Type K failures are diagnosable from their symptom signatures. Start from what the reading is doing — drifting, offset, erratic, dead, or implausible — and the table below maps it to the likely mechanism, the confirming check, and the fix.
| Symptom | Likely cause | Confirming check | Fix |
|---|---|---|---|
| Reads progressively low over weeks at 800–1050 °C, low-oxygen service | Preferential chromium oxidation (“green rot”) in the chromel leg | Pull and inspect: dull green discoloration on the positive leg | Restore oxygen supply, seal the sheath, or move to MI construction or Type N |
| Erratic or suppressed reading that improves when the cold end is dried | Insulation-resistance shunting / moisture ingress forming a virtual junction | Insulation-resistance test conductor-to-sheath; inspect termination seal | Dry, reseal hermetically; replace cable if IR stays low |
| Near-constant offset on an otherwise stable loop | CJC error, or wrong/mismatched extension wire in the run | Measure terminal-block temperature vs instrument CJC; verify wire type at every junction | Correct CJC source; replace with genuine Type KX extension, correct polarity |
| Reading moves the wrong direction when the process heats | Reversed polarity at one connection | Walk the run: KP yellow / KN red (ANSI) or KP green / KN white (IEC) | Re-land the reversed pair |
| Open circuit / overrange ("burnout") indication | Conductor fatigue at the junction or sheath transition; vibration | Continuity check; inspect at the transition and thermowell exit | Replace; add strain relief; verify thermowell wake-frequency margin |
| Noisy or unstable signal, worse near motors/VFDs, grounded junction | Ground loop — more than one ground point in the measuring circuit | Lift the instrument ground or test with an isolated input | Use ungrounded junction or isolated input; keep exactly one ground in the loop |
| Reads between process and ambient, responds slowly | Insufficient immersion — stem conduction biasing the tip | Compare against a deeper-immersed reference probe | Immerse ≥ 10–20× sheath diameter; see installation |
| Small repeatable non-linearity near 350 °C | Crude linearization vs the alumel magnetic (Curie) transition at ~354 °C | Compare instrument output against the ITS-90 table through the region | Use a transmitter/indicator that implements the full ITS-90 polynomials |
| Slow drift of a few °C over a service life, faster near the upper limit | EMF aging: cumulative time-at-temperature, cycling, cold work | Periodic comparison against a reference standard; check class band | Replace when out of class; shorten calibration interval for tight setpoints |
The mechanisms in detail
Preferential oxidation (“green rot”)
In oxygen-starved or marginally oxidizing atmospheres between roughly 800 °C and 1050 °C, chromium in the positive (chromel) leg oxidizes preferentially, depleting the alloy and shifting its EMF — usually low and progressively. The defenses are an adequate oxygen supply, a sealed sheath, or a magnesium “getter”; in reducing service, Type N or a noble-metal type is the structural fix. Mineral-insulated constructions limit atmosphere ingress — see mineral-insulated (MI) Type K probes.
Insulation-resistance loss and shunting
At high temperature the insulation resistance of MI cable falls; combined with moisture ingress at an unsealed termination it creates a parallel leakage path — a “virtual junction” — that pulls the reading toward the temperature of the leak, not the tip. Hermetically sealed terminations and correct sheath selection mitigate it.
The ~354 °C alumel anomaly
Alumel passes through a magnetic (Curie) transition near 354 °C, producing a small, repeatable perturbation in the EMF curve in that neighborhood. It is built into the ITS-90 reference function — so a calibrated instrument handles it — but it surfaces as an apparent non-monotonic drift if a system linearizes with a crude approximation rather than the full polynomial.
Calibration drift and EMF aging
Cumulative time-at-temperature, thermal cycling, and cold-work from handling all shift the EMF output, typically by several degrees over a service life and faster near the upper limit. Periodic comparison against a reference standard is the only way to bound it. Recalibration and certificate services are covered under thermocouple calibration.
Mechanical and vibration fatigue
Flexing at the junction or sheath transition, and flow-induced vibration in a poorly supported thermowell, fatigue the conductors and eventually open the circuit. Match the thermowell to the process velocity and check the wake-frequency margin — see thermowell selection.
If a sensor checks out against its class band but the loop still misreads, the problem is usually downstream: CJC source, extension run, or input linearization — work the run from the terminal block back, not from the tip forward.