Non-invasive liquid-level gauging using guided waves with reference-paths based temperature compensation

This paper presents an accurate, non-invasive liquid-level sensing method based on ultrasonic guided waves generated and received by piezoelectric transducers bonded to the outer wall of a tank. The proposed approach exploits guided-mode conversion between dry and wetted regions, resulting in a dominant quasi-Scholte (QS) mode contribution in the fluid-loaded region. Liquid-level measurement is performed using a calibrated procedure that jointly exploits the differential time of flight and associated phase of the selected QS contribution in the ultrasonic signals, without requiring prior knowledge of the fluid or plate mechanical properties during the estimation stage. A dedicated temperature-compensation strategy based on reference-path measurements is introduced to mitigate thermal drifts affecting physical parameters, and thus wave propagation. Experiments conducted on a representative aluminum tank demonstrate a liquid-level accuracy on the order of 1 mm under the investigated conditions. These results highlight the potential of A₀/QS guided-wave conversion combined with calibration and reference-path-based thermal compensation for compact, accurate, and fully non-invasive level-sensing systems.