The most common thermocouple failure method is
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
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.