Piston provers dead volume evaluation

In piston prover measurements, the term “dead volume” denotes the portion of internal space that remains unreachable by the piston during operation. This volume includes not only the lateral sections of the containing cylinder but also the bottom of the cylinder, as well as auxiliary components such as connecting tubing and recessed areas designed to accommodate measurement instruments. The presence of this volume plays a critical role, as it directly affects the accuracy of the measurements obtained with the device. Specifically, in INRIM facilities where the mass balance method is applied—calculating gas quantity variation by the difference between the estimated gas mass at the beginning and end of a test—having a precise estimate of the initial volume, which inherently includes the dead volume, is essential. However, traditional geometric methods for assessing this volume, based on dimensional measurements, introduce a significant degree of uncertainty. To overcome these limitations, our laboratory has developed an innovative approach that involves injecting a known and controlled amount of gas, measured through the integration of flow data from a calibrated mass flow controller (MFC), and then analyzing the resulting changes in thermodynamic conditions within the piston chamber. This method enables a more accurate characterization of the dead volume, thereby reducing the uncertainty associated with its evaluation. A detailed uncertainty budget analysis for our Microgas and MeGas test facilities will also be presented. In this article, the Authors extend the investigation on the topic of dead volume in piston provers published in the Proceedings of the XXIV IMEKO World Congress [1]