|
| |
| |
| Thermocouples |
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.
| Type | Metals | ASTM Color | EMF (mV) Over Temp Range | Temperature Range | | 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.
| |
|
|
|