An ultrasonic flow meter measures how fast water moves in a pipe or channel using sound pulses, then multiplies velocity by area to give flow.
An ultrasonic flow meter measures the velocity of a liquid with sound waves and multiplies it by the flow area to give a flow rate. It has no moving parts and, in clamp-on form, nothing inside the pipe. There are two main types. A transit-time meter sends ultrasonic pulses between two transducers, one pointing with the flow and one against it. Sound travels faster downstream than upstream, and the difference in travel time is proportional to the mean velocity along the sound path. This type needs fairly clean water. A Doppler meter bounces sound off particles or bubbles in the flow and reads the frequency shift of the echo. It needs suspended solids or air to work, so it suits raw sewage, sludge and turbid water, but it is less accurate in clean water. Utilities use transit-time meters on treated water mains, pump outlets and bulk transfer points. Clamp-on versions are popular for temporary surveys and for checking an existing meter, because they fit on the outside of the pipe without a shutdown. Area-velocity meters for sewers and open channels pair a Doppler velocity sensor with a depth sensor. Installation decides accuracy. The meter needs a settled flow profile, so installers keep a straight run of pipe upstream and downstream of the sensors, often quoted as about 10 pipe diameters upstream and 5 downstream. The pipe must run full. Clamp-on transducers need the right pipe material, wall thickness and lining entered, and good acoustic contact. Old lined or heavily scaled pipes can weaken the signal, and air bubbles disturb transit-time readings. For example, a transit-time meter on a 300 mm main reading a mean velocity of 1.2 m/s gives Q = 1.2 x 0.0707 m² = 0.085 m³/s, or about 85 L/s. Strengths are no pressure loss, a wide range of pipe sizes and easy retrofit. Limits are sensitivity to installation, pipe condition and water clarity.