Explain about Thermocouples?

A thermocouple consists of two wires of dissimilar metals welded together into a junction. At the other end of the signal wires, usually as part of the input instrument, is another junction called the reference junction. Heating the sensing junction generates a thermoelectric potential (emf) proportional to the temperature difference between the two junctions.

This millivolt-level emf, when compensated for the known temperature of the reference junction, indicates the temperature at the sensing tip. Published millivolt tables assume the reference junction is at 0°C. Thermocouples are simple and familiar.

Designing them into systems, however, is complicated by the need for special extension wires and reference junction compensation.

Thermocouple advantages include:

• Extremely high temperature capability: Thermocouples with precious metal junctions may be rated as high as 1800°C.

• Ruggedness: The inherent simplicity of thermocouples makes them resistant to shock and vibration.

• Small size/fast response: A fine-wire thermocouple junction takes up little space and has low mass, making it suitable for point sensing and fast response.

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