  Determination of air density and the quantitative estimates of water vapour
adsorbed or desorbed by the surface of mass standard, remain the principals
sources of uncertainties when weighing mass standard made from stainless steel
or super alloy. The classical method, for air density determination, use the so
called CIPM-1981/91 formula and the measured air parameters. An other approach
is based only on the results of comparisons, realized successively in air and
in vacuum, between special artefacts. The distribution and the state of water
molecules inside the chamber of mass comparator influence the results of
weighing. Also, the instruments used for air moisture measurement are either,
not very sensitive (capacitive hygrometer) or disturbing for environment
(dew-point hygrometer). To control that, a proposal method is given and
observations of water vapour behaviour inside the enclosure for different
conditions are reported. Keywords. mass metrology, air density, water vapour
