Determination of the density of air: a comparison of the CIPM thermodynamic formula and the gravimetric method

Today, the determination of the density of air is the main source of uncertainty (10 mu g) in a comparison of a platinum - iridium alloy national prototype with a stainless steel reference standard. The method currently recommended by the BIPM is to calculate this density, using a formula developed in 1981 and revised in 1991 by the CIPM, from the measured values of the pressure, temperature, hygrometry and molar fraction of CO2 in air. Recent work has found significant differences between the values given by this formula and those given by a method using artefacts having identical masses and very different volumes (6.4 x 10(-5) with a relative uncertainty of 1.2 x 10(-5)). To investigate this difference, the BIPM asked the member laboratories of the Consultatitve Committee for Mass (CCM) to investigate two lines of approach: check the composition of the air, in particular the argon content, which may explain the discrepancies found; this is the object of a study by the chemical metrology unit of LNE (see the article by Sutour et al in this issue), study the behavior of artefacts during the air - vacuum transfer in a comparator that can work both in a controlled atmosphere and in a vacuum, to confirm or not confirm the discrepancies; this latter study is the object of this paper.