電子測量_溫度測量實踐匯編

1、Practical Temperature Measurements001AgendaA1_Background, history_Mechanical sensors_Electrical sensors_Optical Pyrometer_RTD_Thermistor, IC_Thermocouple_Summary & ExamplesWhat is Temperature?_A scalar quantity that determines the direction of heat flow between two bodies_A statistical measureme

2、nt_A difficult measurement_A mostly empirical measurement002How is heat transferred?003_Conduction_Convection_Radiation_Metal coffee cupThe Dewar 004_Glass is a poor conductor_Gap reduces conduction_Metallization reflects radiation_Vacuum reduces convectionThermal Mass005SensorSensor_Dont let the me

3、asuring device change the temperature of what youre measuring._Response time = _fThermal mass_fMeasuring deviceTemperature errors00697.6 98.6 99.6 36.5 37 37.5_What is YOUR normal temperature?_Thermometer accuracy, resolution_Contact time_Thermal mass of thermometer, tongue_Human error in readingHis

4、tory of temperature sensors007_1600 ad_1700 ad_Galileo: First temp. sensor_pressure-sensitive_not repeatable_ Early thermometers_Not repeatable_No good way to calibrate121096_Fahrenheit_Instrument Maker_12*8=96 points_Hg: Repeatable_One standard scaleThe 1700s: Standardization008_1700 ad_1800 ad_Cel

5、sius:_Common, repeatable calibration reference points_Thomson effect_Absolute zero01000100_Centigrade scale1821: It was a very good year009_1800 ad_1900 ad_The Seebeck effect_Pt 100 O deg.C_Davy: The RTDdThe 1900s: Electronic sensors010_1900 ad_Thermistor_2000 ad_1 uA/K_IC sensor_IPTS 1968 _Degree K

6、elvin kelvins_Centigrade Celsius_IPTS 1990 Temperature scales011-273.15Absolute zero0-459.670_Celsius_Kelvin _Fahrenheit_Rankine 0273.1532427.67100373.15212671.67Freezing point H O2Boiling point H O2_Standard is better: _ Reliable reference points_ Easy to understandIPTS 90: More calibration points0

7、12 273.16: TP H2O 83.8058: TP Ar 54.3584: TP O2 24.5561: TP Ne 20.3: BP H2 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 HeAgendaA2_Background, history_Mechan

8、ical sensors_Electrical sensors_Optical Pyrometer_RTD_Thermistor, IC_Thermocouple_Summary & ExamplesBimetal thermometer013_Two dissimilar metals, tightly bonded_Forces due to thermal expansion_Result_Bimetallic thermometer_Poor accuracy_Hysteresis_Thermal expansion causes big problems in other d

9、esigns:_IC bonds_Mechanical interference0100300200400Liquid thermometer; Paints0140100_Liquid-filled thermometer_Accurate over a small range_Accuracy & resolution= f(length)_Range limited by liquid_Fragile_Large thermal mass_Slow_Thermally-sensitive paints_Irreversible change_Low resolution_Usef

10、ul in hard-to-measure areas AgendaA3_Background, history_Mechanical sensors_Electrical sensors_Optical Pyrometer_RTD_Thermistor, IC_Thermocouple_Summary & ExamplesOptical Pyrometer015_Infrared Radiation-sensitive_Photodiode or photoresistor_Accuracy= femissivity_Useful very high temperatures_Non

11、-contacting_Very expensive_Not very accurateAgendaA4_Background, history_Mechanical sensors_Electrical sensors_Optical Pyrometer_RTD_Thermistor, IC_Thermocouple_Summary & ExamplesResistance Temperature Detector016_Most accurate & stable_Good to 800 degrees Celsius_Resistance= fAbsolute T_Sel

12、f-heating a problem_Low resistance_NonlinearRTD Equation_R=Ro(1+aT) - Ro(ad(.01T)(.01T-1)_Ro=100 O C_ a= 0.00385 / - C _ d= 1.49017_R= 100 Ohms O C_Callendar-Van Deusen Equation:_0 200 400 600 800RT300200100NonlinearityFor TOC:for PtMeasuring an RTD: 2-wire methodd018_R= Iref*(Rx + 2* Rlead)_ Error=

