Seebeck Coefficient means a voltage produced by applying difference temperature at two end of material.

*S: Seebeck Coefficient*

*∆V: thermoelectric voltage*

*∆T: difference temperature*

Let join two kind of Conductor, at

*Junction end*has temperature T2 bigger than*Tail end*temperature T1By difference of Temperature T1 and T2, Conductor1 has Seebeck Voltage of V

_{ab}= V_{a}- V_{b}Conductor2 is V

_{cb}= V_{c}- V_{b}Voltage produced at Tail end is:

What if Conductor1 and Conductor2 are same type? They will have same Seebeck coefficient, then same Seebeck Voltage. Therefore V

_{ac}= V_{cb}, V_{ac}= V_{ab}– V_{cb}= 0To build up a Thermocouple, Conductor1 and Conductor2 have to be two different materials; Tail end can produce a voltage then.

So, how to calculate T2?

The temperature vs voltage relationship is given by:

*Where*

*S*

_{12}: Seebeck coefficient of the thermocouple*S*

_{1}: Seebeck coefficient of Conductor1*S*

_{2}: Seebeck coefficient of Conductor2

Voltage of Tail end can be expressed as:

V

_{ac }= V_{a}-V_{c }= S_{12}(T_{2}– T_{1})T

_{2}= V_{ac}/S_{12}+ T_{1}Information of V

_{ac}, S_{12}, T_{1}can be known, therefore T_{2}is also found out by upper equation.T

_{1}, also called as reference temperature, is put on Ice bath to become 0^{o}C, therefore:T

_{2}= V_{ac}/S_{12} In industry, Ice Bath is an integrated circuit called

*Cold Junction compensator*; in fact the cold junction compensator produces a voltage equal to the thermocouple voltage between 0°C and ambient temperature T_{2}Most control module already integrated

*Cold Junction compensator*, user just declares kind of using thermocouple, it will automatically calculated compensation voltage.For example: Yokogawa AST143, Siemens thermocouple module EM 231

There are many kind of Thermocouple

Type R, S and B thermocouples use Pt-base thermoelements and they can operate at temperatures up to 1700°C; however they are more expensive and their voltage output is lower than type K and type N thermocouples, which use Ni-base thermoelements. However, Ni base thermocouples can operate at lower temperatures than the Pt-base ones.

*Voltage vs Temperature relationship for letter-designated thermocouples*

*Approximate composition for thermoelements of letter-designated thermocouples*

*Type K Seebeck coefficient vs Temperature*

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