Thermocouples are widely used for industrial temperature measurement. However, incorrect wiring—especially reversed polarity—can lead to misleading readings. This article explains two common scenarios where thermocouple polarity errors occur and the resulting effects on the displayed temperature.
1. Thermocouple Reversed with Compensation Cable
When a thermocouple is connected to an instrument through extension or compensation cables, the polarity must match correctly to preserve the thermoelectric voltage. If the thermocouple is wired in reverse to the compensation cable, the error depends on the temperature difference between the connection point and the instrument (cold junction).
Possible Outcomes:
🔸 If the connection point is hotter than the instrument/cold junction:
The compensation cable would normally generate a positive thermal EMF.
When reversed, it becomes a negative contribution, reducing the total signal.
✅ Result: Displayed temperature will be lower than actual.
🔸 If the connection point is colder than the cold junction:
The cable should generate a negative EMF under normal wiring.
When reversed, it adds positively to the signal.
✅ Result: Displayed temperature will be higher than actual.
🔸 If the connection point equals the cold junction temperature:
No EMF is generated by the compensation cable.
✅ Result: No effect on the reading, even if reversed.
2. Thermocouple Reversed at Instrument Terminals
If the thermocouple leads are directly connected to the measuring instrument with reversed polarity, the instrument interprets the polarity as reversed current flow.
Result:
The instrument shows a negative temperature (e.g., below zero), regardless of the actual temperature.
This is not a case of “higher or lower” readings but rather a directional inversion.
✅ Result: Displayed temperature appears as a negative value.
Summary Table
| Wiring Error | Condition | Effect on Reading |
|---|---|---|
| Thermocouple vs. Compensation Cable Reversed | Connection point > Cold junction | Reading too low |
| Connection point < Cold junction | Reading too high | |
| Connection point = Cold junction | No effect | |
| Thermocouple vs. Instrument Reversed | Any condition | Reading shows negative |