Measurement stability of oil-filled flexural ultrasonic transducers across sequential in situ pressurization cycles

Recently, flexural ultrasonic transducers (FUTs) for ultrasound measurement toward 200 bar were demonstrated, overcoming the major limitation of commercial variants associated with pressure imbalances due to their rear seals. One solution is through venting approaches, and another is introducing an incompressible fluid to the transducer's interior, thus creating a pressure balance across the vibrating plate. However, this approach has not been validated for repeated pressurization cycles consistent with practical industrial applications. Here, the structural resilience and dynamic responses of oil-filled FUTs (OFFUTs) toward 200 bar are investigated through finite element and experimental methods, including electrical impedance and pitch-catch measurements. Sequential pressurization and depressurization cycles are applied, where the relationship between dynamic response and pressure level is monitored, and the transducer is assessed for its potential longevity in performance. The results demonstrate that via an incompressible fluid in the sensor cavity, stable and reliable ultrasound measurements, across frequency, electrical impedance, and amplitude, are possible across multiple pressurization and depressurization cycles toward 200 bar, where associated pulse envelopes can be used to directly correlate with the environmental pressure level.