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Two EURAMET funded projects help redefine the kelvin, reach the world’s lowest temperature measurement uncertainties and demonstrate a new way to measure temperature

In May 2019 four of the base units of the International System of Units, the SI, were revised, linking them to fundamental constants for the first time. To do this required the development of state-of-the-art techniques in several metrological fields.

One area was for the SI unit of temperature, the kelvin (K). Within EURAMET’s European Metrology Research Programmes (EMRP and EMPIR) the project Implementing the new kelvin 2 (15SIB02, InK 2) built on the work of the earlier project Implementing the new kelvin (SIB01, InK) and led the European contribution to the kelvin redefinition.

As well as helping to deliver the revised unit of temperature InK 2 also undertook research into new primary methods for temperature measurement including Doppler Broadening Thermometry (DBT). In DBT a laser passes through an isothermal region of gas. This is used to measure a spectral absorption line in the gas whose width is broadened due to the thermal motion of the atoms. As the gas atoms move quicker at higher temperatures by measuring changes in the doppler width the temperature of the gas can be determined. These investigations resulted in the world’s lowest measurement uncertainties (<10 ppm) for this technique, which with further development has the potential to become an established primary thermometry approach in the future.

In the future small scale DBT could be developed which, because the method is primary, based on fundamental physics, could be the basis of ‘driftless temperature sensing’. If practical small scale DBT becomes possible then this approach, along with other practical primary thermometry approaches, could be used to facilitate  ‘autonomous’ industrial environments such as those envisaged in Industry 4.0. Furthermore, the two EURAMET projects have developed the methods required for direct traceability for temperature – potentially leading to the realisation and dissemination of thermodynamic temperature over a wide range of temperatures.  

More generally the project results have generated much interest with the project’s coordinator Graham Machin (NPL) being asked to give the keynote address at SMSI 2020 (now 2021) on the topic of realising the redefined kelvin. He has also been discussing organising a specialised session at the International Temperature Symposium (2023) – a conference  that is held every ten years.

Now the kelvin has been successfully redefined it needs to be disseminated to the wider scientific community – work that is continuing in the current EMPIR project Realising the redefined kelvin (18SIB02, Real-K). One main aim of the project is to demonstrate at extreme ends of the temperature scale, greater than 1300 K, and less than 25K, that realisation and dissemination of thermodynamic temperature is a realistic alternative to the current defined scale.

Graham Machin (NPL) – who coordinated all three projects has said about this work

“the rise of practical primary thermometry by DBT and other fundamental approaches is one of the most exciting prospects in sensing at the current time holding out the prospect of in-situ traceability, with “always right” thermometry facilitating autonomous production and optimising energy use in industrial processes”

EMPIR projects are co-funded by the European Union's Horizon 2020 research and innovation programme and the EMPIR Participating States.

EMRP joint research projects are part of EURAMET’s European Metrology Research Programme. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union.

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