transmille training - making good measurements.ppt

8/18/2019 Transmille Training - Making Good Measurements.ppt

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TrainingTrainingMaking accurate ElectricalMeasurements


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IntroductionIntroduction

Sources of errors in electricalmeasurements

Understanding instrument specification

Getting the best from a Digital multimeter and Calibrator


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Making good connectionMaking good connection

Large errors in measurements can be easilycaused by using the incorrect leads.

The key measurements areas here leads effectaccuracy are!"

Low DC voltage

High Value ResistanceLow AC current

Low resistance 4 wire connections


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Measuring Low DC VoltageMeasuring Low DC Voltage

#hen measuring DC $oltage belo %$olt themain source&s of error are thermal generated$oltages 'emf( in the lead connections. These$oltages hich can easily be tens of micro $oltsconstantly change as temperature gradientschange.

Thermo electric $oltages occur hen e$er )different metals are in contact ith a temperaturegradient across the contact. This is ho athermocouple orks.


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Reducing Thermal EMF 1Reducing Thermal EMF 1

There are three ays to reduce EM*!"

Firstly and most important Use connectors made ith metals hich produce lo thermal

$oltages+ like gold+ sil$er.,$oid connectors made+ or plated ith materials like nickel+

hich is often use to plate -banana& type plugs. ote nickel isused in -/ type& thermocouple and can produce up to 00u12C.

The terminals in precision instruments are usually made itho3ygen free copper+ and gold plated and if connected to a

similar material the EM* $oltage ill be less than %u12C.The Transmille precision lead set contains one pair of safety

4mm to 4mm lo thermal leads.


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Reducing Thermal EMF - 2Reducing Thermal EMF - 2


Second ,$oid large temperature gradients across the measurement area.

These can be caused by strong drafts of cold or arm aircoming from air condition units+ the air from such can bese$eral degrees different. ,lso take care ith sun lightthrough a indo or a instrument or soldering iron causinglocal heating.

Try to obtain a stable en$ironment and lea$e connection tothermally stabilise before 5eroing the system. , calibrator orDMM hich as last used on high current may take manyminutes to settle do n.


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Reducing Thermal EMF - Reducing Thermal EMF -


Thirdly

Make sure there is the minimum number ofconnection bet een source and measurementinstrument. Do not e3tended leads by pluggingtogether. Try to keep leads short and close so

they ill be at same temperature.


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Reducing Thermal EMF - !Reducing Thermal EMF - !

,fter you ha$e done all you can to reduce the EM*repeating the measurement se$eral times can gi$e a bettermeasurement as the effects of thermals are usually random.#ith some measurements it is possible to re$erse leads andtake the difference+ but note that holding the connectors

ill arm them up+ e$en the friction caused by plugging incauses heat.

, practical limit is around 6.7u1 and this is a common$alue to used in uncertainty calculations.

Some manufactures to help hen calculating uncertainties includingTransmille try to gi$e a figure for this as in part it is the interaction

bet een the instrument terminal and the lead and in practice it illal ays be present to some degree.


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Making "ccurate #igh ValueMaking "ccurate #igh Value

Resistance Measurements - 1Resistance Measurements - 1There are 0 problems measuring resistance abo$e %66kohm.*irstly is lead insulation resistance. Most test leads are made out of 81C

type plastic hich is not a good insulator. , typical figure for insulationfor this type of test leads could be bet een 76 and %66Gohms. E$en dirton the leads ill reduce insulation.

Test leads ith an insulation resistance of %66Gohm ill gi$e an error of%9 hen measuring %Gohm+ and 6.%9 on %66Mohm.

ote that :nsulation resistance is not connected ith $oltage2current ratingof the lead.

Screened leads made from 8T*E are the best+ ho e$er this is brittle and brakes $ery easily. ,lmost as good is 76 ;hm Coa3 ith a polythene

insulating material.


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Resistance Measurements - 2Resistance Measurements - 2Second problem is noise pick < up in the test leads and the resistor

itself. There is al ays present a strong field from the 8o erline and this can easily s amp out the true signal.

,s this pick up is ,C it presence may not be noticed+ all that may be seen is a more noisy reading. This pick up ill not be a nicesine a$e but a distorted signal+ hich hen a$eraged out bythe DMM&s filter ill gi$e an offset hich ill also result in areading error

, %Gohm resistor measured ith a DMM at %6 $olts ill only

ha$e a current of %6n, flo ing in the test leads+ a noise pick upof =ust %6 8ico amps ill be 6.%9 noise on the reading. :t istherefore $ery important that screened leads are used+ and thecase of the resistor is earthed.


