Abstract: An ultrasonic liquid level detector mounted in a fill pipe cap. The ultrasonic detector mounted in a fill pipe cap (10) is used in determining a level and/or the volume of a liquid (5) stored in an underground storage tank (20). The fill pipe cap contains an ultrasonic ranging unit (50) that measures a time, T.sub.L, required for an ultrasonic pulse to travel round trip between an ultrasonic transducer/receiver (54) and the surface of the liquid in the tank. A reference reflector (25) disposed within a fill pipe (30) is used in determining a reference time, T.sub.R, that is included in a non-linear expression, e.sup..alpha. .multidot.T.sub.L.sup..beta..sbsp.1 .multidot.T.sub.R.sup..beta..sbsp.2, in order to compute the level of liquid in the storage tank. The terms e.sup..alpha., .beta..sub.1, and .beta..sub.2 comprise a correction for errors in the round-trip times T.sub.L and T.sub.R caused by a non-uniform vapor density of the liquid along the path traveled by the ultrasonic pulses.