The measurement of low air flow velocities

Air flow velocity is measured with an acoustic sensor, which can be used especially for measuring low air flow velocities as well as the temperature of the air simultaneously. Two opposite transducers send a sound pulse towards each other. From the difference of the transit times, the air flow velocity is computed. With this method, it is possible to determine the flow direction even at low velocities. Two calibration methods are carried out. First, a set of calibration experiments is carried out in a wind tunnel, in which a uniform air flow is generated. In the centre of the wind tunnel, the sensor and a reference speedometer of Pitot tube are installed to measure the air flow velocity independently. Second, a set of calibration experiments for low air flow velocity is performed with the acoustic sensor in amoving tube with a controlled velocity. The sensor shows good and accurate results corresponding to the speed of the tube