Effect of transducer port cavities in invasive ultrasonic transit-time gas flowmeters

The design of ultrasonic gas flowmeters (UFMs) requires a thorough characterization of the acoustic sound pressure field. For large pipe flowmeters, such as used for custody or flare gas metering, the transducers are operated at frequencies ranging from 20 kHz up to 150 kHz. Thus, we investigate the effect of various acoustic boundary conditions for a 40 kHz air-coupled ultrasonic transducer by varying the mounting position inside the transducer port cavity. We used a volumetric characterization system to measure the sound pressure field for various boundary conditions. A pipe section with a diameter of ten inch and two representative inclination angles for a single-path UFM are used. We observe a maximum bending of the main sound lobe up to 16° and a significant change of the shape from single to multiple lobes. Our results prove that the transducer port cavity has significant effects on the sound pressure field and that these effects need to be taken into account when a flowmeter is supposed to be designed properly in terms of a symmetric or intentionally non-symmetric sound propagation.