For a copper-iron and a chromel-alumel thermocouple, the plots between thermoelectric emf and the temperature q of the hot junction (when the cold junction is at qoC) are found to satisfy approximately the parabola equation with
a = 14 m V o C -1, b = - 0.04m VoC-2 (Copper -iron)a= 41m VoC-1,b = 0.002m VoC-2 (Chromel - alumel)Which of the two thermocouples would you use to measure temperature in the range of about 500oC to 600oC?
The temperature qn (neutral temperature) corresponding to maximum emf is given by
In actual practice, the coefficients a and b vary slightly and the null temperature for Cu-Fe thermocouple is nearly 482oF. A thermocouple cannot be used beyond the null temperature because a given emf corresponds to two different values of q. Thus, for measuring temperatures beyond about 250oC, a Cu-Fe thermocouple is unsuitable. The chromel-alumel thermocouple with the given values of a and b shows no maximum for q > 0. Thus, it can be used for measuring any high temperature and is suitable for the required range (932oF - 1112oF) in the question. In actual practice, its use is limited to nearly 2732oF.
