In 1822, an Estonian physician named Thomas Seebeck, discovered that, the junction between two metals generates a voltage, which is a function of temperature.
Thermocouples use this fact, the so-called Seebeck effect to measure the temperature.
Thermocouples (TC) are based on the principle that, when two dissimilar metals are joined, a proportional voltage is generated which is related to the difference in temperature between the measuring junction and the reference junction.
Because of their easy construction, they are widely used in industry.
Though they are very simple to construct, they have got their own advantages and disadvantages.
Advantages - 1.
Their construction is very simple.
2.
They are very rugged.
So handling in harsh environment is easy.
3.
No external power is needed.
4.
They can operate over wide temperature range.
Disadvantages - 1.
They are not having good accuracy 2.
They provide very small output 3.
The output is non-linear.
Despite of their disadvantages, they are used widely because of their simplicity and large temperature range.
As we have seen the thermocouple uses Seeback effect, any two different metal junction can be used to make a thermocouple.
Some metal combination give more and predictable outputs, so over the period, some of these combinations have found more useful than others.
According to their characteristics, some thermocouples made up of these combinations of metals, have been classified or named.
They are as under.
1.
Type J - This type is made from Iron and Constantan.
They are used to measure temperature from -40 to 750 degree Centigrade.
2.
Type K - This type is made from Chromel and Alumel.
This is far most popular and useful, as its range is wide and it is of low cost.
It operates from -200 to 1200 degree C.
Very good for general purpose use.
3.
Type R - This type is made from Platinum and Rhodium.
This can be used for temperature from -59 C to 1600 C.
Because of its high cost, this is less popular.
4.
Type S - This type of is also made from Platinum and Rhodium.
This can be used for temperature from -50 to 1600 C.
Because of its high cost, this is less popular.
Though this is more stable then Type R.
5.
Type B - This type of is also made from Platinum and Rhodium.
This can be used for temperature up to 1600 C.
Because of its high cost this is less popular.
6.
Type E - This type is made from Chromel and Constantan.
This can be used for temperature -270 C to 790 C.
Because of its capability to measure very low temperature, it is used to measure cryogenic temperature.
7.
Type N - This type is made from Nicrosil and Nisil.
Seen as good improvement over type K, this is used to measure -260 to 1300 C.
Because of its high stability and resistance to high temperature oxidation, this type is used to measure high temperature without using costly thermocouples like R, S or B Types.
Though these are used to measure different type of temperature, choosing the right kind of thermocouple is very important.
The output of any thermocouple though predictable, is non-linear.
The exact temperature has to be derived from the table, that has been defined for all types of thermocouples.
Each one has their own characteristics so whenever you use any TC, make sure to use the relevant table.