電子測量-溫度測量實踐匯編

1、Practical Temperature Measurements001AgendaA1Background, historyMechanical sensorsElectrical sensorsOptical PyrometerRTDThermistor, ICThermocoupleSummary & ExamplesWhat is Temperature?A scalar quantity that determines the direction of heat flow between two bodiesA statistical measurementA difficult

2、measurementA mostly empirical measurement002How is heat transferred?003ConductionConvectionRadiationMetal coffee cupThe Dewar 004Glass is a poor conductorGap reduces conductionMetallization reflects radiationVacuum reduces convectionThermal Mass005SensorSensorDont let the measuring device change the

3、 temperature of what youre measuring.Response time = fThermal massfMeasuring deviceTemperature errors00697.6 98.6 99.6 36.5 37 37.5What is YOUR normal temperature?Thermometer accuracy, resolutionContact timeThermal mass of thermometer, tongueHuman error in readingHistory of temperature sensors007160

4、0 ad1700 adGalileo: First temp. sensorpressure-sensitivenot repeatable Early thermometersNot repeatableNo good way to calibrate121096FahrenheitInstrument Maker12*8=96 pointsHg: RepeatableOne standard scaleThe 1700s: Standardization0081700 ad1800 adCelsius:Common, repeatable calibration reference poi

5、ntsThomson effectAbsolute zero01000100Centigrade scale1821: It was a very good year0091800 ad1900 adThe Seebeck effectPt 100 O deg.CDavy: The RTDdThe 1900s: Electronic sensors0101900 adThermistor2000 ad1 uA/KIC sensorIPTS 1968 Degree Kelvin kelvinsCentigrade CelsiusIPTS 1990 Temperature scales011-27

6、3.15Absolute zero0-459.670CelsiusKelvin FahrenheitRankine 0273.1532427.67100373.15212671.67Freezing point H O2Boiling point H O2Standard is better: Reliable reference points Easy to understandIPTS 90: More calibration points012 273.16: TP H2O 83.8058: TP Ar 54.3584: TP O2 24.5561: TP Ne 20.3: BP H2

7、17 Liq/vapor H2 13.81 TP H2Large gap1234.93: FP Ag 1337.33: FP Au 692.677: FP Zn 429.7485: FP In 234.3156: TP Hg 302.9146: MP Ga 505.078: FP Sn 933.473: FP Al 1357.77: FP Cu 3 to 5: Vapor HeAgendaA2Background, historyMechanical sensorsElectrical sensorsOptical PyrometerRTDThermistor, ICThermocoupleS

8、ummary & ExamplesBimetal thermometer013Two dissimilar metals, tightly bondedForces due to thermal expansionResultBimetallic thermometerPoor accuracyHysteresisThermal expansion causes big problems in other designs:IC bondsMechanical interference0100300200400Liquid thermometer; Paints0140100Liquid-fil

9、led thermometerAccurate over a small rangeAccuracy & resolution= f(length)Range limited by liquidFragileLarge thermal massSlowThermally-sensitive paintsIrreversible changeLow resolutionUseful in hard-to-measure areas AgendaA3Background, historyMechanical sensorsElectrical sensorsOptical PyrometerRTD

10、Thermistor, ICThermocoupleSummary & ExamplesOptical Pyrometer015Infrared Radiation-sensitivePhotodiode or photoresistorAccuracy= femissivityUseful very high temperaturesNon-contactingVery expensiveNot very accurateAgendaA4Background, historyMechanical sensorsElectrical sensorsOptical PyrometerRTDThe

11、rmistor, ICThermocoupleSummary & ExamplesResistance Temperature Detector016Most accurate & stableGood to 800 degrees CelsiusResistance= fAbsolute TSelf-heating a problemLow resistanceNonlinearRTD EquationR=Ro(1+aT) - Ro(ad(.01T)(.01T-1)Ro=100 O C a= 0.00385 / - C d= 1.49017R= 100 Ohms O CCallendar-V

12、an Deusen Equation:0 200 400 600 800RT300200100NonlinearityFor TOC:for PtMeasuring an RTD: 2-wire methodd018R= Iref*(Rx + 2* Rlead) Error= 2 /.385= more than 5 degrees C for 1 ohm Rlead!Self-heating:For 0.5 V signal, I= 5mA; P=.5*.005=2.5 mwatts 1 mW/deg C, Error = 2.5 deg C!Moral: Minimize Iref; Us

