Investigation of the Mechanism Governing Linearity in Guided Wave Ultrasonic Flow Meters

In ultrasonic flow metering, guided waves provide multiple advantages over conventional bulk waves: a longer path length increases transit time differences between upstream and downstream signals, reducing electronic noise requirements; a single acoustic path suffices for measurement, unlike multi-path bulk wave sensors; and they report consistent linearity, i.e. measurement error versus Reynolds number, in both in-line and clamp-on configurations. While bulk wave sensor linearity depends on the flow profile plane intersected by the acoustic beam, guided wave sensors are often designed using a simplistic analysis of dispersion curves, selecting wave modes for ease of time-windowing or faster propagation to minimize sampling windows. This approach fails to predict linearity, resulting in reported values ranging from 2% to 15%. This work demonstrates that the axial component of the excited guided wave mode shape governs sensor linearity. Using the mode shapes of four guided wave modes, linearity curves of commercial devices were predicted and validated through measurements in a flow loop with three different liquids, showing excellent agreement between predicted and measured linearity.