Note how the Doppler effect yields a direct measurement of fluid velocity from each echo received
by the transducer. This stands in marked contrast to measurements of distance based on time-offlight
(time domain reflectometry – where the amount of time between the incident pulse and the
returned echo is proportional to distance between the transducer and the reflecting surface), such
as in the application of ultrasonic liquid level measurement. In a Doppler flowmeter, the time delay
between the incident and reflected pulses is irrelevant. Only the frequency shift between the incident
and reflected signals matters. This frequency shift is also directly proportional to the velocity of
flow, making the Doppler ultrasonic flowmeter a linear measurement device.
Re-arranging the Doppler frequency shift equation to solve for velocity (again, assuming v