Abstract: A non-invasive system and method for measuring liquid level in a vessel utilizes a pair of ultrasonic transducers mounted spaced apart externally on the vessel wall. One of the transducers excites a single pulse of sonic energy that produces a flexural, or elastic, wave in the wall and the second transducer receives the flexural wave to produce an electrical signal waveform corresponding to the travel time of the flexural wave between the two transducers. The electrical waveform signals produced at different times are compared to determine if a phase delay exists between two waveform signals, which indicates that there has been a change in the liquid level condition in the vessel interior space between the two transducers. Measuring the phase delay, or time delay, between the flexural wave components produced at two different times is used to indicate presence or absence of the liquid at a point on the vessel and/or the height of the liquid in the space.