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ADFM® Flow Meter
Section 1 Introduction
1-5
1.4 Operating Principles
Figure 1-1 shows a typical ADFM Pro20 installation for mea-
suring open channel flow in a pipe. The sensor is mounted on the
invert of a pipe or channel. The sensor’s piezoelectric ceramics
emit short pulses along narrow acoustic beams pointing in dif-
ferent directions. Echoes of these pulses are backscattered from
particles or bubbles suspended in the flow. As this material has
motion relative to the transducer, the echoes are Doppler shifted
in frequency. Measurement of this frequency enables the calcu-
lation of the flow speed. A fifth ceramic mounted in the center of
the transducer assembly and aimed vertically is used to measure
the depth.
Figure 1-1 Typical Pro20 Installation (2 of 4 profiles shown)
The ADFM divides the return signal into discrete regular
intervals that correspond to different depths in the flow. Velocity
is calculated from the frequency shift measured in each interval.
The result is a profile, or linear distribution of velocities, along
the direction of the beam. Each of the small black circles in
Figure 1-1 represent an individual velocity measurement in a
small volume known as a velocity bin.
The directions of the velocity profiles are based on the geometry
of the ADFM’s transducer assembly. Figure 1-2 shows a side view
of the transducer assembly. The profiles shown in Figure 1-1 are
generated from velocity data measured by an upstream and
downstream beam pair. The data from one beam pair are
averaged to generate Profile #1, and a beam pair on the opposite
side of the transducer assembly generates Profile #2.
Since Doppler measurements are directional, only the component
of velocity along the direction of transmit and receive is mea-
sured. Narrow acoustic beams are required to accurately
determine the horizontal velocity from the measured component.
The narrow acoustic beams of the ADFM ensure that this mea-
surement is accurate. Also, the range-gate times are short and
the velocity bins occupy a small volume - cylinders approximately
5 centimeters (2 inches) long and 5 centimeters (2 inches) in
diameter. This ensures that the velocity measurements are truly
representative of that portion of the flow, avoiding potential bias
Velocity
profile #1
Sensor
Velocity
profile #2
Velocity
bins
Flow
pattern