1. 2130903 ELECTRICAL & ELECTRONICS MEASURING INSTRUMENTS
PREPARED BY:-
Sabva Vipul

2. THERMOCOUPLE INSTRUMENT/ TRANSDUCER

3. Seeback Effect
In these instruments use of seeback effect is made according to that if the junction of two wires of dissimilar metals ( such as iron and a copper-nickle alloy) is heated and the free or cold ends are connected to a millivoltmeter , an emf is caused.

4. Elements of Thermocouple Instrument
1. Heater element
2. Thermocouple
3. PMMC Instrument

5. Principle of Operation The thermoelectric e. m. f
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
e = a(T1-T2) + b(T1-T2)2
Where a, b = Constants depending on metals.
T1 — T2 Temperature difference of hot and cold junctions. Let  t = T1 — T2 = Difference in temperatures.

6. e= a t + b t (2)
The equation (2) shows that thermo-electric e.m.f. (e) has a parabolic relationship with the temperature difference  t.
The constant a is of the order of 40 to 50 μV per °C difference of temperature.
The constant b is of the order of few tenths or hundredths of a microvolt per (°C)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 I2 R where I is the r.m.s. value of the current and R is the resistance of the heater element.

7. 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.  t = Rise in temperature
And  t  I2 R i.e.  t = K1 I2 R …(3)
Note: The equation (3) shows that thermocouple instruments show the square law response.

8. Practically the value of constant b is very small and can be neglected
Practically the value of constant b is very small and can be neglected.  e=a t e = a K1 I2 R This e.m.f. drives the PMMC instrument to cause the deflection Ѳ proportional to e.m.f. e. Ѳ = Deflection of instrument  e. Ѳ = K2e=K2[a K1 I2 R] Ѳ = K3 I2 where K3 = K1 K2 a R = Constant