Repeatability of a primary thermometer and calibration of resistance thermometers between 5 K and 25 K

Abstract This work describes improvements to our Single Pressure Refractive-Index Gas Thermometry (SPRIGT) system (Gao et al 2020 Metrologia 57 065006), where a new experimental procedure for Constant Refractive-Index Gas Thermometry (CRIGT) was proposed and implemented, and after 2.5 years new values of T – T 90 obtained in the temperature range from 5 K to 25 K. Comparison results indicate that our SPRIGT system is robust with excellent repeatability, reliability, and stability. Not only is the uncertainty in thermodynamic temperature T still below 0.17 mK, but also good reproducibility of T 90 (temperature of the 1990 International Temperature Scale), T and T – T 90 is observed with values of 21 μK, 28 μK and 35 μK, respectively. Furthermore, we have implemented direct dissemination of thermodynamic temperature T by establishing our own thermodynamic temperature wire scale under the new International System of Units (SI) from 5 K to 25 K. The remarkable agreement further enhances our confidence on SPRIGT for implementing the redefined kelvin below 25 K, disseminating thermodynamic temperature T , and ultimately partaking in an international comparison of thermodynamic temperature via highly stable resistance thermometers.