Abstract: An ultrasonic flowmeter which measures gas velocity including a pair of ultrasonic transducers oppositely located around a conduit at a predetermined angle to the flow direction. Each transducer alternates between acting as a transmitter and a detector of ultrasonic frequencies, thus as one transducer transmits, the other receives. A measurement is made of the time of flight of the ultrasonic signal between the transmitter transducer and the receiver transducer and a microcomputer calculates the velocity of the gas by a known mathematical relationship utilizing the difference between the time of flight of the ultrasonic signals traveling upstream and those traveling downstream. The time of flight is determined by the phase shift or difference in phase angle between the signal at the transmitter transducer and the signal detected by the receiver transducer. To prevent ambiguity arising when the phase angle difference exceeds 360.degree.