In the temperature range between 0.65 K and 1 K, the international temperature scale of 1990 (ITS-90) is based on ³He vapour–pressure thermometers and overlaps with the provisional low temperature scale of 2000 (PLTS-2000) defined by the melting pressure of ³He. 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.

在 0.65 K 和 1 K 之间的温度范围内，1990 年的国际温标 (ITS-90) 基于^{3 个}He 蒸气压力温度计，并与由熔化温度定义的 2000 年临时低温标度 (PLTS-2000) 重叠压力为³ He。PTB的间接比较揭示了两个尺度之间的差异，在 0.65 K 时高达 1.5 mK（Engert等人2007 Metrologia 44 40-52）。在 PTB 结果的刺激下，我们在 LNE-CNAM 上进行了从 0.65 K 到 1 K的直接比较T ₉₀ – T ₂₀₀₀。为了测试可重复性，实验进行了两次：2019 年和 2020 年。我们发现差异T ₉₀ – T ₂₀₀₀在 1 K 时为 0.28 mK，在 0.65 K 时增加到 1.58 mK。直接比较消除了由于转移电阻温度计及其校准引起的不确定性分量。除了接近 1 K 的点外，新结果与在 PTB 获得的结果一致（差异小于 0.22 mK），这使得提高 ITS-90 中指定的方程的准确性成为可能。