Main goal of this work was to compare the results of water flow measurement with rotameter and orifice plate. The tested measuring devices are installed in a cavitation pipe system designed for intensifying the process of mixing and processing water. The system is in the development phase as part of co-operation and joint research in the Laboratory for Water, Fuel and Lubricants at the Faculty of Mechanical Engineering and Naval Architecture and the Cabinet for Basic Engineering, Faculty of Food Technology, University of Zagreb. A theoretical part of the task, which describes the history of flow measurement, flow measurement method and mathematical background related to flow measurement, is first made. Then measurements were carried out on the measuring line already in place, where there was no possibility of change except for the change of the orifices plates. Measurements were performed using standard (DN10) and non-standard 3 mm diameter orifice plates, and all results were compared with the results obtained on rotameter. As the rotameter was a constraining element due to the low metering range, the results are comparable to the first four pump strength positions from which we obtained 4 metering points. The goal was to determine the flow rate for each specified point with the standard and nonstandard orifice plate. Flow calculation was performed according to ISO 5167 standard. For all flow values, measurement uncertainties according to ISO GUM method were determined. The Metrological Measurement Laboratory for Process Measurements was used.