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Thermocouples raise reader temperatures
October 24, 2008
In the October print issue, my article "
Thermocouples often described incorrectly" has raised a few eyebrows. In the article, I address the often misunderstood idea that a voltage develops across a thermocouples junction when a voltage actually develops across a wire when subjected to a thermal gradient. Several readers sent e-mails both prasing and criticizing the article. For example:
It was good to see your short piece on thermocouples in TMWorld. I have encountered more than one EE college professor who did not understand how a T/C works. It would be useful to tell your readers just where between the wire ends the emf is actually generated – in regions along the wire that are experiencing a thermal gradient.
This can become crucial to understand when interpreting the calibration of a ‘damaged’ T/C (where the damage may be mechanical, chemical, over-temperature operation, etc.). If in the cal lab the calibration thermal gradient is imposed on an undamaged length of the T/C conductors, then the resulting calibration looks good and misleads the calibration user. When a damaged T/C is calibrated by imposing the calibration thermal gradient on the ‘damaged’ region of the wire, then the results of the damage show up in the calibration.
Others questioned the fact that I didn't discuss cold-junction compensation, which you need because the junctions of thermocouple wires and terminal block form additional thermocouples and instrumentation must compensate for the error voltages that can occur. Because of the nature of the Test Digest page, I didn't have the space to cover that important topic.
Thermocouple information is available through many online sources. To learn more, see the thermocouple section in
Omega Engineering's technical reference section. There, you can follow the "
Using Thermocouples" link for a detailed description of thermocouple measurement principles. Other links discuss thermocouple properties and construction.
Posted by Martin Rowe on October 24, 2008 | Comments (1)
