Characterization of a force-balanced piston gauge as a primary pressure standard

This work characterizes a force-balanced piston gauge (FPG), model FPG8601, as a primary pressure standard over its whole operation range from 1 Pa to 15 kPa. To date, FPGs have been handled as secondary standards that are usually calibrated against pressure balances at pressures above 5 kPa. Starting from dimensional measurements on the FPG piston-cylinder assembly (PCA), 3-dimensional data sets were created that presented the radius of the piston and the cylinder bore along the selected circle and generatrix traces with uncertainties of about 50 nm for the piston and 30 nm for the cylinder. Using this dimensional data and modeling the gas flow in the PCA gap, the effective area of the PCA was determined over the whole pressure range for absolute and gauge operation modes. In the pressure distribution calculation, the methods of the rarefied gas dynamics were applied, and the effect of the change from viscous via transient to molecular flow mode on the effective area was quantified and found to have a relative value of 1.5·10−6. The theoretical effective areas are compared with the experimental areas obtained by means of a calibration against a deadweight pressure balance, and the result shows a reasonable agreement within a relative value of 8·10−6.