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Measuring temperature with a thermocouple is somewhat difficult because the output of the thermocouple is low. It is also complicated because additional thermocouples are created at the point where the thermocouple wires make contact with the copper wires (or traces) that connect to the signal conditioning circuitry. This point is called the cold junction. The junction between metal A and metal B is the main thermocouple junction. Other thermocouples are present where the metal A and metal B wires join with the measuring device’s copper wires or PCB traces. The net effect is that the output voltage of this circuit appears to be the voltage due to the thermocouple minus the voltage of a similar thermocouple at the cold junction temperature. Therefore, if the thermocouple is at +525°C and the cold junction is at +25°C, VOUT will indicate +500°C. To correct for this error, the engineer must measure the cold junction temperature and add that temperature to the value indicated by VOUT: VOUT = VTC – VCJ and therefore VTC = VOUT + VCJ. This is done by placing a temperature sensor at the cold junction location and using the measured temperature to compensate for the cold junction temperature.
PTM Published on: 2015-04-09
