Hach 911 User Manual page 46

Sensor Installation
1 Maximum Range
2 Minimum Distance (must be at least 11 in. (29 cm)
1 Distance from Transducer
44
sensor is "blind" to anything closer than the deadband and stops reading level when the
distance is shorter than that.
4
Figure 22 Invisible Range
Beam Angle
The beam angle from the bottom of the ultrasonic sensor spreads out at an angle of 5° for
the 75 kHz sensor as it travels away from the sensor. Therefore, if the sensor is mounted
too high above a narrow channel, the beam may be wider than the channel when it
reaches the bottom of the channel. 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.
Beam Width Formulas
The maximum beam width for a given distance from the sensor can be calculated as
follows: 
0.087 distance from sensor
2
Figure 23 Beam Width as Distance from Transducer Increases
Protecting the Sensor
Keep the face of the sensor free of accumulating grease and dirt. Since the 940 Flow
Meter listens for the returning echo, a coated sensor will not be able to accurately detect
the echo and may provide inaccurate level measurement.
1
3
3 Highest expected water level
4 Invisible Range (set to ignore tops of channel walls)
. Refer to Figure 23 and
1
2 Beam Width
2
Table 8 on page 45.