Abstract: A transit time ultrasonic flow meter includes a pair of ultrasonic transducers transmitting an ultrasonic signal through a fluid flowing in a conduit and receiving the transmitted ultrasonic signal. An exclusive-OR (XOR) phase comparator then detects a difference in phase between the received ultrasonic signal and a reference signal having a selected phase shift (e.g., 0°, 90°, 180°, or 270°) with respect to the transmitted ultrasonic signal. Control circuitry selects the phase shift of the reference signal based on outputs of the XOR phase comparator in order to maintain the phase shifts detected by the XOR phase comparator within a range in which the comparator has relatively accurate detection (e.g., 45° to 135°). By adjusting the phase of the reference signal and thereby maintaining the XOR phase comparator in its relatively high accuracy region, the present invention enhances the accuracy of detection by the phase comparator.