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Implementing the new kelvin 2

The Project

InK 2 aims to ensure that the new kelvin is implemented effectively across the world.


In 2018 the International Committee for Weights and Measures (the CIPM) aims to redefine all the SI units in terms of fixed values of fundamental constants (for the kelvin this means in terms of the Boltzmann constant) to make a logical coherent and complete system of units for the foreseeable future. This momentous, once-in-a-lifetime change needs to be supported with all the necessary research and background documentation to facilitate a successful transition to the new SI unit system.


For the kelvin this means that by the time of the new redefinition, required primary thermometry methods (i.e. those that are thermodynamically based), and in particular sound results, will need to be in place to facilitate an effective unit redefinition. Especially robust TT90 and TT2000 data are required in the medium term for the MeP-K and in the longer term for the potential new temperature scale, the so called ITS–202x, in the mid-2020s where the current temperature scales (ITS-90/PLTS-2000) are likely to be replaced by a combination of a new scale and practical primary thermometry.

External evidence of the need for this project is:

  1. the proposed redefinition of the kelvin requires that all work necessary for adopting a resolution that would replace the current SI with the revised SI are completed
  2. the essential requirement of a properly founded instructions that allows the definition of the kelvin to be realized in practice (Mise en Pratique, MeP-K), e.g. a complete low uncertainty data set of TT90 and TT2000 values and robustly documented methodology.
  3. the background data for the PLTS–2000 is discrepant and this unsatisfactory situation needs to be resolved before that part of the MeP-K is agreed and any future temperature scale is proposed
  4. Consultatve committe for thermometry (CCT) reports highlighting the need for new primary thermometry in particular ranges 
  5. several published papers  highlighting the need for improved primary thermometry
  6. the recent 2014 CCT declaration, CCT/SUMM-DE2014, "….the CCT encourages NMIs to conduct significant experiments for the determination of thermodynamic temperature, to ensure that the SI unit kelvin is realized and disseminated in an optimum way in the coming decade".


To achieve the ambitious requirements of the project, the following objectives will be met:

  1. To determine T–T90 in the range from ~ 430 K to ~ 1358 K using a variety of techniques and with a target standard uncertainty of 5 mK. High temperature acoustic gas thermometry and lower temperature primary radiometry will be developed and deployed to determine low uncertainty values of TT90.  
  2. To determine TT90 in the range ~5 K to ~200 K using a variety of techniques and with a target standard uncertainty of 0.5 mK. Dielectric constant gas thermometry, refractive index gas thermometry and acoustic gas thermometry will all be deployed to determine low uncertainty values of TT90.
  3. To establish novel primary thermometry approaches to re-determine T and TT90, to identify and minimise systematic uncertainties. Three novel primary thermometry methods will be investigated to develop techniques that can provide independent confirmation of established thermodynamic temperature values both from InK ‑ 1 and also here in InK-2.
  4. To perform research in the ultralow temperature thermometry regime (0.9 mK to around 1 K) – so as to demonstrate primary thermometer dissemination of T and identify the cause of the PLTS-2000 background data discrepancy (6 % at lowest temperatures). The focus of this activity will be to examine TT2000 in the lowest part of the range from <20 mK to 0.9 mK by a number of primary thermometry methods (the region above 20 mK was characterised in InK 1). The overall objective is to develop a completely new data set from 1 K to 0.9 mK with an uncertainty of around 1%.
  5. To facilitate the take up of the technology and measurement infrastructure developed by the project by the measurement supply chain (other NMIs, accredited laboratories, instrumentation manufacturers, etc.). The NMI partners will ensure that the project outcomes will be rapidly and effectively incorporated within the MeP-K-19 and recommendations on best practice. 

The proposed research in this project represents a step change in capability and achievable measurement uncertainty beyond the current state of the art in primary thermometry. It will, for the first time, explore parts of the current temperature scales (ITS–90 and PLTS–2000) that have not been properly investigated since their inception in 1990 and 2000 respectively, and this will be performed by at least two independent primary thermometry methods for each region. In addition it will develop a number of emerging primary thermometry techniques which will be unique and have the potential to confirm previous measurements using primary thermometry providing additional confidence to those measurements. Finally, at very low temperatures, practical primary thermometers will be developed to the point where they could be implemented in manufacturers' dilution refrigerators, giving for the first time a robust and direct dissemination of thermodynamic temperature at temperatures below 1 K.


The InK-2 project will have profound impact on the whole of the international thermometry community through performing the necessary research required to ensure an effective and trouble free implementation of the kelvin redefinition in 2018.

The main immediate beneficiary will be the International Committee for Weights and Measures (CIPM) as the work performed in this project will facilitate a sound and reliable introduction of the redefinition of the kelvin. The Consultative Committee for Thermometry (CCT) will be strongly impacted by this project, with direct input into the MeP-K through very reliable low uncertainty thermodynamic temperature values, TT90 and T–T2000, and also through advancing the practice of a number of primary thermometry methods to the stage where their uncertainty is low enough to allow them to be admitted into the MeP-K.

The longer term impact of this project on the practice of thermometry, the realisation and dissemination of the unit, cannot be overstated. Through the vehicle of the MeP-K-19 the redefined kelvin will "come to life" and be put in place around the world in National Metrology Institutes, accredited calibration laboratories and through them to the wider temperature measurement community in commerce, health, environment and science. Thus, any user who needs reliable fit for purpose temperature measurement or derived quantities such as humidity and moisture, and thermal properties as well as any scientific discipline requiring a stable temperature baseline such as meteorology and climate change research will benefit from the research outputs.