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 Section 4. VELOCITY MODIFIED FLOWMETERS
1. INTRODUCTION. A velocity modified flowmeter is a single-unit sensing
device utilizing velocity, depth, and diameter data to determine volumetric
flow in open channels. Special units can be obtained for use in full pipe
flows. A typical unit is made up of a steel band to which is attached an
electromagnetic flow-velocity sensor and a pressure transducer to measure
water depth (Figure 7-9).
1.1 Operating Principles. Using a plug-in module correlated to the pipe
diameter, the processor applies velocity and depth data to the continuity
=
equation (Q A x V) and computes volumetric flow without need for pipe slope,
surface roughness, or empirical formulas. Even though volumetric flow is the
principle information computed and recorded, velocity and depth data may be
displayed.
METER DESIGNS. A typical unit is a steel band , sensor units, and cabling
2.
between the power source and the data transmitter.
2.1 Steel Band. An expandable , stainless steel band recessed into the flow
conduit and expanded against the interior wall. Attached to the steel band is
a capsule containing the velocity and depth sensors. The capsule shape is
streamlined to prevent fouling of debris.
2.2 Velocity Sensor. A solid state, electromagnetic sensor attached to the
steel band to correlate existing velocities to mean velocity of the flow
conduit. These data are transmitted to the unit processor.
2.3 Depth Sensor. Water depth is measured by a submerged pressure transducer
encapsulated with the velocity sensor.
Water depth data are transmitted to
the unit processor.
3. LIMITATIONS.
The limitations of a typical velocity modified flowmeter are
as follows:
Conduit diameter range is from 8 to 102 inches.
q
limits are from -lC (-30F) to +32C (+90F).
q Temperature
q Liquids
must have a conductivity of at least 5 micromhos/cm.
q Diameter
of pipe or cross-section of channel must be known.
VAC, 60 Hz power is required.
q 120
INSTALLATION. Velocity modified flowmeters are installed inside pipes
4.
using an expandable steel band sized to the pipe diameter. Velocity and depth
sensors are an integral part of the band. Cabling is attached to the sensors
from a power supply and to recording equipment. Installation u ay be permanent
or temporary.
7-13
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