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Types of Thermocouples

A thermocouple consists of two dissimilar metals, joined together at one end, which produce a small voltage when heated (or cooled). This voltage is measured and used to determine the temperature of the junction of the dissimilar metals. The voltage for any one temperature is unique to the combination of metals used.
 
Thermocouples are available in different combinations of metals, usually refered to by a letter, e.g. type J thermocouple, type K etc. Each combination has a different temperature range and is therefore more suited to certain applications than others. Although it is worth noting that the maximum temperature varies with the diameter of the wire used in the thermocouple.
 
Guidance for the selection and use of devices for measuring temperature is given in the relevant sections of British Standards Specification, BS 1041, Temperature Measurement.
 
 
ASTM
Type
Conductor
Combination
Temperature Range
°F °C
B Platinum 30% Rhodium /
Platinum 6% Rhodium
2500 to 3100 1370 to 1700
E Nickel-chromium / Constantan 32 to 1600 0 to 870
J Iron / Constantan 32 to 1400 0 to 760
K Nickel-chromium / Nickel-aluminum 32 to 2300 0 to 1260
N Nicrosil / Nisil 32 to 2300 0 to 1260
R Platinum 13% Rhodium /
Platinum
1600 to 2640 870 to 1450
S Platinum 10% Rhodium /
Platinum
1800 to 2640 980 to 1450
T Copper / Constantan -75 to +700 -59 to +370

 

Type K Thermocouples

Type K are the most common thermocouple in the Oil & Gas, and refining industries due to their wide measuring range and low cost. They are occasionally referred to as Chromel-Alumel thermocouples, though this is a dated term. Note that above about 750°C oxidation leads to drift and the need for recalibration.
Practical Temperature Measurement
Practical Temperature Measurement
Handbook of Temperature Measurement
Handbook of Temperature Measurement
Thermocouples: Theory and Properties
Thermocouples: Theory and Properties


 

 
 
















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