Question

In the construction of a thermoelectric thermometer the thermocouple must be selected such that the range of temperatures to be measured must be:
A) less than cold junction temperature
B) In between cold junction temperature and neutral temperature
C) In between neutral temperature and temperature of inversion
D) Greater than the temperature of inversion

Answer 1

Hint: Thermocouple of one junction at a lower temperature is called temperature cold junction and the other side at a higher temperature is called temperature of the hot junction. It doesn't measure less than cold junction temperature as well as Greater than the temperature of inversion

Complete step by step answer:
A thermocouple is a device having two wire legs made from a different metal. It is used to measure temperature. The wire legs are connected at one and create a junction. The temperature is measured in this junction. When the temperature changes, a voltage is created in that device.
Thermocouple of one junction at a lower temperature is called temperature cold junction and the other side at a higher temperature is called temperature of the hot junction.

Now we consider the given options below,

A) Range of temperatures to be measured must be less than the cold junction temperature – this is not right as a thermocouple measures the temperature change between lower range to higher range.

B) Range of temperatures to be measured must be In between cold junction temperature and neutral temperature. This is the correct answer because thermocouple measures the temperature change between lower range to higher range as one junction at a lower temperature is called temperature cold junction and the other side at a higher temperature is called the temperature of the hot junction.

C) Range of temperatures to be measured must be In between neutral temperature and temperature of inversion – this is not the correct option.

D) Range of temperatures to be measured must be greater than the temperature of inversion – this is not right. The thermocouple measures the temperature change between lower range to higher range.

Hence, option(B) is the correct option.

Note:
There are two types of thermocouples – type $J$, $K$, $T$, and $E$ are the most common types of thermocouple called ‘base metal’ thermocouples. And type $R$, $S$, and $B$ thermocouples are called ‘noble metal thermocouples that are used in applications of high temperature.