131913-2000 EDT
Why is anyone concerned about wire resistance in a thermocouple for instrumentation purposes?
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I think the discussion was changed over to RTD's at some point once it was brought out that switching TC's via a relay is not in most cases feasible because of the errors that would be introduced by adding new uncontrolled junctions made of materials of a different composition then the TC conductors are made of.
Thermocouples work because there is two junctions at two different temperatures called the Seebeck effect (named after the physicist Thomas Johann Seebeck), the hot junction and the cold junction, the cold junction is the compensated junction in most high temperature measurements and is located in the measuring equipment, in the case of very low temperature measurements the hot junction is the compensated junction.
Since the voltage developed is more dependent upon the materials used to make the TC including the conductors to the cold junction (reference junction) because the measuring device is of such a high impedance the resistance of the circuit is not important, but the type of materials are very important and is why installing a relay or switch between the two points is in most cases is out of the question unless they are specially made for that type of TC, like a K, R or S type TC, at work we use extension cables that run to our PLC TC inputs or TC to 4-20ma transmitters, but even these extension cables and the connectors are specially designed for the type of TC that is being used, using any other type of conductors, terminals, or relay contacts, (even installing crimp on connectors on the leads) not designed for the type of TC will remove the ability to correctly measure the temperature at the hot junction.
And yes because of the above they do make crimp on connectors for each type of TC and for each polarity and are color coded for each wire and type, just not very common.
