Thermocouples

Thermocouples contain two electrical conductors made of different materials which are connected at one end. The end of the conductors which will be exposed to the process temperature is called the measurement junction. The point at which the thermocouple conductors end (usually where the conductors connect to the measurement device) is called the reference junction. When the measurement and reference junctions of a thermocouple are at different temperatures, a millivolt potential is formed within the conductors. Knowing the type of thermocouple used, the magnitude of the millivolt potential within the thermocouple, and the temperature of the reference junction allows the user to determine the temperature at the measurement junction. The millivolt potential that is created in the thermocouple conductors differs depending on the materials used. Some materials make better thermocouples than other because the millivolt potentials created by these materials are more repeatable and well established. These thermocouples have been given specific type designations such as Type E, J, K, N, T, B, R and S.

Thermocouple Types :

Type Material Composition Temperature Range EMF
positive (+) negative (-)
T Copper Constantan -270°C to +400°C -6.2 to 20.87 mV
E Chromel Constantan -270°C to +1000°C -9.8 to 76.37 mV
J Iron Constantan -210°C to +760°C -8.09 to 69.55 mV
K Chromel Alumel -270°C to +1370°C -6.4 to 54.88 mV
R Pt-13% Rh Platinum -50°C to +1760°C 0.2 to 21.10 mV
B Pt30% Rhodium Pt 6% Rhodium 0°C to + 2000°C 0.0  to 13.82 mV
S Platinum/10% Rhodium Platinum -50°C to +1750°C -0.23 to 18.69 mV
N Nicrosil Nisil -270°C to 1300°C -4.34 to 47.51 mV

 

Thermocouple Junction Types :

Thermocouple-Junction-Types-text


Thermocouple Accuracy :

Type Limits of Error ( Whichever is Greater )
  Standard Special
K ± 2.2 ˚C  or ±0.75% ±1.1˚C  or ±0.4%
T ± 1.0˚C  or ±0.75% ± 0.5˚C  or ±0.4%
J ± 2.2 ˚C  or ±0.75% ± 1.1˚C  or ±0.4%
N ± 2.2˚C  or ±0.75% ± 1.1˚C  or ±0.4%
E ± 1.7˚C  or ±0.50% ± 1.0˚C  or ±0.4%
S ± 1.5˚C  or ±0.25% ± 0.6˚C  or ±0.1%
R ± 1.5˚C  or ±0.25% ± 0.6˚C  or ±0.1%