Thermocouple Troubleshooting & Common Failures
Thermocouples fail in a handful of recognizable ways: open circuits, drift, reversed polarity, and alloy degradation such as green rot. This guide lists the common symptoms, their causes, and how to confirm and correct each one.
Common Failure Modes
| Symptom | Likely cause | Check / fix |
|---|---|---|
| Reading pegged high or shows open/overrange | Open circuit — broken junction or wire | Measure continuity; a healthy thermocouple reads a few ohms. Replace if open. |
| Reading drifts low over weeks/months | Decalibration / drift from aging, oxidation | Compare against a reference (comparison calibration); recalibrate or replace. |
| Reads lower when heated / moves wrong way | Reversed polarity in wiring | Verify +/− against color codes; red is negative in ANSI. |
| Erratic, noisy, jumping reading | Ground loop or intermittent connection | Switch to an ungrounded junction; check terminals and shielding. |
| Type K reads low after service at 800–1050°C | Green rot — selective chromium oxidation in low-oxygen atmosphere | Replace; use Type N or a sealed/ventilated sheath for that range. |
| Offset roughly equal to room-temperature error | Faulty cold-junction compensation | Verify CJC sensor and terminal temperature; see how CJC works. |
| Slow response | Build-up on sheath, loose fit in thermowell, or ungrounded junction | Clean sheath, improve thermal contact, or select a grounded junction. |
Why Thermocouples Drift and Fail
Green Rot
In Type K, when oxygen is present but limited (e.g. inside a leaky sheath) at 800–1050°C, chromium in the chromel leg oxidizes selectively, lowering its EMF and causing large negative errors. Use Type N, a fully sealed sheath, or a vented sheath to avoid it.
Drift & Decalibration
Thermoelectric output changes as alloys oxidize, inhomogeneities develop, and grain structure shifts with thermal cycling. Drift is gradual and type-dependent; it is why in-service calibration intervals exist. Note that recalibrating a used thermocouple often yields irrelevant results because the inhomogeneity sits in the gradient zone.
Open Circuit
Vibration, thermal fatigue, or corrosion breaks the junction or a lead. The instrument typically drives the input to full scale (burnout indication). Confirm with a continuity check.
Installation Error
Insufficient immersion depth, poor thermowell contact, or reversed extension wire produce steady offsets that masquerade as sensor faults. Rule these out before replacing the thermocouple — see sources of error.
Quick Diagnostic Order
1) Check polarity and extension-wire type. 2) Verify immersion depth and thermowell contact. 3) Confirm cold-junction compensation. 4) Continuity-test for an open circuit. 5) Compare against a reference to quantify drift. Most "bad sensor" calls are actually wiring or installation issues found in the first three steps.