Thermocouple is a device which is widely used as a pyrometer on board ship for continuous measurement of temperature for various machineries like Main engine, Auxiliary engine, gas turbines etc.
It is absolutely important to choose the correct thermocouple material for different temperature range operation, depending upon the machinery and thermocouple location, where the parameter has to be measured.
Construction of Thermocouple
A thermocouple consists of two dissimilar homogeneous materials connected together. The materials used depend upon the application and usage. Normally, following materials are used for different temperature range:
Copper – constantan (copper nickel alloy) for range of -200 to 400 °C
Iron – constantan for temperature range of −40 to +750 °C
Chrome – alumel (alloy of nickel, manganese, aluminium and silicon) for temperature range of −200 to +1350 °C.
These materials are connected together in a ceramic sheath covered again by a metal sheath and fitted at desired locations like exhaust manifold etc. The one end of the thermocouple is placed in hot junction and other end is kept in a constant cold junction. These materials are led to a temperature indicator through amplifier and compensator lead. The amplifier and compensator lead (normally made up of copper and dose not effect the circuit) helps in transmitting the output to a remote location.
Principle & Working
Thermocouple works with the principle of “seebeck effect” which states that-“Temperature between two dissimilar metals in a circuit converts into electric current”
When two dissimilar metal wires joined, suppose iron and constantan, are coupled together and exposed to difference in temperature at both the ends, then emf is generated and current flows from hot to cold side.
The magnitude of the current depends on the temperature difference between the junctions.
If one junction is kept at constant temperature, then the value of temperature for the other junction can be easily determined.
Advantages of Thermocouple
- Can be used for remote temperature sensing.
- It can be used for continuous temperature sensing.
- Temperature difference can be measured with high accuracy.
- They are inexpensive and easily interchangeable.
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