Trains passing tunnels cause pressure-waves. This can adversely affect the tunnel drainage. Under certain circumstances, the direction of flow could be reversed. For the purpose of investigation a measurement system is needed, which is able to simultaneously measure speed as well as direction for both air and water. The measurement system should be suitable for measuring small velocities of several cm/s with high precision, while having a range of several m/s. Furthermore, the data needs to be captured with a rate of at least 1 Hz to guarantee a sufficiently high temporal resolution. The sensor has to be placed directly in the drainage pipe. As a consequence, it has to be robust and water-resistant.

For the calibration of the measurement system a verifiable and constant flow has to be generated. Since the generation of a constant and laminar flow is very difficult due to different hydro-mechanical effects a measurement setup was chosen where the sensors are moved through a stationary medium (Figure 2). Thus, a nearly constant flow can be generated. Two ultrasonic sensors where positioned at a certain distance from each other and at a certain angle to the flow direction. A linear axis was used to simulate different flow velocities. A vibrometer served as reference. The performed measurements achieved an uncertainty of 0.1 cm/s. Additionally, the suitability of the system for the measurement of higher flow velocities in the range of m/s was confirmed qualitatively.