Ultrasonic flowmeters measure fluid velocity by passing high-frequency sound waves along the fluid
flow path. Fluid motion influences the propagation of these sound waves, which may then be
measured to infer fluid velocity. Two major sub-types of ultrasonic flowmeters exist: Doppler and
transit-time. Both types of ultrasonic flowmeter work by transmitting a high-frequency sound wave
into the fluid stream (the incident pulse) and analyzing the received pulse.
Doppler flowmeters exploit the Doppler effect, which is the shifting of frequency resulting from
waves emitted by or reflected by a moving object. A common realization of the Doppler effect is the
perceived shift in frequency of a horn’s report from a moving vehicle: as the vehicle approaches the
listener, the pitch of the horn seems higher than normal; when the vehicle passes the listener and
begins to move away, the horn’s pitch appears to suddenly “shift down” to a lower frequency. In
reality, the horn’s frequency never changes, but the velocity of the approaching vehicle relative to
the stationary listener acts to “compress” the sonic vibrations in the air. When the vehicle moves
away, the sound waves are “stretched” from the perspective of the listener.
The same effect takes place if a sound wave is aimed at a moving object, and the echo’s frequency
is compared to the transmitted (incident) frequency. If the reflected wave returns from a bubble
advancing toward the ultrasonic transducer, the reflected frequency will be greater than the
incident frequency. If the flow reverses direction and the reflected wave returns from a bubble
traveling away from the transducer, the reflected frequency will be less than the incident frequency.
This matches the phenomenon of a vehicle’s horn pitch seemingly increasing as the vehicle approaches
a listener and seemingly decreasing as the vehicle moves away from a listener.
A Doppler flowmeter bounces sound waves off of bubbles or particulate material in the flow
stream, measuring the frequency shift and inferring fluid velocity from the magnitude of that shift