Abstract: A storage tank level monitoring apparatus and method for determining fluid levels in a storage tank. The apparatus of the present invention includes a magnet coated with rubber for holding a pivotally mounted housing to the interior of the storage tank and an ultrasonic transducer contained within the housing for generating acoustic pulses and for receiving reflections of the acoustic pulses for producing an analog signal proportional to the reflections, a string carrying a number of reference disks vertically oriented below the housing for producing a series of reflected acoustic signals from each reference disk, and a processor connected to the ultrasonic transducer for activating the transducer and for analyzing the reflected pulses for calibrating the apparatus for environmental changes in the atmosphere above the stored fluid and for determining the level of the stored fluid in the tank.