Question

Assertion: A thermocouple cannot be used for measuring temperature beyond neutral temperature
Reason: Neutral temperature is independent of temperature of cold junction.
A. Both A and R are true and R is the correct explanation of A.
B. Both A and R are true and R is not the correct explanation of A.
C. A is true R is false.
D. A is false R is true.

Answer 1

Thermocouple is used to measure temperature and it’s a type of a sensor. In accordance to the variation of thermo emf with temperature, the thermo emf which is generated due to the Seeback effect of the thermocouple, it is of the maximum value when it is at neutral temperature at the junction which produces the emf.

Complete answer:
A Thermocouple is a type of sensor which is used to measure temperature. Thermocouple consists of two wired legs each made from different metals. The wires legs are welded together from one end, hence creating a junction. This junction is the place where temperature is measured. When the junction experiences a change in temperature, a voltage is created.

In accordance to the variation of thermo emf with temperature, the thermo emf is of maximum value at neutral temperature beyond this it decreases so beyond this temp thermocouple is not used for measuring temp. Hence the assertion is correct. Now, the neutral temperature of a thermocouple is given by:
${T_n} = \dfrac{{{T_i} + {T_c}}}{2}$
Here, ${T_n}$ is the neutral temperature, while ${T_i}$ is the inversion temperature of the thermocouple. Tc is the temperature of the cold junction. Thus, the neutral temperature is dependent upon the cold junction.

Hence option C is the correct answer.

Note: Seeback effect states that when two different conductors are joined to form a circuit just like in thermocouples and the two junctions are held at the different temperature then an emf is produced which results in the flow of the electric current throughout the circuit. Magnitude of thermo-electric emf depends only upon the nature of the two metals and also on the temperature difference between terminals.