Metrological features of the Large Piston Prover at INRIM

INRIM realizes its flow rate standard using three distinct facilities, aimed at measuring different flow rate ranges; in particular, for the largest flow range rate (10–2600 L/min) a piston prover is used. This machine is of the volumetric type, therefore its traceability can be obtained through dimensional calibration of the piston, which has a nominal diameter of 1000 mm and a nominal stroke of 1200 mm. The present paper describes in detail the features of the standard, its traceability chain and the uncertainty budget of the measurements it can perform. The uncertainty budget directly determines the Calibration and Measurement Capabilities claim in the range available to the test rig. A detailed analysis of the various uncertainty components will be presented and discussed. Special attention will be dedicated to the dimensional calibration of the piston, since it is of paramount importance for the determination of the main uncertainty component. This calibration is particularly challenging since, due to the large size of the piston, it must be carried on in-situ, thereby requiring a set of special adaptations with respect to a standard calibration of a cylinder. It will be shown that the calibration of the piston recently performed is in good accordance with the one that was carried out at the piston initial installation in 1999, thus confirming the stability of the standard.