Electrical and Electronic Measurements&Instrumentation10EE35
Resistancetemperaturedetector(RTD)
Resistancetemperaturedetector is a primaryelectrical transducer which is used to measurethe change in the temperature.It is commonlyknown as resistancethermometer. Theresistancethermometersarebasedontheprinciplethattheresistanceofthe conductor
changes when ~hetemperature changes.Basicallythe resistancethermometerdetermines the' changein theelectrical resistanceof the conductor subjected to the temperature changes.Thetemperaturesensingelementusedinthisthermometershouldexhibita relativelylargechangeinresistanceforagivenchangeintemperature.Alsothesensing elementshouldnotundergopermanentchangewithuseorage.Anotherdesirable characteristicforthesensingelementisthelinearchangeinresistancewithchangein temperature.Whenthesensingelementissmallerinsize,lessheatisrequiredtoraiseits temperature.Thisissuitableformeasurementofrapidvariationsintemperature. Platinum, nickel, and copper are the metals mostcommonlyusedto measure temperature. Therelationship betweentemperature and resistanceof conductor isgivenbyequation:
Almostallmetallicconductorshaveapositivetemperaturecoefficientsothattheir resistanceincreaseswithanincreaseintemperature.Ahighvalueofaisdesirableina temperature sensing element so that a substantial changein resistance occurs for a relativelysmall changein temperature. This changein resistance[L\R]can be measured with a Wheatstone bridge, the output of which can be directlycalibrated to indicate the temperature which caused the changeis resistance.
Most of the metals show an increasein resistivitywith temperature,whichis first linearandthenincreasesinanacceleratedfashion.Themetalsthatexhibitgood sensitivityandreproducibilityfortemperaturemeasurementpurposesarecopper,nickel, andplatinum.Amongthese, copper has the highest temperature coefficientwith the most linear dependence. However, copper isgenerallynot used due to certain practical problems. Because of its low resistivity,
Electrical and Electronic Measurements&Instrumentation10EE35
the size of the resistance element increases to get reasonablesensitivity.Intherangebelow400K,agoldsilveralloycanbeusedwhich has the same characteristic as platinum.
ConstructionofRTD
Thewireresistancethermometerusuallyconsistsofacoilwoundonamicaor ceramicformer, as shown in the Fig.The coil iswound in bifilar form soas to makeitnoinductive.Suchcoilsareavailableindifferentsizesandwithdifferentresistance
valuesrangingfrom10ohmsto25,000ohms.Toavoidcorrosionofresistiveelement, usuallyelementsareenclosedinaprotectivetubeofpyrexglass,porcelain,quartzor nickel,dependingontherangeoftemperatureandthenatureofthefluidwhose temperatureistobemeasured.Thetubeisevacuatedandsealedorfilledwithairorany otherinertgasandkeptaroundatmosphericpressureorinsomecasesatahigher pressure.