Abstract: Apparatus (1) for measuring fluid flow is disclosed. The apparatus comprises first, second and third ultrasonic transducers (8), the second and third ultrasonic transducers arranged to receive an ultrasonic wave (13) from the first ultrasonic transducer, wherein the second and third ultrasonic transducers are spaced apart by a given distance. The apparatus further comprises a circuit (6) comprising a phase comparator (21) arranged to compare first and second signals (20) from the second and third ultrasonic transducers.

Abstract: A flexural ultrasonic transducer is disclosed. The ultrasonic transducer comprises a sealed case which includes a flexible membrane and which defines a sealed cavity. The sealed case has an inside and an outside. The ultrasonic transducer comprises an active element, for example, a piezoelectric element, inside the sealed case supported on the flexible membrane. The ultrasonic transducer includes a liquid in the cavity. The sealed case includes a resilient portion between the inside and outside of the case for allowing equalization of pressure between the inside and the outside of the case. The resilient portion may comprise resilient walls or a thin-wall section, the flexible membrane or the case.