13、 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; Use 4-wire method_If you must use 2-wire, NULL out the lead resistance100RleadV-+I ref= 5 mAPtdRxRleadThe 4-Wire technique019_R= Iref *

14、Rx_Error not a function of R in source or sense leads_No error due to changes in lead R_Twice as much wire_Twice as many scanner channels_Usually slower than 2-wire100Rlead=1V-+I ref= 5 mAddRxOffset compensation020_Eliminates thermal voltages_Measure V without I applied_Measure V I appliedR=VIWith10

15、0V-+I ref (switched)VoffsetdBridge method021V_High resolution (DMM stays on most sensitive range)_Nonlinear output_Bridge resistors too close to heat source100d100d1000d1000d3-Wire bridge022V10001001001000_Keeps bridge away from heat source_Break DMM lead (dashed line); connect to RTD through 3rd se

16、nse wire_If Rlead 1= Rlead 2, sense wire makes error small_Series resistance of sense wire causes no errorRlead 1Rlead 2Sense wire3-Wire PRTDddddAgendaA5_Background, history_Mechanical sensors_Electrical sensors_Optical Pyrometer_RTD_Thermistor, IC_Thermocouple_Summary & ExamplesElectrical senso

17、rs: Thermistor_Hi-Z; Sensitive: 5 k 25C; R = 4%/deg C0235kV-+I= 0.1 mA_2-Wire method: R= I * (Rthmr + 2*Rlead)_ Lead R Error= 2 /400= 0.005 degrees C_Low thermal mass: High self-heating_Very nonlinearRlead=1ddddRlead=1d_Limited rangeI.C. Sensord+-024VI= 1 uA/K5Vd100960= 1mV/KAD590_High output_Very l

18、inear_Accurate room ambient_Limited range_CheapSummary: Absolute T devices025_Expensive_Slow_Needs I source_Self-heating_4-wire meas.RTD_Most accurate _Most stable_Fairly linearThermistor_High output_Fast_2-wire meas._Very nonlinear_Limited range_Needs I source_Self-heating_FragileAD590I.C._High out

19、put_Most linear_Inexpensive_Limited variety_Limited range_Needs V source_Self-heatingAgendaA6_Background, history_Mechanical sensors_Electrical sensors_Optical Pyrometer_RTD_Thermistor, IC_Thermocouple_Summary & ExamplesThermocouples The Gradient Theory026TxTaVV= e(T) dTTaTx_The WIRE is the sens

20、or, not the junction_The Seebeck coefficient (e) is a function of temperatureMaking a thermocouple027_Two wires make a thermocouple_Voltage output is nonzero if metals are not the sameV= e dTTaTx + e dTTaTxABTxTaVTaABGradient theory also says.028_If wires are the same type, or if there is one wire,

21、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 temperatures029_Theoretically, Vab= fTx-Tab_But, try to measure it with a DMM:TxConFeVCuCu=ConaTxFebCuConFeTxCuVSolution: Reference Thermoco

22、uple030_Problems: a) 3 different thermocouples, b) 3 unknown temperatures_Solutions: a) Add an opposing thermocouple b) Use a known reference temp.CuVCu FeTref= 0 CConFeTxoIsothermal blockCuVCuFeTrefConFeTxAddThe Classical Method031CuVCu FeTref= 0 CConFeTxo_If both Cu junctions are at same T, the tw

23、o batteries cancel_Tref is an ice bath (sometimes an electronic ice bath)_All T/C tables are referenced to an ice bath_V= fTx-Tref_Question: How can we eliminate the ice bath?Eliminating the ice bath032TrefCuVCuFeConFeTx_Dont force Tref to icepoint, just measure it_Compensate for Tref mathematically

24、:V=f Tx - Tref _If we know Tref , we can compute Tx.TiceTiceTiceEliminating the second T/C033_Extend the isothermal block_If isothermal, V1-V2=02CuVCuFeConFeTx1CuVCuConFeTx21TrefTrefThe Algorithm for one T/C_Measure Tref: RTD, IC or thermistor_Tref = Vref O C for Type J(Fe-C)_Know V, Know Vref: Comp

