Question 15.3: Refer to the thermocouple circuit in Figure 15–22 . Suppose ...
Refer to the thermocouple circuit in Figure 15–22 . Suppose the thermocouple is measuring 200°C. Again, the circuit board is in an area where the ambient temperature can vary from 15°C to 35°C. The reference thermocouple is held at exactly 0°C. Determine the voltage across the circuit input terminals at the ambient temperature extremes.
From Table 15–2 in Example 15–2 , the thermocouple voltage is 0 V at 0°C. Since the reference thermocouple produces no voltage at 0°C and is completely independent of ambient temperature, there is no error in the measured voltage over the ambient temperature range. Therefore, the voltage across the circuit input terminals at both temperature extremes equals the measuring thermocouple voltage, which is 9.286 mV .
P R A C T I C E EXERCISE
If the reference thermocouple were held at -100°C instead of 0°C, what would be the voltage across the circuit input terminals if the measuring thermocouple were at 400°C?
| TABLE 15–2 • Type-T thermocouple voltage. | |
| TEMPERATURE (°C) | OUTPUT (mV) |
| -200 | -5.603 |
| -100 | -3.378 |
| 0 | 0.000 |
| +100 | 4.277 |
| +200 | 9.286 |
| +300 | 14.860 |
| +400 | 20.869 |