13、e 4-wire methodIf you must use 2-wire, NULL out the lead resistance100RleadV-+I ref= 5 mAPtdRxRleadThe 4-Wire technique019R= Iref * RxError not a function of R in source or sense leadsNo error due to changes in lead RTwice as much wireTwice as many scanner channelsUsually slower than 2-wire100Rlead=

14、1V-+I ref= 5 mAddRxOffset compensation020Eliminates thermal voltagesMeasure V without I appliedMeasure V I appliedR=VIWith100V-+I ref (switched)VoffsetdBridge method021VHigh resolution (DMM stays on most sensitive range)Nonlinear outputBridge resistors too close to heat source100d100d1000d1000d3-Wir

15、e bridge022V10001001001000Keeps bridge away from heat sourceBreak DMM lead (dashed line); connect to RTD through 3rd sense wireIf Rlead 1= Rlead 2, sense wire makes error smallSeries resistance of sense wire causes no errorRlead 1Rlead 2Sense wire3-Wire PRTDddddAgendaA5Background, historyMechanical

16、sensorsElectrical sensorsOptical PyrometerRTDThermistor, ICThermocoupleSummary & ExamplesElectrical sensors: ThermistorHi-Z; Sensitive: 5 k 25C; R = 4%/deg C0235kV-+I= 0.1 mA2-Wire method: R= I * (Rthmr + 2*Rlead) Lead R Error= 2 /400= 0.005 degrees CLow thermal mass: High self-heatingVery nonlinear

17、Rlead=1ddddRlead=1dLimited rangeI.C. Sensord+-024VI= 1 uA/K5Vd100960= 1mV/KAD590High outputVery linearAccurate room ambientLimited rangeCheapSummary: Absolute T devices025ExpensiveSlowNeeds I sourceSelf-heating4-wire meas.RTDMost accurate Most stableFairly linearThermistorHigh outputFast2-wire meas.

18、Very nonlinearLimited rangeNeeds I sourceSelf-heatingFragileAD590I.C.High outputMost linearInexpensiveLimited varietyLimited rangeNeeds V sourceSelf-heatingAgendaA6Background, historyMechanical sensorsElectrical sensorsOptical PyrometerRTDThermistor, ICThermocoupleSummary & ExamplesThermocouples The

19、 Gradient Theory026TxTaVV= e(T) dTTaTxThe WIRE is the sensor, not the junctionThe Seebeck coefficient (e) is a function of temperatureMaking a thermocouple027Two wires make a thermocoupleVoltage output is nonzero if metals are not the sameV= e dTTaTx + e dTTaTxABTxTaVTaABGradient theory also says.02

20、8If wires are the same type, or if there is one wire, and both ends are at the same temperature, output= Zero.V= e dTTaTx + e dT = 0TaTxAATxTaVTaAANow try to measure it:Result: 3 unequal junctions, all at unknown temperatures029Theoretically, Vab= fTx-TabBut, try to measure it with a DMM:TxConFeVCuC

21、u=ConaTxFebCuConFeTxCuVSolution: Reference Thermocouple030Problems: a) 3 different thermocouples, b) 3 unknown temperaturesSolutions: a) Add an opposing thermocouple b) Use a known reference temp.CuVCuFeTref= 0 CConFeTxoIsothermal blockCuVCuFeTrefConFeTxAddThe Classical Method031CuVCuFeTref= 0 CConF

22、eTxoIf both Cu junctions are at same T, the two batteries cancelTref is an ice bath (sometimes an electronic ice bath)All T/C tables are referenced to an ice bathV= fTx-TrefQuestion: How can we eliminate the ice bath?Eliminating the ice bath032TrefCuVCuFeConFeTxDont force Tref to icepoint, just meas

23、ure itCompensate for Tref mathematically:V=f Tx - Tref If we know Tref , we can compute Tx.TiceTiceTiceEliminating the second T/C033Extend the isothermal blockIf isothermal, V1-V2=02CuVCuFeConFeTx1CuVCuConFeTx21TrefTrefThe Algorithm for one T/CMeasure Tref: RTD, IC or thermistorTref = Vref O C for T

