Abstract: A timing means for determining the time of travel of ultrasonic pulses between a transmitter and a receiver immersed in a fluid. The first time instant is determined when the transmitter transmits an ultrasonic pulse in the fluid. A second time instant is determined when the ultrasonic wavelet has been detected, converted to electrical signal, and the zero crossing time of the electrical wavelet after a selected positive peak is determined. Timing means are provided to determine the time interval between the first and second time instants. This is done by having a coarse time clock and counter. The number of full clock periods before the second instant is determined, and a ramp of voltage is generated having a known rate of rise. The ramp is negatively biased and the bias voltage is varied until the ramp voltage reaches zero at the second instant. The transmitter upstream is fired, and the wavelet is detected at the downstream transducer.