Abstract
The Czech national standard of low gas flow is based on the dynamic gravimetry principle, defining the flow rate as the mass loss of gas from a pressure bottle which is continuously weighed. This weighing is performed in ambient atmosphere, which brings two problems.
First, the buoyancy corrections are the main and unavoidable source of uncertainty. Second, condensation of the water vapor on the pressure bottle can occur, due to its cooling below dew point at higher flow rates.
A weighing carried out in a hermetically closed atmosphere suppresses the problematic buoyancy effects. A weighing in a vacuum does the same, and also eliminates any danger of water condensation.
The paper describes the hermetic and vacuum mode modifications together with the relevant uncertainty analyses.