Abstract: A non-intrusive acoustic sensor system for detecting and displaying the location of the level of a liquid in a tank uses a piezoelectric transceiver mounted to a bottom exterior location on the tank. The transceiver is an element of a driving and signal processing circuit which also includes a readout device for displaying liquid level location in selected terms. The system circuitry includes a microprocessor programmed to enable the velocity of sound in a liquid in the tank to be determined by the system at the site of use from a single distance measurement descriptive of the usage situation. The system automatically selects and uses the transceiver acoustic emission frequency which is best for the tank and liquid combination of interest at any time. The system, in a dual-transceiver configuration described, determines and uses speed of sound in the liquid as it in fact exists at any time.