How to detect Thermocouple burnout?

The most common thermocouple failure method is open failure, otherwise known as “burning out.”

An open thermocouple is difficult for any voltage-measuring tool with elevated input impedance because the absence of a full input circuit allows electrical noise from nearby sources (energy lines, electrical engines, variable-frequency engine drives) to be identified by the tool and misinterpreted as a wildly variable temperature.

For this reason, in the lack of a full circuit, it is prudent to design some provision into the thermocouple tool to generate a coherent state. This is called a thermocouple tool’s burnout mode.

A simple thermocouple circuit equipped with burnout detection is shown in this diagram:

In the case of an open thermocouple, the resistor in this circuit offers a route for current. When the thermocouple circuit is complete, it is sized in the mega-ohm range to minimize its impact during normal operation.

Only when the thermocouple fails open will the miniscule current through the resistor have any substantial effect on the voltmeter’s indication.

The SPDT switch offers a selectable burnout mode: in the case of a burnt-out thermocouple, we can configure the meter either to read elevated temperatures (produced by the inner mili-voltage source of the instrument) or to set low temperatures (grounded), depending on which failure mode we consider to be safest.