Wednesday, October 31, 2018

Dissimilar Metal Junctions (How Thermocouples Work)

When two dissimilar metal wires are joined together at one end, a voltage is produced at the other end that is approximately proportional to temperature. That is to say, the junction of two different metals behaves like a temperature-sensitive battery. This form of electrical temperature sensor is called a thermocouple:

This phenomenon provides us with a simple way to electrically infer temperature: simply measure the voltage produced by the junction, and you can tell the temperature of that junction. And it would be that simple, if it were not for an unavoidable consequence of electric circuits: when we connect any kind of electrical instrument to the thermocouple wires, we inevitably produce another junction of dissimilar metals. The following schematic shows this fact, where the iron-copper junction J1 is necessarily complemented by a second iron-copper junction J2 of opposing polarity:


Junction J1 is a junction of iron and copper – two dissimilar metals – which will generate a voltage related to temperature. Note that junction J2, which is necessary for the simple fact that we must somehow connect our copper-wired voltmeter to the iron wire, is also a dissimilar-metal junction which will also generate a voltage related to temperature. Further note how the polarity of junction J2 stands opposed to the polarity of junction J1 (iron = positive ; copper = negative). A third junction (J3) also exists between wires, but it is of no consequence because it is a junction of two identical metals which does not generate a temperature-dependent voltage at all.

The presence of this second voltage-generating junction (J2) helps explain why the voltmeter registers 0 volts when the entire system is at room temperature: any voltage generated by the iron-copper junctions will be equal in magnitude and opposite in polarity, resulting in a net (series-total) voltage of zero. Only when the two junctions J1 and J2 are at different temperatures will the voltmeter register any voltage at all.


Reprinted from "Lessons In Industrial Instrumentation" by Tony R. Kuphaldt – under the terms and conditions of the Creative Commons Attribution 4.0 International Public License.