Development of novel self-validating thermocouples
self-validation
NPL is developing new, self-validating thermocouples which are designed to solve the problem of validating temperature measurements where removal of the thermocouple for calibration is impossible or impractical.
In the space or nuclear industries, for example, where thermocouples are used in extremely harsh or toxic environments - this new technology is crucial to maintaining optimal performance of the sensor. Furthermore, for less harsh environments where the thermocouple must perform reliably for a particularly long timescale or to particularly narrow tolerances, self-validating thermocouples have been shown to be a significant improvement over conventional options.
Self-validating thermocouple assembly
These are devices with two major components - the thermocouple (typically a Pt/Pd thermocouple) and a fixed-point unit (typically a 2 cm x 1 cm cylinder) placed around the measuring junction of the thermocouple.
Method
The fixed-point unit remains permanently in place. Upon exposure to a temperature rise or fall passing through the fixed-point phase transition temperature, a repeatable and reliable measurement check can be made. The deviation of this result from the previous calibration can be used to adjust the interpretation of the thermocouple readings. This is a form of active control, and is based on NPL's in-depth knowledge of the phase transition behaviour of pure metals and their alloys.
Fixed-point phase transitions
Every time the fixed-point material undergoes a phase transition (e.g. melting or freezing), the thermocouple output 'hesitates'. Therefore where a linear temperature increase is applied to the active area, a plateau in the thermocouple output is registered - as the energy entering the system is used to achieve the phase transition instead of changing the temperature. Under repeatable conditions, this is known to occur at exactly the same temperature every time - and therefore can be used to validate the sensor reading.
Options
These devices are currently prototypes, but development is ongoing, with the aim of producing robust, modular devices that can contain one or more fixed-point units. Furthermore, with the development of supporting automation, these devices will make the process transparent to the user.
Working with the European Space Agency (ESA)
NPL has recently completed a project with ESA-estec to develop self-validating temperature sensors for applications up to 2300 °C. For the project details and results, please click here
Contact
For further information, please contact Jonathan Pearce or Claire Elliott