question_answer

The temperature of inversion of a thermocouple is \[620{}^\circ C\] and the neutral temperature is \[300{}^\circ C\].  What is the temperature of cold junction?                                                              [AIPMT (S) 2005]

A)  \[20{}^\circ C\]

B)                   \[320{}^\circ C\]

C)  \[-20{}^\circ C\]

D)                   \[40{}^\circ C\] Correct Answer: C Solution : [c] Key Idea: The temperature of inversion is higher than the neutral temperature by the same amount as the neutral temperature is higher than the temperature of cold junction. As per Key Idea

\[{{t}_{n}}-{{t}_{0}}={{t}_{i}}-{{t}_{n}}\]

\[\Rightarrow \]   \[{{t}_{0}}=2{{t}_{n}}-{{t}_{i}}\]

Given,   \[{{t}_{i}}={{620}^{o}}C,\,\,{{t}_{n}}={{300}^{o}}C\]

Hence, \[{{t}_{0}}=\] temperature of cold junction

\[=2\times 300-620\]

\[=600-620=-20{}^\circ C\]

Note:    Neutral temperature does not change for a metal whether the temperature of cold junction and inversion temperature change by any value.