THE MAGAZINE FOR COMPUTER APPLICATIONS
Circuit Cellar Online offers articles illustrating creative solutions
and unique applications through complete projects, practical
tutorials, and useful design techniques.

THE BASICS OF THERMOCOUPLES

by Bob Perrin

Start ı A Tail of Two Metals ı Breaking the Loop ı Measuring Temperature ı Practical Matters ı Circuits ı Closing Time ı Sources and PDF

A TAIL OF TWO METALS

Figure 1a shows a thermocouple. One junction is designated the hot junction. The other junction is designated as the cold or reference junction. The current developed in the loop is proportional to the difference in temperature between the hot and cold junctions. Thermocouples measure differences in temperature, not absolute temperature.

Figure 1aıTwo wires are all that are required to form a thermocouple.

To understand why a current is formed, we must revert to physics. Unfortunately, Iım not a physicist, so this explanation may bend a concept or two, but Iıll proceed nonetheless.

Consider a homogenous metallic wire. If heat is applied at one end, the electrons at that end become more energetic. They absorb energy and move out of their normal energy states and into higher ones. Some will be liberated from their atoms entirely. These newly freed highly energetic electrons move toward the cool end of the wire. As these electrons speed down the wire, they transfer their energy to other atoms. This is how energy (heat) is transferred from the hot end to the cool end of the wire.

As these electrons build up at the cool end of the wire, they experience an electrostatic repulsion. The not-so-energetic electrons at the cool end move toward the hot end of the wire, which is how charge neutrality is maintained in the conductor.

The electrons moving from the cold end toward the hot end move slower than the energetic electrons moving from the hot end move toward the cool end. But, on a macroscopic level, a charge balance is maintained.

When two dissimilar metals are used to form a thermocouple loop, as in Figure 1a, the difference in the two metalıs affinity for electrons enables a current to develop when a temperature differential is set up between the two junctions.

As electrons move from the cold junction to the hot junction, these not-so-energetic electrons are able to move easier in one metal than the other. The electrons that are moving from the hot end to the cold end have already absorbed a lot of energy, and are free to move almost equally well in both wires. This is why an electric current is developed in the loop.

I may have missed some finer points of the physics, but I think I hit the highlights. If anyone can offer a more in-depth or detailed explanation, please e-mail me. One of the best things about writing for a technical audience is learning from my readers.

PREVIOUS • NEXT

Circuit Cellar provides up-to-date information for engineers. Visit www.circuitcellar.com for more information and additional articles.
For subscription information, call (860) 875-2199, subscribe@circuitcellar.com or subscribe online. ıCircuit Cellar, the Magazine for Computer Applications. Posted with permission.