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Working Of Thermocouple

 



A thermocouple circuit made up from joining two rivers A and B made of  dissimilar metals. Due to the Seebeck effect, a net e.m.f is generated in the circuit which depends on the difference in temperature between the hot and cold junctions and is , therefore, a thermometric property of the circuit. This e.m.f  can be measured by a microvoltmeter to a high degree of accuracy. The choice of the metals depends largely on the temperature range to be investigated, and copper- constantan, chromel - alumel and platinum - platinum - rhodium are typical combinations in use. 

A thermo couple is calibrated by measuring the thermal e.m.f  at various known temperatures, the reference junction being kept  at 0*C. The results of such measurements on most thermocouples can usually be represented by a cubic equation of the form :

                               E = a + bt + cr2 + dt3 

Where E is the thermal e.m.f  and the constants a,b,c and d are different for each thermocouple. 

The advantage of a thermocouple is that it comes to thermal equilibrium with the system, whose temperature is to be measured, quite rapidly, because its mass is small. 

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