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Thermocouples work on the
Seebeck principle, discovered accidentally in 1822 by an Estonian physician named Thomas Seebeck. Basically, the
Seebeck principle explains how two dissimilar metals generate a temperature-dependent flow of current at their
welded junction when attached to a closed circuit. Seebeck discovered that a compass needle would be
deflected when a closed loop was formed of two metals joined in two places with a temperature difference between
the junctions. The metals respond to temperature by assuming different voltage potentials, thereby generating a
current related to the temperature of the junction. This table lists the properties of the most
common thermocouple junctions.
| B |
Platinum30% Rhodium (+) Platinum 6% Rhodium (-) |
Gray Red |
0 to 13.820 |
0 to 1700°C (32 to 3092°F) |
| C |
W5Re Tungsten 5% Rhenium (+) W26Re Tungsten 26% Rhenium (-) |
White Red |
0 to 37.066 |
0 to 2320°C (32 to 4208°F) |
| E |
Chromel (+) Constantan (-) |
Violet Red |
-9.835 to 76.373 |
-200 to 900°C (-328 to 1652°F) |
| G |
Tungsten 26% (+) Rhenium W-26% (-) |
White Red |
0 to 38.564 |
0 to 2320°C (32 to 4208°F) |
| J |
Iron (+) Constantan (-) |
White Red |
-8.095 to 69.553 |
0 to 750°C (32 to 1382°F) |
| K |
Chromel (+) Alumel (-) |
Yellow Red |
-6.458 to 54.886 |
-200 to 1250°C (-328 to 2282°F) |
| N |
Nicrosil (+) Nisil (-) |
Orange Red |
-4.345 to 47.513 |
270 to 1300°C (-450 to 2372°F) |
| R |
Platinum 13% Rhodium (+) Platinum (-) |
Black Red |
-0.226 to 21.101 |
0 to 1450°C (32 to 2642°F) |
| S |
Platinum 10% Rhodium (+) Platinum (-) |
Black Red |
-0.236 to 18.693 |
0 to 1450°C (32 to 2642°F) |
| T |
Copper (+) Constantan (-) |
Blue Red |
-6.528 to 20.872 |
-200 to 350°C (-328 to 662°F) |
Thermocouple types R, S, and B are constructed of platinum and rhodium, and are referred to as noble metal
thermocouples. They are more accurate and more stable than base metal types, but are more expensive.
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