<p><em>The ITS-90 temperature scale is used worldwide but unresolved problems could have limited its useful lifespan. These have been addressed by a completed EMPIR project</em></p>
The ITS-90 temperature scale is used worldwide but unresolved problems could have limited its useful lifespan. These have been addressed by a completed EMPIR project
Temperature scales
In 2019 the SI unit for temperature, the kelvin (K), was redefined in terms of the fundamental Boltzmann constant (k). This raised the possibility of bringing primary temperature measurements from the laboratory to their point of use, and also disseminating thermodynamic temperature directly.
However, these outcomes may take a decade to realise and thus the current temperature scale – the International Temperature scale of 1990 (ITS-90) and the specialised low temperature scale – the Provisional Low Temperature Scale of 2000 (PLTS-2000) have to be used until then.
The ITS-90 contains many sub-ranges covering a wide range of temperatures, from 0.65 K (-272.5 oC) to >1300 K (~>1030 oC). A set of fixed-points of defined temperature (melting, freezing, triple, or boiling points) of various pure substances, for e.g. the triple point of water (273.16 K/0.01 oC) and the freezing point of tin (505.078 K/231.928 oC) or silver (1234.93 K/961.78 oC)) are used to calibrate Standard Platinum Resistance Thermometers (SPRTs) and establish the ITS-90.
Life-limiting problems ITS-90
The current scale has provided reliable, traceable temperature measurements with low measurement uncertainty for over 30 years. However, there a several potential issues in the system which could limit its life in the future.
One problem is that one fixed-point which the scale utilises is the triple point of mercury (Hg, 234.156 K/-38.8344 oC). This is a toxic metal banned by the Minamata convention. If this ban is extended to its use in ITS-90 - as no viable alternative is available - this will mean a large part of the range, with the lowest uncertainties, would not be obtainable.
A second problem with the ITS-90 lies in measurement uncertainty, especially that relating to Type 1 and Type 3 non-uniqueness and relates to the use of Standard Platinum Resistance Thermometers.
Type 1 non-uniqueness, is associated with the difference between the interpolations over different, overlapping ITS 90 subranges for the same Standard Platinum Resistance Thermometer and Type 3 arises from the difference between individual thermometers over the same subrange. The latter is due to the fact that the resistance characteristics of thermometers are not identical, and the interpolations cannot take these small differences fully into account.
A lack of knowledge on these factors will ultimately limit the uncertainties achievable, which in turn may impact such things as energy efficiency savings of high temperature processes or the development of cryogenic technologies.
The now completed EMPIR project Realising the redefined kelvin (18SIB02, Real-K) addressed these two issues.
New fixed-point cells
To replace the mercury fixed point the project investigated new cell types and delivered two viable alternatives based on carbon dioxide (CO2) and sulfur hexafluoride (SF6) and addressed their integration into the ITS-90. If the use of mercury in fixed-points is banned viable replacements are now available.
Non-uniqueness
The project performed a comprehensive investigation on Type 1 and Type 3 non-uniqueness using Standard Platinum Resistance Thermometers allowing the overall ITS-90 measurement uncertainty to be reduced by > 30 %.
End users now have, for the first time, reliable estimates for Type 1 and Type 3 non-uniqueness uncertainties. This will allow the usability of the scale to be extended until primary thermometry methods mature.
Other project achievements
New instrumentation & measurements
- 3 primary practical thermometry techniques were developed for measurements at cryogenic temperatures (<25 K), primary Magnetic Field Fluctuation Thermometry (pMFFT), Coulomb Blockade Thermometers (CBT) and the fast-Acoustic Gas Thermometer (fast-AGT).
- Low uncertainty temperatures for four High temperature fixed points (HTFPs) spanning 1426 K to – 3020 K were definitively determined for the first time. These provide lower uncertainties than can be routinely achieved through absolute primary radiometry and will facilitate the realisation and dissemination of thermodynamic temperature with low uncertainties.
- A reliable and comprehensive set of ab initio values of key thermophysical properties for thermometric gases (e.g. Ar, Ne, He) covering 10 K to 350 K were developed. This work simplifies gas thermometry calibrations from a week per temperature to less than one day and lays the foundations for practical gas-based primary thermometry to deliver temperature traceability directly to the kelvin.
Many of the developments have been listed in a review article from the project.
This research continues in two European Metrology Partnership projects:
Graham Machin (NPL) who coordinated the project said about the work:
“The Real-K project made tremendous strides in realising the redefined kelvin. This has been through advancing gas thermometry beyond the state of the art, and also providing new high temperature fixed-points which will be the foundation for disseminating thermodynamic temperature in the future. The life extension work of the ITS-90 has been a crucial achievement in giving time for these more fundamental thermometry approaches to be (and continue to be) developed”
This EMPIR project was co-funded by the European Union's Horizon 2020 research and innovation programme and the EMPIR Participating States.
The Metrology Partnership projects received funding from the European Partnership on Metrology, co-financed by the European Union Horizon Europe Research and Innovation Programme and from the Participating States.
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