methods for measuring liquid levels and leak rates in a storage tank , using an acoustic transducer and a quasi - static reference device that positions an acoustic target within a predetermined distance range below the surface of the liquid . a method for measuring the height of the surface of a liquid , i . e ., liquid level . in a tank comprising the steps of positioning the emitting and receiving acoustic transducers below the surface of the liquid , positioning a plurality of fiducials separated by known distances such that at least two of the fiducials lie below the surface of the liquid , emitting acoustic signals , receiving and measuring the travel times for the reflections of the acoustic signals from the liquid surface and the fiducials , estimating the speed of sound in the liquid using the travel times for the acoustic signals in the liquid between at least two of the fiducials , and calculating the height of the liquid surface by multiplying the speed of sound in the liquid by the travel time from the liquid surface to a known height above the bottom of the liquid , and adding the known height to the product of the speed of sound and travel time .