Hach 911, User Manual

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Section 3 Getting Started
3.1 Installing the Flow Meter-Considerations
3.1.1 Choosing the Proper Site
The accuracy of flow measurement depends on the suitability of individual monitoring
sites. Select sites that have normalized flow and minimal turbulence. Turbulence can
make it difficult to detect an average velocity in the flow stream. Obstructions, vertical
drops, pipe bends, and elbows can create turbulence.
Table 3 contains suggestions for
preventing turbulence.
3.1.2 Dealing with Difficult Sites
Some sites may be difficult to monitor due to poor site conditions. The direction and speed
of particles in the flow stream contribute to the signal received by the velocity sensor. If the
turbulence near the measurement point is excessive, it may be difficult for the sensor to
determine the average velocity of the stream. The flow meters provide several unique
features to help deal with these problem sites.
3.1.3 Choosing the Appropriate Meter and Sensor Combination
Use Table 4 to determine which sensor to use. See Section 6 on page 43 for sensor
installation instructions.
Table 3 Suggestions for Preventing Turbulence
Site Condition Suggested Remedy
Outfalls Attach the sensor at least ten times the maximum expected level upstream of the outfall.
Vertical drops in the
channel floor
Attach the sensor at least ten times the maximum expected level upstream of the vertical drop.
Attach the sensor at least ten times the maximum expected level downstream of the vertical drop.
Elbows, sharp turns,
and “Y” connections
Locate the sensor at least ten times the maximum expected level upstream of the obstruction.
Locate the sensor at least ten times the maximum expected level downstream of the obstruction.
Table 4 Flow Meter and Sensor Configurations
Flow Meter Model Level and Velocity Sensor Configurations
911 Meter plus one Submerged Area/Velocity Sensor or one Velocity-Only Sensor
940
Meter plus two Submerged Area/Velocity Sensors or
Meter plus one Submerged Area/Velocity Sensor and one In-Pipe/Downlook Ultrasonic Sensor.