Molar mass of dry air in mass metrology

Since 1980, the different national metrology institutes use the same procedure and formula for air density determination. The formula employed assumes some hypotheses on the composition of dry air and is expressed in terms of its molar mass and the four environmental parameters: air pressure, air temperature and concentrations of carbon dioxide and water vapour. In this formula (known as CIPM-1981/91) recommended by the Comité International des Poids et Mesures, the mole fraction of argon is fixed at 9.17 × 10−3. This value is now questioned and new measurements involving different techniques are of great interest.This paper describes the experimental set-up used to evaluate the concentration of argon relative to that of nitrogen contained in air sampled from mass laboratory. The method uses a flexible capillary tube, maintained at constant temperature, for gas admission. With this system, air from different samples of atmospheric air is introduced into the vacuum chamber to be analysed. Preliminary measurements show that the value of Pp(Ar)/Pp(N2)Pp(Ar)/Pp(N2) is closer to 1.196 × 10−2, given in some publications, than the value of 1.174 × 10−2, used in the current method for air density determination. In the future, comparison between argon and oxygen concentration will allow a more precise measurement with regard to the existing values.