Abstract: A sonar transducer mounted on a probe inserted in a vessel or container emits a sonar wave for measuring the level of fluid interfaces within the vessel or container. The sonar transducer includes a housing closed by a diaphragm at one end. A piezoelectric crystal located within the housing is biased against the diaphragm and is electrically connected to an energizing source. The travel time of the sonar wave emitted by the piezoelectric crystal and reflected by the fluid interfaces is measured. The sonic wave measurements are communicated to a microprocessor and the levels of the liquid interfaces within the vessel or container are calculated.