Direct comparison of ITS-90 and PLTS-2000 from 0.65 K to 1 K at LNE-CNAM

International audienceIn the temperature range between 0.65 K and 1 K, the international temperature scale of 1990 (ITS-90) is based on 3 He vapour-pressure thermometers and overlaps with the provisional low temperature scale of 2000 (PLTS-2000) defined by the melting pressure of 3 He. An indirect comparison at PTB revealed differences between the two scales of up to 1.5 mK at 0.65 K (Engert et al 2007 Metrologia 44 40-52). Stimulated by the PTB results, we have performed a direct comparison T 90-T 2000 from 0.65 K to 1 K at LNE-CNAM. To test repeatability, the experiment was conducted twice: in 2019 and 2020. We find differences T 90-T 2000 of 0.28 mK at 1 K, increasing to 1.58 mK at 0.65 K. The direct comparison, eliminates the uncertainty component due to the transfer resistance thermometer and its calibration. Except for a point near 1 K, the new results are in accordance with those obtained at PTB (differences of less than 0.22 mK), which makes it possible to improve the accuracy of the equation specified in ITS-90. Keywords: international temperature scale of 1990 (ITS-90), provisional low temperature scale of 2000 (PLTS-2000), vapour pressure of 3 He, melting curve of 3 He, low temperature thermometr