Thermocouple Faults and How to Troubleshoot Them

Overview

A thermocouple loop almost never fails at random. No reading, drifting readings and unstable readings each point to a specific part of the loop — the sensor, the extension cable, the reference junction or the instrument. Working through the loop in that order is the fastest way to find the fault.

Key Points

No reading or over-range. An open circuit in the sensor or the wiring is the usual cause; check continuity of the sensor and each terminal in turn.
Reading drifts high or low. Look for wrong extension or compensating cable, reversed polarity, or a sensor that has been contaminated and shifted its calibration.
Unstable or noisy signal. Usually poor shielding, a grounded loop, or moisture in a terminal head.
Slow response. Often caused by a thermowell with too much air gap, or by heavy scaling on the sheath.

Summary

Before replacing a sensor, measure the loop resistance and check the insulation resistance between the wires and the sheath. In corrosive or high-velocity service, mineral-insulated construction and a corrosion-resistant sheath material are the most effective way to extend service life.

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