Blue-White Sonic-Pro Hybrid Ultrasonic Flowmeters - Manual (Page 6)

The Doppler measurement method requires that particles be present in the flow stream
to “reflect” the sound waves. The particles must be of sufficient size, volume and type to
reflect the sound waves. For example; although orange juice may contain pulp that is of
sufficient size and volume, the properties of the pulp do not allow the sound waves to
reflect. Therefore, the meter will not operate in the Doppler mode with orange juice
containing pulp.
The meter may be operated in the Doppler mode when the fluid contains
particles that are at least 50 micron in size and have sufficient
sound reflection properties.
Doppler measurement method, a continuous-wave ultrasonic
signal passes from a transmitting transducer into a pipe carrying a moving fluid. The
signal is reflected by particles in the fluid and is picked up by a receiving transducer on
the opposite side of the pipe.
When using the Doppler method, the transducers are installed directly opposite each
other along the pipe axis, one each side of the pipe, either with or against the flow of
fluid. The ultrasonic signal is subject to a Doppler shift when it is reflected from the
particles moving at (nominally) the same velocity as the fluid. Therefore, the frequency
at the receiving transducer will be slightly different from the transmitted frequency.
Signal Processing Unit) measures this difference in frequency and calculates the
fluid velocity.
Note that the meter is actually measuring the velocity of the particles and not the actual
fluid. For this reason, measurement error will occur if the particles do not move at the
same velocity as the fluid, such as when the particles are heavier than the fluid or when
the fluid is moving at such a low velocity that the particles drop out of suspension. Large
“gulps” of air or particles will temporarily disrupt the flow measurement resulting in
error.
0.02% to 15%
(200 to 150,000 ppm) of
When operated in the
The
SPU (
1.3
Doppler
Operation
Page 6 Sonic-Pro
Industries, Ltd. Industries, Ltd.
ProSeries
by Blue-White Ind.
TM
DOPPLER MEASUREMENT PRINCIPLE