Direct calibration of resistance thermometers between 10 K and 25 K by absolute acoustic gas thermometry in helium

We have implemented absolute acoustic gas thermometry at 10 K, 13.8 K, 19 K and 24.6 K to evaluate the performance of this primary method for the direct thermodynamic calibration of capsule-type resistance thermometers, including both platinum and rhodium-iron types. Our implementation is based on speed of sound measurements in helium at a single pressure, chosen in the range 65 to 130 kPa, with non-ideality corrections relying on accurate ab initio calculations of the thermodynamic properties. The overall accuracy achieved in the determination of the thermodynamic temperature T varies between a minimum of 0.1 mK at 13.8 K to a maximum of 0.2 mK at 24.6 K. From the acoustic results and the calibration of thermometers on the international temperature scale ITS-90 providing T90, we determined the differences (T-T90), finding them in good agreement with the 2022 consensus estimates within the combined uncertainties. These results include a determination of the thermodynamic temperature at the triple point of neon TNe = (24.55502 ± 0.00030) K. This new value of TNe is consistent with other recent determinations obtained with various primary methods. Finally, we provide an example of a rapid, yet accurate, simplified thermodynamic calibration procedure. This article is part of the Theo Murphy meeting issue 'The redefined kelvin: progress and prospects'.