An ultrasonic probe utilized in an ultrasonic ranging system for determining fluid volume and/or fluid leakage in an underground storage tank, as well as monitoring fluid inventory, is provided which is characterized by (i) having its calibration reflectors positioned whereby each of the secondary echo packets formed by the calibration reflectors of the ultrasonic probe becomes imbedded into the trailing edge of a primary echo packet of another calibration reflector or to cause its secondary echo packet to be detected by the transducer after the transducer has detected a primary echo packet associated with the fluid surface, (ii) each of the calibration reflectors being positioned perpendicular to and offset from the longitudinal axis of the probe body, (iii) an ultrasonic sound wave dampening member fixedly positioned in the probe body between the transducer assembly and the tank bottom, the wave dampening member having a surface facing toward the transducer assembly wherein the surface is configured to dampen sound waves striking the surface before the sound waves are reflected from the surface, or (iv) the transducer assembly comprising a transducer body in which is housed a piezoelectric crystal oriented perpendicular to the longitudinal axis of the prove body, wherein the transducer body is structured to dampen selected ultrasonic sound waves which emanate from the crystal.