Abstract: An improvement in systems for measuring the rate of fluid flow in a pipe or an open unrestricted channel as a current at sea or ocean wherein the time delay between acoustic pulses transmitted upstream and downstream in a fluid passing along a path of travel is multiplied by repeated transmissions over predetermined time intervals. The correction for the speed of sound in the medium is founded on the realization that in a multiple time difference ultrasonic flowmeter, there is a pulse (N.sub.c .DELTA.t) in the generated N.DELTA.t train which is directly proportional to the velocity of the fluid. Accordingly, the time interval during which the train N.DELTA.t last is automatically adjusted so that the time difference, or multiple of .DELTA.t, be in fact N.sub.c .DELTA.t.