thermocouple instruments are based on the seebeck effect which states that if the two dissimilar metals having different work functions are joined together to form a junction and if the junction is subjected to change in temperature then voltage is generated at the junction. Such a junction is called thermocouple. The e.m.f. generated is proportional to the temperature difference and is called thermo-electric e.m.f.
The essential elements of a thermocouple instruments are,
1. The heater element : This is nothing but a fine wire which carries the current to be measured. The heater wire is made up of an alloy which has almost zero temperature coefficient of resistance.
2. Thermocouple : This is a junction of two dissimilar metals such as iron and copper-nickel alloy. Its junction is in thermal contact with the heater elements and gets subjected to rise in temperature due to heat produced by the current to be measured. The e.m.f. is available at the output of thermocouple.
3. PMMC Instrument : A sensitive permanent magnet moving coil instrument is necessary to sense the e.m.f. generated at the junction. The deflection of this instrument is proportional to the thermo-electric e.m.f.
The arrangement of thermocouple instrument is shown in the Fig. 1.
|Fig. 1 Basic arrangement of thermocouple instrument|
The e.m.f. produced is proportional to heat i.e. r.m.s. value of the current to be measured. Thus scale of the PMMC instrument can be calibrated to read the current to be measured. The combination of heater element alongwith thermocouple is called thermo-element.
1.1 Principle of Operation
The thermoelectric e.m.f. generated in a thermocouple is proportional to the difference of temperatures of hot and cold junctions. This relation is parabolic in nature and given by,
where a, b = constants depending o metalsT1 - T2 = Temperature difference of hot and cold junctions
Let Δt = T1 - T2 = Difference in temperatures
The equation (2) shows that thermo-electric e.m.f. e has a parabolic relationship with the temperature difference Δt. The constant a is of the order of 40 to 50 μv per oC difference of temperature. The constant b is of the order of few tenths or hundredths of a micorvolt per (oC)2.
The heater element carries the current to be measured and heat produced is proportional to square of the r.m.s. value of the current. Thus the rise in temperature of hot junction is proportional to I2R where I is the r.m.s. value of the current and R is the resistance of the heater element.
If the cold junction is maintained at ambient temperature then the rise in temperature of hot junction is equal to temperature rise of hot junction above the ambient temperature.
Note : The equation (3) shows that thermocouple instruments show the square low response.Practically the value of constant b is very small and can be neglected.
This e.m.f. drives the PMMC instrument to cause the deflection proportional to e.m.f. e.
where K3 = K1 K2 a R = constant
The above square law is practically not exact as,
1. The heater resistance R is not constant but varies with temperature due to positive or negative temperature coefficient of the heater element. Hence constant K3 is not exactly constant.
2. Practically e.m.f. varies as Δt2 and not only Δt. But the effects of above are not significant and thus if air gap field of PMMC instrument is uniform, the scale can be calibrated interms of I2.