < for the value of uncertainty usually in m Use column ; for a descrition of tye of =robability distribution Use column > for the divisor, < for a normal dist, '.?;< for rect Use column * for a coeficient used to convert say millvolts to microvolts Use column @ column-divisor % column*-coeff

Use the sum of s/uare to total column @ to get the Combined 5tandard

Uncertainty

To obtain the E%anded uncertainty -B


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MeasurementMeasurementE%amle


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Using $roCal to calculateUsing $roCal to calculate

UncertaintiesUncertainties

=roCal use three key elements to dynamically calculate

uncertainties as the test is run

' A table with imorted uncertainties and calibrator secifications


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Set Up Instrument Spec andSet Up Instrument Spec and

Imported UncertaintiesImported Uncertainties

Use =roset, select instrument Traceability in the file menu. 5elect the "nstrument re/uired 5elect the 6uncertainties button to access the table Enter the data, note default table e%ists for


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Set Up $rocedure TemplateSet Up $rocedure Template

Use =ro5et, select 6laboratory rocedures8 in the file menu. 5elect the "nstrument re/uired 5elect the rocedure sreadsheet temlate say for 0C voltage Enter the arameters to be calculated, note the imorted and

reference secification will always be added automatically. ote the rocedures for the main functions of the calibrator are

"nstalled as default


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Set Up Cali#ration $rocedureSet Up Cali#ration $rocedure

Use =roEdit, edit a rocedure and go to the 6"nstruments8 tab. Check the calibration instrument, lab. =rocedure and

uncertainty line. "f a Transmille calibrator is in use these items will be set

automatically, without having to be selected.


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Cali#rating %n InstrumentCali#rating %n Instrument Use =roCal, run a calibration "nut the reading and select enter the noise flicker in the dro

down bo% dislayed. "f re/uired, click on the UCE!T button to view the

uncertainty calculation.


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&here to 'et More Information&here to 'et More Information

UBA5 -www.UBA5.co.uk

ational =hysical 4aboratory

Transmille 4td -www.transmille.com

transmille training - uncertainties.ppt

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