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

A grounded junction is recommended for the measurement of static or flowing corrosive gas and liquid temperatures and for high-pressure applications. Grounded thermocouples have a very good response time because the thermocouple is making direct contact with the sheath, allowing heat to transfer easily. However they are much more susceptible to electrical noise and ground loops.

An ungrounded junction is recommended for measurements in corrosive environments where it is desirable to have the thermocouple electronically isolated from and shielded by the sheath. Ungrounded thermocouples offer better resistance to electrical noise and ground loops.

An exposed junction is recommended for the measurement gas temperatures where fast response time is required. The response time is very quick, but exposed thermocouple wires are more prone to corrosion and degradation.

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%
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