Abstract: The present invention relates to a method and a device to measure reliably and accurately a set of data needed to test, operate and monitor the performance of turbines. The invention proposes to determine the performance parameters of a turbine applicable to its entire operating envelope and for a condition when it is producing net output power, by merely measuring its characteristic performance parameters only in the mode of no net output-power production. In the method the performance response of a turbine under no-load conditions and/or no-speed conditions is measured when subject to known values of input for flow, head, hydraulic energy and fluid properties. From this the condition and the performance of the turbine can be determined. Furthermore it is possible to determine best efficiency point for operating the turbine from the recorded no-load and no-speed characteristic.