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| Numerical 08 |
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08. 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? |
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| Suggested solution: |
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| The temperature qn (neutral temperature) corresponding to maximum emf is given by |
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| 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. |
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