THERMOCOUPLE

∙ Principle of Operation∙ Materials Used∙ Advantages∙ Applications∙ Comparison with RTD∙ Limitations

By

Anand BongirGirjashankar

Mishra

• A thermocouple is a junction between two different metals that produces a voltage related to a temperature difference.

Thermocouples are based on the principle that two wires made of dissimilar materials connected at either end will generate a potential between the two ends that is a function of the materials and temperature difference between the two ends (also called the Seebeck Effect).

Principle of Operation

Seebeck Effect

Materials Used

Type E:Chromel – Constantan

• 401 to 900° C

• 68 µV/°C

Type K:Chromel – Alumel

• Range: −200 °C to +1350 °C

• Sensi: 41 µV/°C

Type J:Iron – Constantan

• −40 to +750 °C

• 55 µV/°C

Type N:Nicrosil – Nisil

• >1200 °C

• 39 µV/°C

Advantages• It is rugged in construction• Covers a wide temperature range• Using extension leads and compensating cables, long

transmission distances for temperature measurement possible. This is most suitable for temperature measurement of industrial furnaces

• Comparatively cheaper in cost• Calibration can be easily checked• Offers good reproducibility• High speed of response• Satisfactory measurement accuracy

Limitations

• For accurate temperature measurements, cold junction compensation is necessary

• The emf induced versus temperature characteristics is somewhat nonlinear

• Stray voltage pickup is possible• In many applications, amplification of signal is

required

Applications

• Type B, S, R and K thermocouples are used extensively in the steel and iron industries to monitor temperatures and chemistry throughout the steel making process.

• Gas-fed heating appliances such as ovens & water heaters.

• In the testing of prototype electrical and mechanical apparatus

Comparison with RTDParameter Thermocouple RTD

Typical measurement range

-267°C to 2316°C -240°C to 649°C

Characteristic Linear Nonlinear

Sensitivity Medium Medium

Speed of response High High

Type of transducer Active Passive

Accuracy Moderate High

Size Small Large

Cost Low High

Material Used Copper-constantan, Iron-constantan etc.

Nickel, Copper, Platinum etc.

Compensation Cold junction compensation required

Not required

Applications Suitable for applications which require wide temperature range

Suitable where speed of response and accuracy are

more important