Abstract: The speed of flow of a fluid is measured by transmitting across the flowing fluid upstream and downstream ultrasonic energy beams, transmitted beams, deriving electrical outputs therefrom, detecting the outputs to provide upstream and downstream detected signals representing noise imparted to the transmitted radiant energy beams by the flowing fluid, digitizing the detected signals, and imposing a time delay or delays on the upstream signal corresponding to the time delay between the upstream and downstream signals due to the spacing of the receiver means. A shift register imposes the time delay or delays and the transmission time of the shift register is controlled in response to the product of the signals.