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1、Unit15TemperatureSensorsTextKeywordsTechnicaltermsExpressionsNotestothetextTextOneoftheprimarymethodsforelectricalmeasurementoftemperatureinvolveschangesintheelectricalresistanceofcertainmaterials.Inthis,aswellasothercases,theprinciplemeasurementtechniqueistoplacethetemperature-sensingdeviceinconta
2、ctwiththeenvironmentwhosetemperatureistobemeasured.Thesensingdevicethentakesonthetemperatureoftheenvironment.Thusameasureofitsresistanceindicatesthetemperatureofthedeviceandtheenvironmentmustwaituntilthedevicecomesintothermalequilibriumwiththeenvironment.ThetwobasicdevicesusedastheResistance-Temper
3、atureDetector(RTD),basedonthevariationofmetalresistancewithtemperature,andthethermistor,basedonthevariationofsemiconductorresistancewithtemperature.Thereexistsanotherdependenceofelectricalbehaviorofmaterialsontemperaturethatformsthebasisofalargepercentageofalltemperaturemeasurement.Thiseffectischar
4、acterizedbyavoltage-generatingsensorinwhichanelectromotiveforce(emf)isproducedthatisproportionaltotemperature.Suchanemfisfoundtobealmostlinearwithtemperatureandveryrepeatableforconstantmaterials.Devicesthatmeasuretemperatureonthebasisofthisthermoelectricprinciplearecalledthermocouples(TCs).Herewe’l
5、lbrieflydiscussthecharacteristicsofResistance-TemperatureDetector(RTD).AResistance-TemperatureDetector(RTD)isatemperaturesensorthatisbasedontheprinciplesthatmetalresistanceincreasingwithtemperature.Metalusedinthesedevicesvaryfromplatinum,whichisveryrepeatable,quitesensitive,andveryexpensive,tonicke
6、l,whichisnotquitsasrepeatable,moresensitive,andlessexpensive.SensitivityAnestimateofRTDsensitivitycanbenotedfromtypicalvaluesofa0,thelinearfractionalchangeinresistancewithtemperature.Forplatinum,thisnumberistypicallyontheorderof0.004/℃,andfornickelatypicalvalueis0.005/℃.Thus,withplatinum,forexample
7、,achangeofonly0.4Ωwouldbeexpectedfora100ΩRTDifthetemperatureischangedby1℃.Usually,aspecificationwillprovidecalibrationinformationeitherasagraphofresistanceversustemperatureorasatableofvaluesfromwhichthesens
