Potential Impact of Flow Meter Selection on Air Compressor Performance Analysis

Air compressors are among the most energy-intensive cross-cutting technologies used in the industry, making accurate performance evaluation essential for improving energy efficiency. ISO 1217, the internationally recognized standard for displacement compressor acceptance testing, has remained largely unchanged since its last revision in 2009. The standard specifies that flow-rate measurement should be performed as indicated in ISO 5167-1 and ISO 9300. However, it does not prescribe a specific flow-meter technology, allowing different technologies to be selected for compressor performance measurements. Since flow-rate measurement is one of the most critical parameters for compressor performance assessment, the choice of flow-meter technology can directly influence the evaluated compressor performance. Furthermore, the importance of energy efficiency has increased, while flow-measurement technologies have continued to develop since the last revision of ISO 1217 in 2009. Therefore, the influence of flow-meter technology and its measurement accuracy should be considered more explicitly in the ongoing revision of the standard to support reliable performance evaluation and the development of more energy-efficient displacement compressors. To investigate the influence of flow-meter technology, this paper reviews the physical operating principles of commonly used flow-meter technologies for compressed-air applications and discusses their respective advantages and limitations. In addition, four flow meters from different brands, utilizing different technologies, were experimentally compared on a dedicated laboratory test bench under controlled conditions at compressor discharge pressures between 5 and 8 bare, representing typical industrial operating pressures. To ensure comparability, all measured flow rates were normalized to common reference conditions according to ISO 2533. The results reveal substantial differences among the investigated technologies. The rotary displacement meter showed the closest agreement with the Venturi reference meter, with deviations generally below 3%. In contrast, thermal and ultrasonic flow meters exhibited larger systematic deviations, particularly when operated at low load factors relative to their measurement ranges. Based on the test results obtained with different flow-meter brands, the choice of flow-meter technology was found to significantly influence compressor performance calculations according to ISO 1217, resulting in calculated isentropic efficiencies ranging from approximately 58% to 72% under the same compressor operating conditions. These findings demonstrate that both flow-meter technology and sensor sizing can substantially affect the result of compressor performance evaluations. The results highlight the importance of appropriate flow-meter selection and indicate a need for clearer guidance regarding flow-measurement instrumentation in revisions of ISO 1217.