25、ute Vx_Solve for using VxTx034CuVCuConFeTxTref0 TrefVxVrefTxComputeVx=V+VrefVo oLinearization035_Polynomial: T=a +a V +a V +a V +. a V_Nested (faster): T=a +V(a +V(a +V(a +.)_Small sectors (faster): T=T +bV+cV _Lookup table: Fastest, most memory212320 123399000 TrefTxoVTSmall sectorsCommon Thermocou

26、ples0360 500 1000 2000 mVdeg C204060E ERNKJESTPlatinum T/CsBase Metal T/Cs_All have Seebeck coefficients in MICROvolts/deg.CCommon Thermocouples037SeebeckCoeff: uV/CTypeMetalsJKTSENFe-ConNi-CrCu-ConPt/Rh-PtNi/Cr-ConNi/Cr/Si-Ni/Si504038105939_Microvolt output is a tough measurement_Type N is fairly n

27、ew. more rugged and higher temp. than type K, but still cheapExtension Wires038Large extension wiresSmall diametermeasurementwires_Possible problemhere_Extension wires are cheaper, more rugged, but not exactly the same characteristic curve as the T/C._Keep extension/TC junction near room temperature

28、_Where is most of the signal generated in this circuit?Noise: DMM Glossary039DMMInput ResistanceNormal Modedc SIGNALNormal Modeac NOISEDMMInput ResistanceCommon Modeac NOISEHIHILOLO_Normal Mode: In series with input_Common Mode: Both HI and LOterminals driven equallyGenerating noise040Normal Mode_La

29、rge surface area, high Rlead: Max. static coupling_Large loop area: Max. magnetic couplingDMMInput Resistancedc SIGNALDMMInput ResistanceHIHILOLOElectrostaticNoiseMagneticNoiseCommon Mode ac sourceR leadR leakCommon Mode Current_Large R lead, small R leak: Mmon mode noiseEliminating noise041Normal M

30、odedc SIGNAL_Filter, shielding, small loop area(Caution: filter slows down the measurement)_Make R leak close to DMMInputRDMMInput RHIHILOLOElectrostaticNoiseMagneticNoiseCommon Mode ac sourceR leakCommon Mode Current- +Magnetic Noise042_Magnetic couplingDMMInputResistanceInduced I_Minimize area_Twi

31、st leads_Move away from strong fieldsReducing Magnetic Noise043_Equal and opposite induced currentsDMMInputResistance_Even with twisted pair:_Minimize area_Move away from strong fieldsElectrostatic noise044DMMInputResistance_Stray capacitance causes I noise_DMM resistance to ground is importantStray

32、 resistancesAC NoisesourceStray capacitancesInoiseReducing Electrostatic Coupling045DMMInputResistance_Shield shunts stray current_For noise coupled to the tip, Rleak is still importantAC Noise sourceHILORleakA scanning system for T/CsOHMsConv.046HILOFloating CircuitryGrounded CircuitryIsolatorsuPuP

33、 I/O(HP-IB,RS-232) ToComputerROMLookupIntegrating A/D_One thermistor, multiple T/C channels_Noise reduction_CPU linearizes T/C_DMM must be very high qualityErrors in the systemOHMsConv.047HILOFloating CircuitryGrounded CircuitryIsolatorsuPuP I/O(HP-IB,RS-232)ROMLookupIntegrating A/DThermal emfLinear

34、ization algorithmReferenceThermistorOhmsmeasurementRef. Thermistor cal, linearityT/C Calibration & Wire errorsRef. Block Thermal gradientDMM offset, linearity, thermal emf, noiseExtension wirejunction errorPhysical errors048_Shorts, shunt impedance_Galvanic action_Decalibration_Sensor accuracy_T

35、hermal contact_Thermal shuntingPhysical Errors049_Water droplets cause galvanic action; huge offsets_Hot spot causes shunt Z, meter shows the WRONG temperature_Exceeding the T/Cs range can cause permanent offset_Real T/Cs have absolute accuracy of 1 deg C 25C: Calibrate often and take carePhysical error: Thermal contact050Surface probe_Make sure thermal mass is much smallerthan that of object being measuredPhysical errors: Decalibration0511000 C200 C300 C350 C975 C100 CThis section produces theENTIRE signal_Dont ex