Realizing the redefined Kelvin: Realization and dissemination of the Kelvin below 25 K.

With the redefinition of the kelvin in 2019 and the new mise en pratique for the definition of the kelvin (MeP-K) [1], the realization of the kelvin is no longer exclusively confined to the International Temperature Scales (ITS-90, PLTS-2000). Primary thermometry has become possible as an alternative method for the realization of the kelvin. For that purpose, different primary thermometers, specifically the primary magnetic field fluctuation thermometer (pMFFT), the Coulomb blockade thermometer (CBT) and the acoustic gas thermometer (AGT) have been optimized in the EMPIR project Real-K [2] for an extended operation range to cover, at least partly, the temperature range from 1 K to 25 K. The performance of the thermometers was checked by comparison with the ITS-90 at various arbitrary temperatures. Two reference points served as highly stable and well characterized references, the superfluid lambda transition in 4He (λ-point) at 2.1768 K and the Ne triple point at 24.5561 K. With pMFFT and CBT we also demonstrated a smooth overlap between the PLTS-2000 range below 1 K and the ITS-90 range above 1 K. All the investigated primary thermometers proved to be very practical alternatives to the established thermometric methods of the ITS-90 in the range below 25 K. [ABSTRACT FROM AUTHOR]