Ultrasonic Flow Meters    Ultrasonic flow meters use a method of measurement that measures the velocity of a fluid with ultrasonic sound waves to calculate the fluid flow. Using a pair of ultrasonic transducers, the flow meter measures the average velocity along the ultrasonic path, by averaging the difference in the transit time (time of flight) between the emitted and recieved pulses propagating into and against the direction of the flow.   This time difference is a measure of the average fluid velocity along the path of the ultrasonic beam. By using the transit times both the averaged fluid velocity and the speed of sound can be calculated. Using the two transit times (in the up stream and downstream directions), the axial distanc...