Gravimetric flow standard in the vacuum and hermetic modes

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.

Gas flow standards and their uncertainty

“…The temperature of the tank is measured with non-contact thermometers to avoid extra weight and parasitic forces from connecting wires. Some implementations of the dynamic gravimetric method make buoyancy corrections negligible by housing the collection tank in a vacuum chamber [86,89]. Making the mass measurements in vacuum also eliminates forces due to natural convection currents flowing over the surfaces of the collection tank caused by temperature differences between the tank and the environment [90].…”

Time-Correction Model Based on Diverter Speed for a pVTt Gas Flow Primary Standard

“…When the fluid is a gas, there are two basic groups of primary standards. Gravimetric standards use scales or mass comparators to evaluate the mass change of a depleted/filled pressure vessel [ 2 , 3 , 4 , 5 ] with the best uncertainties defined as 0.019% [ 2 ] of the measured flow rate. The volumetric method uses the definition of gas mass as a product of the gas density and a known volume that the gas occupies.…”