Fault Finding Guides · Free PDF
Temperature Measurement Fault Finding Guide
A one page guide to temperature measurement problems with thermocouples, RTDs and transmitters.
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A one page A4 guide to print for the workshop or keep on your phone in the field. The full content is also shown below.
Download PDF A4 · 54 KBBefore you start
- Permit, isolation and control room informed.
- Put the loop in manual or inhibit trips as authorised.
- Use certified test equipment in hazardous areas.
- Record as found readings before changing anything.
Quick checks
- Sensor type and wiring match configuration?
- Resistance or mV at ambient sensible?
- Element touches well bottom?
- Compare with a portable probe.
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Symptoms, causes and fixes
| Symptom | Likely causes | Checks and fixes |
|---|---|---|
| Reads upscale or fault | Open element or wire, burnout detection | Check continuity at head; replace element |
| Moves wrong way | Thermocouple polarity reversed | Swap at transmitter; check colour code |
| Few °C offset (TC) | Copper or wrong extension cable, CJC error | Use correct cable to CJC point |
| Few °C offset (RTD) | 2 wire leads, wrong alpha or sensor type, mismatched 3 wire leads | Check wiring and configuration |
| Reads near ambient | Element not reaching well bottom, wrong length | Spring load element; check length |
| Slow response | Air gap in thermowell, heavy well | Fit correct element; consider tapered well |
| Noisy | Grounded TC with earth loop, VFD noise | Use isolated transmitter; shield and segregate |
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Quick values
- Pt100: 100 Ω at 0 °C, 109.73 Ω at 25 °C, 138.51 Ω at 100 °C.
- Type K: about 1.0 mV at 25 °C, 4.096 mV at 100 °C.
- Add CJC in mV, never in °C.
Related Tools, Procedures and Calculators
Frequently Asked Questions
Why does a thermocouple read room temperature?
Often an open circuit without burnout detection, or the element is not in the well.
How do I check a Pt100 quickly?
Measure resistance and compare with the table.
Why is my TC reading wrong by a few degrees?
Wrong extension cable or a cold junction error are common causes.
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