NPL is leading an international project that will revolutionise the International Temperature Scale realisation and dissemination at high temperatures. This will be based on new fixed-points. The thermodynamic temperature of these fixed points will be determined radiometrically.

The improved temperature scale will be based on a set of new fixed-point cells – blackbody cavities surrounded by melting metal-carbon eutectics. For the improved scale, the cavities’ thermodynamic temperatures need to be determined with low uncertainties. Optical temperature measurements using filter radiometry provide the most promising method to determine these temperatures.

NPL is co-ordinating the overall international project that will create this step change improvement in high temperature measurement standards. NPL’s Thermal and Optical Technologies teams are involved in all parts of this plan and are working closely with each other and with our partner national metrology institutes.

The Thermal team is building on considerable experience in constructing these fixed-points. We are developing the cell design and operational conditions and testing the long-term repeatability and reproducibility of these cells. The cells have already been used for international comparisons and to help develop new materials for high efficiency jet turbine engines and to calibrate small spectrometers.

NPL’s Optical Technologies team is developing filter radiometric techniques and investigating an entirely novel technique based on a different calibration route. Our measurements will be combined and compared with measurements by our colleagues at other National Metrology Institutes to get an international agreement amongst the very best laboratories prior to the definition of the fixed-point temperatures. We are expecting to lead this final international measurement campaign and its analysis in 2009-2011.

Filter Radiometry for Eutectics

Filter radiometry is an established technique for determining the radiance of a blackbody cavity that was pioneered at NPL in the early 1990s. To determine thermodynamic temperature above 2000 K to uncertainties approaching the reproducibility of the eutectic cells (<100 mK), we have had to revisit the filter radiometer instruments and the entire calibration chain – characterising the components 10 times more accurately than has previously been required.

The Double Wavelength Technique

The double wavelength technique is a novel, alternative technique to determining thermodynamic temperature. Filter radiometry provides thermodynamic temperatures traceable to a cryogenic radiometer. An alternative technique is possible that provides absolute thermodynamic temperature through a different traceability chain; ultimately back to wavelength/frequency (only), with the radiometric measurements purely relative. If this technique could be experimentally realised with similar uncertainties to those associated with filter radiometry, then this would provide a metrologically valuable “second opinion” that would test for systematic effects common to all filter radiometry measurements.

For further information, please contact: Emma Woolliams