Acoustic flow measurement systems have long established themselves as a reliable and convenient method of determining turbine efficiency. Measurements in several planes according to international standards are recommended for this task. With this method, there is no need for calibration and a deterioration of turbine or hydraulic efficiency can be detected at an early stage. A further area of application is the detection of leaks, where at least two systems are permanently installed. Because of the accuracy of the Ductus system, even small leakages can be detected instantaneously.