Methods to determine the density of moist air

Improved knowledge of air density will be useful to support various experiments in the mass field. A study was carried out at the Bureau International des Poids et Mesures to reduce the uncertainty on the air density determination by comparison of the performance of the following three methods: application of the CIPM- 81/91 formula; direct determination using air buoyancy artefacts having the same surface area but a large volume difference; refractometry, exploiting the high correlation between air density and air index of refraction, using a novel refractometer well adapted to our requirements (relative determination). The coherence among the three methods was satisfactory, but a discrepancy of the order of 10^-4 kg·m^-3 was observed between the CIPM-81/91 formula and methods employing artefacts. Additional measurements made in the framework of Euromet mass project 144 confirm this difference, which needs to be clarified by future studies using gravimetric measurements and on further investigations of the air composition.