What is Thermocouple Burnout?
The most common failure mode for thermocouples is for them to fail open, this is known as thermocouple burnout. Burnout can result from a variety of reasons including the thermocouple experiencing a temperature in excess of that for which it was designed, metal fatigue, and oxidation.
Why is Thermocouple Burnout a Potential Safety Issue?
In a temperature control loop if a thermocouple burns out the controller will loose its input signal, which as far as the controller is concerned, is equivalent to the measured temperature becoming very low. This fake low temperature would fool the controller into increasing the heat input to the process until a potentially dangerous process upset could occur.
It is worth noting that something as simple as a wire in the instrument loop becoming disconnected will produce the same effect as burnout, therefore selecting a burnout mode and enabling burnout protection, as discussed below, is good practice for all thermocouple applications.
Burnout Protection Modes
To avoid process upsets in the event of a thermocouple burnout it is prudent to design into the thermocouple instrument loop some provision for generating a consistent state in the absence of a complete circuit. This form of burnout protection is called the burnout mode of a thermocouple instrument.
In the event of a thermocouple burning out we can configure the meter to either have upscale burnout i.e. read high temperature or downscale burnout i.e. read low temperature. This choice of configuration is termed burnout mode, and the choice of upscale or lowscale burnout mode will depend on which failure mode is deemed safest for any particular application. Generally, in heating applications upscale burnout is selected to prevent overheating.
How is Burnout Protection Achieved?
In its simplest form, burnout protection modes can be provided as shown below:
The Burnout Mode Switch allows selection of burnout mode – hi for upscale, and lo for downscale.
Upscale burnout is achieved by supplying a small current (sourced by the instrument’s internal milli-voltage source) to a thermocouple’s input. In the diagram above, the resistor in the circuit provides a path for current in the event of an open thermocouple. In real life applications the small current supplied to the circuit would be offset by the zero and span adjustments. Typical values transmitted from the field to the control system are ≤3.2 mA (for down scale burnout) and ≥21.6 mA (for up scale burnout).