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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.