Developing low uncertainty, thermodynamic temperature capabilities
Non-contact temperature measurement is based on the principle of blackbody radiation where a hot object emits electromagnetic radiation by virtue of its temperature. The actual spectrum of the emitted light depends on the temperature of the object being measured, with more shorter wavelength light emitted as the temperature of the object increases. This thermal radiation is detected using optical or infra-red detectors, similar in principle to those found in digital cameras. The temperature can be inferred from the intensity of the light emitted, either in total or at particular wavelengths.
Most thermometer calibration is done in terms of the International Temperature Scale or 1990 (ITS-90). This is only an approximation to 'true' (thermodynamic or primary) temperature, but has the advantage of being straightforward to achieve and guarantee worldwide standardisation. However, there are circumstances where it is necessary to know the true temperature, so we are researching how best to deliver thermodynamic calibrations.
Comparison with ITS-90
It is important to know how ITS-90 differs from thermodynamic temperature. By comparing sensors calibrated in terms of ITS-90 with primary temperature measurement based on Planck’s radiation law, this difference can be evaluated. The comparison is done using heat-pipe sources with extremely good stability, staying constant over time by better than five thousandths of a degree Celsius at up to 1000 °C. This is part of a global campaign to establish accepted differences.
High temperature improvement
New high-temperature reference standards, or fixed points, with values agreed as part of the International Systems of Units (the SI), open new possibilities for temperature metrology in the range 1000 °C to 3000 °C. We are researching improvements in the necessary furnaces and the interplay of various related uncertainty components. This allows us to supply high temperature fixed points to be part of thermodynamic temperature measurement capability to National Measurement Institutes (NMIs) and Designated Institutes (DIs) in other counties and to industrial calibration laboratories.
The availability of high temperature fixed points changes the requirements for the design of radiation thermometers that give the lowest uncertainty. This is, of course, also balanced against complexity and cost. NPL is developing a thermometer that can match our best measurement capability in a robust, cost-effective and compact package. Either on its own, or complemented by high temperature fixed points, this allows a level of uncertainty for the problematic high temperature range that is currently only achieved at top level NMIs.
Reference cells for in-process calibration
Pyrometers are often used in high temperature industrial environments, and focused on objects which are behind one or more windows, to protect the detector and the staff. This typically means that the signal is degraded over time, for example as the windows get 'dirty'. We are developing techniques to allow in-situ pyrometer validation, where suitable corrections for varying window transmission can be identified and applied without interfering with the ongoing industrial process.