Hach 911, User Manual

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Appendix A Working with Primary Devices
A.1 Working with Primary Devices and Sensor Operation
Installing the sensor in any primary device requires familiarity with the proper location for
head measurement in that particular primary device. Just as the proper installation of the
primary device itself is critical to obtaining optimum flow measurement accuracy, the
appropriate location of the submerged or ultrasonic sensor also greatly affects flow
measurement accuracy.
Always consult the manufacturer of the primary device, whenever possible, for details
concerning the proper location of the sensor.
A.2 Setting an Offset (For Use in a Weir)
When installing a submerged sensor behind a weir, the user may need to “offset” the
actual reading from the submerged sensor to compensate for the difference in height
between the submerged sensor and the crest of the weir. This allows the user to place the
sensor at an arbitrary height in the weir, as long as it is located below the crest of the weir
(and at the proper distance upstream from the weir plate). See
Figure 40 on page 76 for
details on head measurement locations in a weir.
In the text example below:
• The distance between the crest of the weir (bottom of the ‘V’) and the submerged
sensor is 4 in. (10 cm).
• The distance between the surface of the water and the submerged sensor is 6 in. (15
cm).
• The level of water which is contributing to flow is 2 in. (5 cm) (water flowing over the
weir).
Entering 2 in. (5.1 cm) using the
LEVEL ADJUST
key provides the desired offset. As the
water level falls to the bottom of the ‘V” the flow meter reads zero level and zero flow. If the
level falls below the crest of the weir, due to evaporation or other reason, the level reads a
negative number and the flow remains at zero.
The key to proper level setting is to adjust the level (using the
LEVEL ADJUST
key) to the
level that is contributing to flow. In a round pipe the level contributing to flow is the distance
from the surface of the water to the invert (bottom) of the pipe. In a flume, the level
contributing to flow is the distance from the surface of the water to the floor of the flume.
A.3 Types of Primary Devices
Use the primary device illustrations that follow as general guides for proper head
measurement location in commonly used primary devices. Refer to
Figure 41 on page 77
for Parshall Flume, Figure 42 on page 78 for Palmer-Bowlus Flume (with Integral
Approach), Figure 43 on page 79 for Leopold-Lagco Flume, Figure 44 on page 80 for
H-Flume and Figure 45 on page 80 for Round Pipes. Contact the primary device
manufacturer for more details.