24、ype J(Fe-C)Know V, Know Vref: Compute VxSolve for using VxTx034CuVCuConFeTxTref0TrefVxVrefTxComputeVx=V+VrefVo oLinearization035Polynomial: T=a +a V +a V +a V +. a VNested (faster): T=a +V(a +V(a +V(a +.)Small sectors (faster): T=T +bV+cV Lookup table: Fastest, most memory212320123399000TrefTxoVTSma

25、ll sectorsCommon Thermocouples0360 500 1000 2000 mVdeg C204060EERNKJESTPlatinum T/CsBase Metal T/CsAll have Seebeck coefficients in MICROvolts/deg.CCommon Thermocouples037SeebeckCoeff: uV/CTypeMetalsJKTSENFe-ConNi-CrCu-ConPt/Rh-PtNi/Cr-ConNi/Cr/Si-Ni/Si504038105939Microvolt output is a tough measure

26、mentType N is fairly new. more rugged and higher temp. than type K, but still cheapExtension Wires038Large extension wiresSmall diametermeasurementwiresPossible problemhereExtension wires are cheaper, more rugged, but not exactly the same characteristic curve as the T/C.Keep extension/TC junction ne

27、ar room temperatureWhere is most of the signal generated in this circuit?Noise: DMM Glossary039DMMInputResistanceNormal Modedc SIGNALNormal Modeac NOISEDMMInputResistanceCommon Modeac NOISEHIHILOLONormal Mode: In series with inputCommon Mode: Both HI and LOterminals driven equallyGenerating noise040

28、Normal ModeLarge surface area, high Rlead: Max. static couplingLarge loop area: Max. magnetic couplingDMMInputResistancedc SIGNALDMMInputResistanceHIHILOLOElectrostaticNoiseMagneticNoiseCommon Mode ac sourceR leadR leakCommon Mode CurrentLarge R lead, small R leak: Mmon mode noiseEliminating noise04

29、1Normal Modedc SIGNALFilter, shielding, small loop area(Caution: filter slows down the measurement)Make R leak close to DMMInputRDMMInput RHIHILOLOElectrostaticNoiseMagneticNoiseCommon Mode ac sourceR leakCommon Mode Current- +Magnetic Noise042Magnetic couplingDMMInputResistanceInduced IMinimize are

30、aTwist leadsMove away from strong fieldsReducing Magnetic Noise043Equal and opposite induced currentsDMMInputResistanceEven with twisted pair:Minimize areaMove away from strong fieldsElectrostatic noise044DMMInputResistanceStray capacitance causes I noiseDMM resistance to ground is importantStray re

31、sistancesAC NoisesourceStray capacitancesInoiseReducing Electrostatic Coupling045DMMInputResistanceShield shunts stray currentFor noise coupled to the tip, Rleak is still importantAC Noise sourceHILORleakA scanning system for T/CsOHMsConv.046HILOFloating CircuitryGrounded CircuitryIsolatorsuPuP I/O(

32、HP-IB,RS-232) ToComputerROMLookupIntegrating A/DOne thermistor, multiple T/C channelsNoise reductionCPU linearizes T/CDMM must be very high qualityErrors in the systemOHMsConv.047HILOFloating CircuitryGrounded CircuitryIsolatorsuPuP I/O(HP-IB,RS-232)ROMLookupIntegrating A/DThermal emfLinearization a

33、lgorithmReferenceThermistorOhmsmeasurementRef. Thermistor cal, linearityT/C Calibration & Wire errorsRef. Block Thermal gradientDMM offset, linearity, thermal emf, noiseExtension wirejunction errorPhysical errors048Shorts, shunt impedanceGalvanic actionDecalibrationSensor accuracyThermal contactTher

34、mal shuntingPhysical Errors049Water droplets cause galvanic action; huge offsetsHot spot causes shunt Z, meter shows the WRONG temperatureExceeding the T/Cs range can cause permanent offsetReal T/Cs have absolute accuracy of 1 deg C 25C: Calibrate often and take carePhysical error: Thermal contact050Surface probeMake sure thermal mass is much smallerthan that of object being measuredPhysical errors: Decalibration0511000 C200 C300 C350 C975 C100 CThis section produces theE