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.
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.
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