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Resistance Measurements - Resistance Measurements - To o$ercome+ or at least reduce the lead and pick up problem it is

$ery common to measure high $alue resistors at higher $oltagesto obtain higher currents to measure. The $oltage is generated

by a calibrator and the current is measured by a 8ico amp meteror electrometer. Transmille )666 series can also be used forthis measurement.

This ho e$er introduces the effect of -$oltage coefficient& in the

resistor here the actual $alue of the resistor changes ith theapplied $oltage+ this is not due to self heating as in a po erresistor but due to -$oltage stress& and is an instantaneouseffect. :t may be necessary to characterise the resistor for its$oltage coefficient.


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Low "C Current MeasurementsLow "C Current Measurements

The main problem ith this is current leakage to earththrough capaciti$e coupling through mains transformers+capacitance to case etc+ in fact any here there is an un

guarded path to earth.

ote this is not a DC path but an ,C path hich isfre>uency dependant.

,gain 8ick < up is also a problem+ use screened leads butkeep the distance short as the capacitance of screenedcable is high and this ill again leak some current a ay.


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Low $alue ResistanceLow $alue ResistanceTo measure lo ohms a 4 ire /el$in connection must be used+ other isethe resistance of the connections ? leads used ill also be measured.

/el$in connection ork by using separate leads for the measurementcurrent and for measuring the $oltage drop across the resistor. ,ny$oltage dropped along the leads carrying the current is not measured+ and

there is no current flo ing in the $oltage sense leads the resistance of theleads ill make no difference.Thermal EM* in the $oltage sense leads ill make a difference and thesame precautions as used for any lo DC $oltage measurement must betaken.

Note some DMM’s can automatically compensate for thermal EMFs byturning the measurement current on and off and measuring the voltage

difference.


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Instrument %&eci'ications(Instrument %&eci'ications(Different manufacturers present specifications in manydifferent ays. ;ften the different ord is term is used forthe same thing.

@asically ho e$er all specification come do n to the same0 terms and a temperature coefficient

1: A percentage of the actual ValueSometime called % or ppm of reading or output

2: A percentage of the Full scale of that rangeSometime called % or ppm of range or full scale3: A fi e! allowance " or Floor

Often e pressed as a number of digits or counts!Or directly in units! e ample "u#


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Instrument %&eci'ications - 1Instrument %&eci'ications - 1To ork out the allo ance at any point simple ork outeach $alue and add them up. ote that the -counts& or-Digits& refers to the last digit in the display < the resolution

E3ampleMeasuring % $olt on a DMM ith a spec of .679 of reading A0counts

% 1olt at 6.679 B 7m10 counts on %. 1 display B 0m1Total error allo ed B m1


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Instrument %&eci'ications - 2Instrument %&eci'ications - 2

88M and 8ercentageThese terms ork in the same ay+ but percentage is based

on %66+ ppm is based on one million+'ppm B parts per million(6.6%9 B %66ppm


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Instrument %&eci'ications - Instrument %&eci'ications - ,n instrument accuracy specification is based on 0 factors

%( The instruments change ith time)( The Design hich may limit initial accuracy0( The uncertainties to hich the laboratory calibration can be

performed

The specification normally pro$ided by the manufacturer are % and )combined+ this of course re>uires that the manufacture supplies alsothe time+ often one year that the specification applies for.

This specification is relati$e to calibration standards+ hich means noaccount has been taken of the uncertainties to hich the instrumenthas been calibrated to.

,bsolute accuracy can be calculated by adding this figure to theuncertainty to hich the calibrator is certified. Transmille beli$e itis unhelpful for the manufacture to gi$e an absolute figure based onthe manufactures o n laboratory+ as this limits the choice of thecustomer to select the calibration source.


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)etting the *est out o')etting the *est out o'modern high &er'ormancemodern high &er'ormanceDMMs and cali*ratorsDMMs and cali*rators

Lea$e on in a stable en$ironment

un any auto"cal 2 self cal before useConnect using the best leads for the measurementSelect the best range and settings

ull 2 5ero any offset errors first,llo time to stabilise after use on high po erranges

transmille training - making good measurements.ppt

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