Abstract: A transit time gas flow meter comprising a pair of transducers mounted at an oblique angle to a gas flow in a duct for measuring transit time of an ultrasonic pulse in both directions between the transducers and a computer to determine the difference between transit times as a measure of the velocity of a gas stream flowing in the duct. Each transducer is carried in a mounting assembly which is comprised of a U-shaped frame member having a base and leg members and an ultrasonic reflection surface formed on the base of a U-shaped frame member. A transducer is mounted between said legs and at a reference target carried on the U-shaped frame member and is positioned at the edge of the gas, and the ultrasonic transit time to each said reference target is subtracted by the computer means from the transit times from each transducer to the other transducer, to zero flow calibrate the transit time flow meter and to improve the accuracy measurement.