Thermocouple Instrument

THERMOCOUPLE INSTRUMENT

It is used for the measurement of both AC and DC.

It gives very accurate output about 50MHZ.

It consist of two parts

1. Thermoelectric element and 2. indicating instrument.

The current to be measured is pass through the heater element. It produces heat. This heat is sensed by a thermocouple placed near to the heater. Thermocouple works on the principle of seebeck effect and produces emf in MV range.

Here PMMC meter forms the indicating instrument. Because the output of Thermocouple is DC. The meter indicates the rms value of the current.

This measuring Thermocouple is placed in one arm of a bridge circuit. It produces deflection in the Galvanometer. Feedback is taken and given to another heater element. This heater element is sensed by a compensating thermocouple placed in another arm of the bridge circuit. So this nullifies the meter movement. Now the output dc emf value is the original indication of measured current.

This arrangement is used to eliminate the non linear behaviour of Thermocouple.

Torque equation:-

Thermal emf developed in the circuit is given by

E = a (T₁-T₂) + b (T₁ - T₂)² ……….(1)

where T₁ is the temperature of hot junction in °C.

T₂ is the temperature of cold junction in °C.

a and b are thermocouple constants.

Change in temperature, ΔT = (T₁-T₂)

Heat α I² R

Heat is given by rise in temperature or ΔT.

ΔT α I² R

ΔT = K I²R …..... (2)

Deflection of PMMC meter Ɵ = k₂ E

From equation (1) put the value of E

Ɵ = k₂ [aΔT + bΔT²]

since b is very small neglect the second term.

Ꮎ = k₂ α ΔT

Use equation (2) in the above equation.

Ꮎ = k₂ a (k₁ I²R)

Ꮎ = k₁ k₂ a R I²

Ꮎ = k₃ I²

Ꮎ α I² This instrument exhibits square law response.

Error:

At high frequency current is forced to pass through the outer surface of the conductor. Because the reactance of the inner parts of the conductor is greater than the outer parts. This increases the resistance. This effect is called skin effect.

Heater is suffer from skin effect. In order to eliminate this effect, heater wires are designed with small crossectional area or a fine non magnetic material with very high resistivity is used for making heater elements.

Advantages:

1. High sensitivity

2. Used for the measurement of both AC and DC.

3. Frequency error, stray magnetic field errors are absent.

4. Very accurate for frequency greater than 50MHZ.

5. Current range is 0.5 to 20A.

6. It indicates rms value of current.

7. It gives accurate output for complex waveforms like non sinusoidal inputs, distorted or noisy inputs

Disadvantage:

1. Overload capacity is small.

2. At normal rated current, heater attains a temperature of 300 °C. So heater blows out before fuse blows.