Thermocouple is a temperature measuring device which works on the principle of thermoelectric effect. The temperature difference between the two junctions is converted to voltage. According to German physicist Thomas Johann Seebeck when two different metals are connected at one end and create a temperature difference between two ends.
Principle of working
A thermocouple has two plates of different metals which are connected at one end called a junction. There are two junctions in a thermocouple one is called a hot junction and the other is a cold junction. Then hot junction is always placed on the surface where we want to measure the temperature. Then other end is at lower temperature and is called a cold junction.
According to the Seeback effect the temperature difference between the two different metals induces the potential differences between two points of the thermocouple plates.
There is a very small amount of current in the circuit. The voltmeter measures the voltage which is the function of temperature difference between the two junctions. The metals used are selected such that the change in temperature and voltage produced are linear.
Types of Thermocouple
T – Type
They are used for low temperatures. Then Positive wire is of Cu and Negative wire of Constantan. most importantly it can be used up to 350 degree C.
E – Type
They are very sensitive and can measure up to 850C. Then Positive wire is of Chromel Negative wire of Constantan.
J – Type
This is the most common type of thermocouple which has Positive wire of Iron and Negative wire of Constantan. They can also be used upto 1000oC. Also, they are highly stable.
K — Type
They are most commonly used as it is cheap. Then Positive wire is made of Chromel and Negative wire of Alumel. Also it can be used up to 1200oC.
S – Type
Moreover, they are highly accurate. Then Positive wire is made of Platinum 10% and Rhodium and Negative wire of Platinum. It can also be used up to 1400oC.
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