Developing solutions to contact thermometry challenges
Contact thermometry is temperature measurement using sensors in contact with the medium to be measured. Although contact thermometry techniques, using either resistance thermometers (PRTs), thermistors or thermocouples, may appear to be well established, there is a constant need to improve the technology. Ongoing research is needed to reach wider temperature ranges and operate with confidence under increasingly harsh conditions. We also use thermal modelling to improve our understanding.
Amongst other things, NPL is developing new measurement techniques, such as in-situ measurement validation and improving upon existing technology to meet increasingly demanding industrial requirements. These include making impurity corrections for temperature fixed points and assessing thermoelectric inhomogeneity for thermocouples.
There is an online community forum focused on thermocouples on LinkedIn to share developments and needs, and NPL hosts regular meetings.
In-situ measurement validation
When temperature sensors are used, over time the measured temperature often drifts unpredictably away from the original calibration. This is usually due to unavoidable degradation and means that the sensors need to be removed and either recalibrated or replaced.
To avoid this problem, we are developing in-situ validation techniques suitable for a variety of industries and environments. This is typically achieved through embedding a material which has a known, and unchanging, melting temperature. These developments are underpinned and optimised by thermal and thermodynamic modelling.
The first technology, in-situ self-validation ('inseva') fits within a 7 mm outer diameter thermocouple sheath and has been proven in industrial environments over six months. This technology has now been licenced to CCPI Europe and NPL is continuing to support the commercialisation.
The second technology is a validation unit for use with satellite radiometers in space. This is based on the same basic concept, but must be as small and light as possible. This is being developed in collaboration with RAL-Space.
The conventional technique of calibrating SPRTs in highly-accurate 'fixed-point' cells, which contain an ingot of a very pure element with a known melting or freezing temperature, is very reliable. But as the demands for better calibration uncertainty grow, it is important to improve the technique.
We are studying the effect of impurities at the level of parts per million on the performance of the cell, in order to be able to understand the effect they have on the freezing temperature and hence the calibration.
Thermocouple inhomogeneity scanning
NPL is in collaboration with the Measurement Standards Laboratory of New Zealand (MSL) to develop a technique for scanning thermocouples to measure the thermoelectric inhomogeneity along their length. This is an important factor in thermocouple uncertainty budgets, but the changes with time are not yet well understood as they vary with dimensions and exposure to both temperature and contaminants.