Simplified Notes For Electrical Measurement And Instrumentation

LECTURE 7Thermal (Temperature) Sensors and TransducersTemperaturemeasurementisveryimportantespeciallyintheprocessindustryformonitoringandcontrolofvariousprocesses.Temperatureismeasuredbywayofheatexchangebetweentheobjectandtheprobe(inthesensingelement)throughheatconduction,convectionorradiation.Theprobewarmsuporcoolsdownandthischangeiscapturedelectrically.Needtoconverttemperaturesignalintoanelectricalsignalrequiressignalconditioningcircuits/interfaces.Couplingbetweentheprobeandtheobjectwhosetemperatureisbeingmeasuredwillalwaysintroduceerrorsintothemeasurementandmustbeconsideredduringsensordesign.Temperaturemeasurementalsoinvolvestimeconstants,nonlinearities,noise,etc.Applicationconsiderationsinclude:range,accuracy,resolution,environment2Temperature sensors Traditionally,temperaturesensorswerebasedonliquidexpansion(hencewideavailabilityofliquidinglassthermometers)Electricaltransductionemploysvariousmethodsthermoresistive,thermoelectric,semiconductive,optical,acoustic,pyroelectricandpiezoelectric.Temperaturesensoristhermallycoupledtoobjectfortemperaturemeasurement,eitherphysically(contact)orremotely(non-contact)Theremustbeestablishedathermalcouplingforanymeasureableelectricalresponse3Categorization of temperature sensors4ThermometersThermometersarethemostcommontemperaturesensorsencounteredinsimple,everydaymeasurementsoftemperature.TwoexamplesofthermometersaretheFilledSystemandBimetalthermometers.FilledSystemThermometerThefamiliarliquidthermometerconsistsofaliquidenclosedinatube.Thevolumeofthefluidchangesasafunctionoftemperature.Increasedmolecularmovementwithincreasingtemperaturecausesthefluidtoexpandandmovealongcalibratedmarkingsonthesideofthetube.Thefluidshouldhavearelativelylargethermalexpansioncoefficientsothatsmallchangesintemperaturewillresultindetectablechangesinvolume.Acommontubematerialisglassandacommonfluidisalcohol.Mercuryusedtobeamorecommonfluiduntilitstoxicitywasrealized.Althoughthefilled-systemthermometeristhesimplestandcheapestwaytomeasuretemperature,itsaccuracyislimitedbythecalibrationmarksalongthetubelength.5Becausefilledsystemthermometersarereadvisuallyanddontproduceelectricalsignals,itisdifficulttoimplementtheminprocesscontrolsthatrelyheavilyonelectricalandcomputerizedcontrol.BimetalThermometerInthebimetalthermometer,twometals(commonlysteelandcopper)withdifferentthermalexpansioncoefficientsarefixedtooneanotherwithrivetsorbywelding.Asthetemperatureofthestripincreases,themetalwiththehigherthermalexpansioncoefficientsexpandstoagreaterdegree,causingstressinthematerialsandadeflectioninthestrip.Theamountofthisdeflectionisafunctionoftemperature.Thetemperaturerangesforwhichthesethermometerscanbeusedislimitedbytherangeoverwhichthemetalshavesignificantlydifferentthermalexpansioncoefficients.Bimetallicstripsareoftenwoundintocoilsandplacedinthermostats.Themovingendofthestripisanelectricalcontact,whichtransmitsthetemperaturethermostat.6Thermoresistive SensorsThesearemodulatingtransducersbasedonthethermoresistivetransductionprinciple.Energyconversionisthermaltoelectricalbyfirstdetectingachangeinresistance.Haveadvantagesofsimplicityofinterfacecircuits(e.g.Wheatstonebridges),sensitivityandlong-termstabilityThreemaingroups:RTDs,Thermistors,semiconductordetectors(siliconbased)7Resistance Temperature Detectors (RTDs)Asecondcommonlyusedtemperaturesensoristheresistancetemperaturedetector(RTD,alsoknownasresistancethermometer).Unlikefilledsystemthermometers,theRTDprovidesanelectricalmeansoftemperaturemeasurement,thusmakingitmoreconvenientforusewithacomputerizedsystem.AnRTDutilizestherelationshipbetweenelectricalresistanceandtemperature,whichmayeitherbelinearornonlinear.RTDsaretraditionallyusedfortheirhighaccuracyandprecision.However,athightemperatures(above700oC)theybecomeveryinaccurateduetodegradationoftheoutersheath,whichcontainsthethermometer.Therefore,RTDusageispreferredatlowertemperatureranges,wheretheyarethemostaccurate.8RTDsTherearetwomaintypesofRTDs,thetraditionalRTDandthethermistorTraditionalRTDsusemetallicsensingelementsthatresultinalinearrelationshipbetweentemperatureandresistance.Asthetemperatureofthemetalincreases,increasedrandommolecularmovementimpedestheflowofelectrons.Theincreasedresistanceismeasuredasareducedcurrentthroughthemetalforafixedvoltageapplied.Thethermistorusesasemiconductorsensor,whichgivesapowerfunctionrelationshipbetweentemperatureandresistance.9RTDsAsshowninfig,theRTDcontainsanoutersheathtopreventcontaminationfromthesurroundingmedium.Ideally,thissheathiscomposedofmaterialthatefficientlyconductsheattotheresistor,butresistsdegradationfromheatorthesurroundingmedium.Theresistancesensoritselfisresponsibleforthetemperaturemeasurement,asshowninthediagram.Sensorsaremostcommonlycomposedofmetals,suchasplatinum,nickel,orcopper.ThematerialchosenforthesensordeterminestherangeoftemperaturesinwhichtheRTDcouldbeused.Forexample,platinumsensors,themostcommontypeofresistor,havearangeofapproximately-200oC-800oC.Connectedtothesensoraretwoinsulatedconnectionleads.Theseleadscontinuetocompletetheresistorcircuit.10Resistance Temperature Detectors (RTDs) Basedontemperaturedependenceoftheresistivitiesofallmetalsandmostalloys.Makesuseofmetalorthinfilmsandsomesemiconductormaterials.Mostlyusesplatinum(linear,predictableresponse,long-termstability,anddurability),butalsonickel,copper,tungsten(rare)andalloys.Resistivityincreaseswithtemperature(positivetemperaturecoefficient).Temperature-resistancerelationshipisgivenbyCallendar-vanDusenapproximations:Fortherangefrom-200to0C11For the range from 0 to 630 oC it becomes Alternatively, the above equations can be written as:A, B, C and , , are material thermal coefficients of resistance (also given in Tables) 12Relation between temperature and resistance13Thermoresistive effectConductivity depends on temperatureConductors and semiconductorsResistance is measured, all other parameters must stay constant.Thermoresistive effect (cont.)Resistance of a length of wire:Conductivity is:Resistance as a function of temperature: a - Temperature Coefficient of Resistance (TCR) [C]Thermoresistive effect (cont.)T is the temperature [C ] 0 is the conductivity of the conductor at the reference temperature T0. T0 is usually given at 20C but may be given at other temperatures as necessary. a - Temperature Coefficient of Resistance (TCR) [C] given at T0RTD characteristicsAccurate (0.1 C) Highly linear over limited temperature range (platinum) Wide temperature range Long term stability Repeatable Resistant to contamination/corrosion (Pt)Disadvantages of RTDs: Expensive wire wound but low cost film Low sensitivity (Pt100; 0.4/C) Available in limited values (Pt100, Pt500, Pt1000) Slow response time Sensitive to shock and vibration 17Self heat in RTDsRTDs are subject to errors due to rise in their temperature produced