Reduction of a Gas Prover Uncertainty

INRIM owns a gas piston prover for small gas flow rates (MICROGAS) which can provide flow rates lower than 0.1 SCCM with uncertainties of the order of 0.05% due to several technological features which include the two-sides temperature control of the gas chamber and the interferometric measurement of the piston displacement. The most important uncertainty contribution in the overall uncertainty estimate for this test rig is the dimensional uncertainty of the piston. The MICROGAS piston was initially measured in the length section of the then-IMGC at three cross-sections. Due to the relatively low number of measured cross sections, the diameter uncertainty was conservatively estimated by considering the piston diameter as nominally constant. When the piston was measured again, improvements in the methodology at the length section of INRIM allowed to reduce the measurement uncertainty and to increase the number of cross sections where the diameter was measured. This allows to provide a more detailed description of the section evolution along the piston, and therefore a smaller uncertainty of the diameter at the various sections. This improvement can be used to improve the overall uncertainty of the test rig by associating a different diameter, together with an associated uncertainty, to the various vertical positions measured for the piston. The paper describes the measurements performed on the piston diameter and a possible method for describing the volume displacement evolution along the piston run with its effect on the uncertainty estimate of the gas flow rate measurement. The actual implementation of the method is planned for the near future.