Ultrasonic Flow Meters - Principles of Operation

Transit-Time Ultrasonic Flow Meters

Ultrasonic waves are transmitted in the direction of flow.  These are accelerated slightly by the velocity of the liquid in the pipe.  When ultrasound is transmitted in the opposite direction, the flow of the liquid causes the transmitted sound to decelerate.  The subsequent time difference is directly proportional to the flow velocity in the pipe.  Having measured the flow velocity and knowing the cross-sectional area, the volumetric flow can be easily calculated.

Doppler Ultrasonic Flow Meters

A Doppler ultrasonic flow meter operates on the Doppler shift principle, whereby the transmitted frequency is altered linearly by being reflected from particles and bubbles in the fluid.  A transmitter transmits a high frequency ultrasonic signal into the pipe which is reflected off particulate matter or gas bubbles in the fluid and picked up by the receiving sensor.  The fluid velocity in the pipe is directly proportional to the change in frequency between the transmitted and reflected ultrasonic signals.  With knowledge of the pipe size, the electronics of the flow meter will correlate the fluid velocity into volumetric flow.