Abstract: Apparatus for measuring the mean velocity of fluid flow down a pipe, or quantities related to it such as mean mass flow. Two similar beams of ultrasonic energy are transmitted across the fluid, one beam with an upstream and the other with a downstream component of direction. The output of the apparatus is derived from the difference in time taken by the two beams to pass through the fluid, for instance by the phase difference at the receivers of beams that were transmitted synchronized. By locating the beams so that they are offset from the pipe axis, instead of intersecting it as has been customary, it is possible to depress the effect upon the output of variation in the flow profile, (e.g. laminar, turbulent or uniform) of the fluid. In a circular pipe, such profile effects upon the output will often be minimized if the beams lie in planes parallel to the axis and separated from it by a distance equal to half the radius of the pipe or slightly more.