Near-Field Diffraction and Reception Effects in Finite Element Modeling of Ultrasound Measurement Systems for Gas. Comparison to Measurements in Air

Ultrasonic transit time flow meters are promising for fiscal high-precision energy flow rate measurement of natural gas in subsea applications. For accredited operation, improved control with the measurement uncertainty is required, e.g., by using finite element (FE) modeling. Prior FE modeling typically applies simplifications such as far-field spherical reciprocity, piston-type diffraction correction, and free-field sensitivity. In this work, an FE modeling approach that avoids these limitations is implemented. Comparison with prior FE modeling and controlled measurements in air shows notably improved agreement with measurements for both magnitude and phase angle—for an example transmit-receive system with piezoelectric disks in the frequency range of their lower radial resonances (50-300 kHz), at separation distances 0.15 and 0.75 m. This can be used to improve sound velocity measurements in gas, and reduce uncertainty also in other applications based on transit time measurement in fluids.