21.4.10Burnout detection

Another consideration for thermocouples is burnout detection. The most common failure mode for thermocouples is to fail open, otherwise known as “burning out.” An open thermocouple is problematic for any voltage-measuring instrument with high input impedance because the lack of a complete circuit on the input makes it possible for electrical noise from surrounding sources (power lines, electric motors, variable-frequency motor drives) to be detected by the instrument and falsely interpreted as a wildly varying temperature.

For this reason it is prudent to design into the thermocouple instrument some provision for generating a consistent state in the absence of a complete circuit. This is called the burnout mode of a thermocouple instrument. A simple thermocouple circuit equipped with burnout detection is shown in this diagram:

The resistor in this circuit provides a connection to a stable voltage19 in the event of an open thermocouple. It is sized in the mega-ohm range to minimize its e ect during normal operation when the thermocouple circuit is complete. Only when the thermocouple fails open will the miniscule current through the resistor have any substantial e ect on the voltmeter’s indication. The SPDT switch provides a selectable burnout mode: in the event of a burnt-out thermocouple, we can configure the meter to either read high temperature (sourced by the instrument’s internal millivoltage source) or low temperature (grounded), depending on what failure mode we deem safest20 for the application.

19For those readers familiar with digital logic gate circuits, this resistor fulfills the same function as a pullup or pulldown resistor on the input of a digital gate: providing a stable logic state in the event of a floating input condition.

20This is a good application of fail-safe design, where we choose the transmitter’s failure mode based on the safest outcome. For example, if our temperature transmitter were being used to sense the temperature of a furnace where excessive temperature was more dangerous than insu cient temperature, we would want to configure it for “high” burnout. This way if the thermocouple fails open, the transmitter will report a dangerous (but false) measurement of furnace temperature to the controller, which in turn will automatically act to decrease the furnace’s actual temperature (i.e. the safer condition.)