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Section 4
Page 79
Downlook Ultrasonic Sensor
4.10.1 Installing an Ultrasonic Sensor (Downlook) at a Primary Device
Note: Do not suspend the sensor by
the cable. Only use appropriate
mounting hardware.
Site selection can greatly enhance system performance. Several important
factors should be considered when installing the ultrasonic sensor at any
primary device. Refer to Appendix A for more information on primary devices.
• Locate the ultrasonic sensor at the proper head measurement point for
that primary device.
• Determine the appropriate sensor height. Refer to Figure 33 on page 80.
• Suspend the ultrasonic sensor over the center of the flow stream where
there is the least surface turbulence.
• Mount the sensor to a stable, secure location, isolated from vibration.
Figure 31 Ultrasonic Sensor (Downlook)
4.10.1.1 Choosing the Appropriate Sensor Height (Ultrasonic “Downlook” Sensor)
The beam angle is half the band width at -10 dB. The beam angle from the
bottom of the ultrasonic sensor spreads out at an angle of 12° for the 75 kHz
sensor as it travels away from the sensor. Therefore, if you mount the sensor
too high above a narrow channel, the beam may be wider than the channel
when it reaches it. This can cause false echoes from the tops of the channel
walls rather than from the water surface. To prevent false echoes, determine
the appropriate sensor height so that the entire beam falls within the channel
and does not strike any obstructions
To determine the appropriate sensor mounting height
1. Take the measurements described in Figure 32.
2. Calculate the angle of obstruction from the axis.
is accurate within one degree.
3. Plot the measurements on the graph in Figure 33 to determine if the
beam width falls within the possible false echo or safe area.
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