Area Velocity Sensor Installation - Teledyne 4250 Installation And Operation Manual

4250 Flow Meter
Section 3 Installation
3.6 Area Velocity Sensor
Installation
3-12
Before installing the AV sensor, please consider the following:
• Exact dimensions for the channel and the correct level of
the flow stream are essential for accurate results.
The importance of accurate measurements for both area
and level cannot be overstated. The 4250 makes all
subsequent calculations based on these measure-
ments.
• Errors in level measurement are more significant at low
flows, while dimensional errors become significant at
higher flows.
Even minor errors in measurement will have a signif-
icant and cumulative effect. For example, in a nominal
10" diameter pipe with a 3" liquid level, a measurement
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error of only / " in the diameter of the pipe and
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the level will result in a cumulative flow error of more
than 14%!
• Damage to the pressure transducer, ultrasonic trans-
ducers, electronics, or cable will ruin the sensor.
Handle the sensor with reasonable care when installing.
Do not drop or hit it. The pressure transducer is
sensitive to shock. The sensor is encapsulated in a
plastic potting compound and if damaged, cannot be
opened for repair.
• The standard AV sensor accurately detects levels above
approximately 0.1 foot (30 mm) and velocities for
streams with a minimum depth of 2 – 4 inches (50-100
mm).
This value is selected in programming. See Measuring
Minimum Depth on page 2-14. Below the minimum depth,
velocities are approximated, based on previous measure-
ments. Although the standard sensor can measure levels
less than 0.1 foot, (about one inch, or 25 mm) accuracy in
this range is not guaranteed.
Shallower streams should be measured using the
low-profile AV sensor. Streams that run consistently
below one inch are not a good application for the 4250.
• Velocity measurements depend on the presence of some
particles in the water, either air bubbles or suspended
solids.
If the flow stream is so clear that there are neither air
bubbles nor suspended solids, the velocity sensor cannot
function properly and will not be able to read velocity. In
such cases, it may be necessary to aerate the water
upstream from the sensor with a small pump or other
apparatus to ensure there is something to reflect the
ultrasonic waves.
• When installing the AV sensor in a pipe or invert, mount
the sensor upstream from the outfall. For the most
reliable readings, place the sensor at the bottom center
of the flow stream.
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/ " in
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