Since the redefinition of the base unit kelvin via fixing the Boltzmann constant in 2019, it has been possible to realise the unit applying different gas-thermometry methods in accordance with the ‘Mise en pratique for the definition of the kelvin in the SI’. For this application, the use of data for the gas properties resulting from ab initio calculations is of special interest because it makes primary thermometry much easier. But since a rigorous estimation of the uncertainty of theoretical calculations is at least very complex, if not impossible, a check of the results by a comparison with highly-accurate experimental values is mandatory. Such a check is performed for the second virial coefficient of helium, which is a widely used measuring gas, in the temperature range from 3.7K to 273K. For obtaining highly-accurate second-virial-coefficient values (relative uncertainty at a few tenth of a percent level or even better), isotherms were measured with the PTB dielectric-constant gas thermometer. The highly-accurate isotherm data were evaluated by fitting, applying an extended working equation for the dependence of the gas pressure on the dielectric constant. The comparison with the results of the latest ab initio calculations shows coincidence within the combined uncertainty estimates.

自从 2019 年通过固定玻尔兹曼常数重新定义开尔文基本单位以来，已经可以根据“ Mise en pratique for the definition of the SI ”实现应用不同气体温度测量方法的单位。对于此应用，使用从头算产生的气体特性数据计算特别有趣，因为它使初级温度测量更容易。但是由于对理论计算的不确定性的严格估计至少是非常复杂的，如果不是不可能的话，通过与高精度实验值的比较来检查结果是强制性的。在 3.7K 至 273K 的温度范围内，对氦气的第二维里系数进行这种检查，氦气是一种广泛使用的测量气体。为了获得高精度的第二维里系数值（相对不确定度为十分之几甚至更好），使用 PTB 介电常数气体温度计测量等温线。高精度等温线数据通过拟合进行评估，应用扩展工作方程来确定气体压力对介电常数的依赖性。ab initio计算显示了组合不确定性估计中的巧合。