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Working with Primary Devices
Appendix A Working with Primary Devices
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 your primary device, whenever possible,
for details concerning the proper location of the sensor.
Setting an Offset (For Use in a Weir)
When installing a submerged sensor behind a weir you 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 you 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 50 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 will provide the desired
offset. As the water level falls to the bottom of the ‘V” the flow meter will read
zero level and zero flow. If the level falls below the crest of the weir, due to
evaporation or other reason, the level will read a negative number and the flow
will remain at zero.
The key to proper level setting is to adjust your level (using the
LEVEL ADJUST
key) to the level which 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.
Types of Primary Devices
Use the primary device illustrations that follow as general guides for proper
head measurement location in commonly used primary devices. Contact your
primary device manufacturer for more details.