In 2000, the Bureau National de Métrologie (BNM, France) decided to develop a new watt balance experiment. Among numerous design studies, the choice of the transfer mass is particularly important. Because of the proximity to a source of high magnetic intensity, this mass must have a magnetic susceptibility as weak as possible. Gold-platinum alloy seems to meet this requirement, as well as additional requirements for density and hardness values, making it a possible candidate for mass standard realization. Five different gold-platinum alloys were studied, their volume magnetic susceptibility ranging from −2.8 × 10 −5 to −2.1 × 10 −5 for two of them and from +1.1 × 10 −5 to +8.8 × 10 −5 for the other three.

The mole fraction of argon in ambient dry air has been measured using gas phase chromatography calibrated with gravimetric gas standards prepared at the Laboratoire National de Métrologie et d'Essais. Different air samples, operators and gas standards were used to obtain a value of 0.9330 × 10 −2 mol mol −1 ± 0.0032 × 10 −2 mol mol −1 . This value is significantly different from the conventional value (0.917 × 10 −2 mol mol −1 ) used in the 81/91 formula of the Comité International des Poids et Mesures (CIPM) (Davies 1992 Metrologia 29 67-70) for the determination of air density during comparisons of mass standards.This value confirms the result obtained by the Korea Research Institute of Standards and Science, which found a concentration of 0.9332 × 10 −2 mol mol −1 ± 0.0006 × 10 −2 mol mol −1 by mass spectrometry in 2004.This new value explains the difference between the CIPM formula method and the artefact method used for the determination of air density.

The Istituto di Metrologia "G. Colonnetti" (IMGC, Italy), the Laboratoire National d'Essais (LNE, France) and the Mittatekniikan Keskus (MIKES, Finland) support primary calibration facilities used both for liquid density measurements and for hydrometer calibrations. In 1998, two independent bilateral comparisons of the calibration of standard hydrometers were carried out between the IMGC and the LNE and between the IMGC and the MIKES, the main purpose being to assess the present state of mutual compatibility in the density range 600 kg/m 3 to 2000 kg/m 3 among the three laboratories. The results show agreement within the uncertainties declared by the laboratories, although the MIKES measurements are in general 0.2 kg/m 3 lower than those of the IMGC and the LNE measurements are systematically 0.04 kg/m 3 higher than those of the IMGC. Both differences are smaller than half of the least-scale division of the transfer hydrometer used.

In mass metrology, the standards currently used are calibrated by a chain of comparisons, performed using mass comparators, that extends ultimately from the international prototype (which is the definition of the unit of mass) to the standards in routine use. The differences measured in the course of

Today, the determination of the density of air is the main source of uncertainty (10 µg) 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 × 10−5 with a relative uncertainty of 1.2 × 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.