THERMOCOUPLES Temperature DetectorsFigure 5 shows the internal constructionFigure 5 Internal Construction of aTypical Thermocoupleof a typical thermocouple. The leads ofthe thermocouple are encased in a rigidmetal sheath. The measuring junction isnormally formed at the bottom of thethermocouple housing. Magnesium oxidesurrounds the thermocouple wires toprevent vibration that could damage thefine wires and to enhance heat transferbetween the measuring junction and themedium surrounding the thermocouple.ThermocoupleOperationThermocouples will cause an electriccurrent to flow in the attached circuitwhen subjected to changes in temperature.The amount of current that will beproduced is dependent on the temperaturedifference between the measurement andreference junction; the characteristics ofthe two metals used; and thecharacteristics of the attached circuit.Figure 6 illustrates a simple thermocouple circuit.Heatingthe measuringFigure 6 Simple Thermocouple Circuitj u n c t i o n o f t h ethermocouple produces avoltage which is greaterthan the voltage across thereference junction. Thedifference between the twovoltages is proportional tot h e d i f f e r e n c e i ntemperature and can bemeasured on the voltmeter(in millivolts). For ease ofoperator use, somevoltmeters are set up toread out directly intemperature through use ofelectronic circuity.IC-01 Page 6 Rev. 0
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