**Adam Adamkowski, Waldemar Janicki and Mariusz Lewandowski \***

The Szewalski Institute of Fluid-Flow Machinery, Department of Hydropower, Polish Academy of Sciences, Fiszera 14, 80-231 Gdansk, Poland; adam.adamkowski@imp.gda.pl (A.A.); waldemar.janicki@imp.gda.pl (W.J.) **\*** Correspondence: mariusz.lewandowski@imp.gda.pl

Received: 13 July 2020; Accepted: 28 August 2020; Published: 9 September 2020

**Abstract:** This article presents the original procedures for measuring the flow rate using the pressure-time and the volumetric gauging method in the case of performance tests of a reversible hydraulic machine in either turbine or pump modes of operation. Achieving the lowest possible measurement uncertainty was one of the basic conditions during implemented machine tests. It was met using appropriate measuring procedures and high-class measuring equipment. Estimation of the uncertainty for both methods was made on the basis of an analysis consistent with current requirements in this respect. The pressure-time method was supplemented by the computational fluid dynamics (CFD) analysis that allowed reducing the impact of the pipeline complex irregular geometry on the uncertainty of flow measurement. Appropriate modifications of the calculation procedure enabled accurate measurements of flow during the pump mode of operation of the tested machine as well. The volumetric gauging method, thanks to a special procedure used for accurate measurement of the water level in the upper reservoir of the power plant, allowed measuring the discharge through the tested reversible machine with very low uncertainty. The obtained results allowed for a detailed comparison and mutual verification of the methods used to measure the discharge of the tested reversible machine in both modes of its operation. The most possible causes of obtained results are discussed and summarized in the paper. The need for further research was pointed out to explain the differences obtained and their influence on the accuracy of discharge measurement using the pressure-time method in pump operation mode.

**Keywords:** reversible hydraulic machines; penstocks; pressure pipelines; performance tests; flow rate measurements; volumetric gauging method; pressure-time method; water-hammer
