Part 3 - Methods - Area/Velocity, New Developments
Flow can be
measured without a hydraulic structure such as a weir or flume. In
the area velocity method, the mean velocity of the flow is
calculated at a cross-section, and this value is multiplied by the
flow area. Normally, this method requires that two measurements be
made: one to determine mean velocity, and another measurement to
determine depth of flow. Flow rate Q is determined according to the
Q = V x
The area velocity method is used when it is not
practical to use a weir or flume, and for temporary flow
measurements. Examples include influx and infiltration studies and
sewer flow monitoring.
The velocity measurement is made using
a variety of technologies, including:
Using Doppler technology, a flowmeter with a velocity sensor
transmits high frequency sound waves into the flowing stream. These
waves are reflected off air bubbles and suspended particles in the
flow. The sensor detects the reflected waves and determines their
frequencies. By processing the reflected frequencies, the flowmeter
determines the velocity of the flowing stream.
Transit time technology is based on the idea that sound impulses
traveling with a flow stream move faster when traveling in the
direction of a flow stream and decelerate when traveling against the
flow stream. Transit time flowmeters have pairs of transducers that
generate sound pulses traveling at an angle to the flow. By
measuring the transit time of the sound pulses, the flowmeter
calculates velocity of flow.
Electromagnetic probes use Faraday’s principle to measure flow
velocity. According to this principle, when a conductor moves
through a magnetic field, it generates a voltage proportional to the
velocity of the conductor. The electromagnetic probes create a
magnetic field. When the liquid flows through this field, it creates
voltage. The probe measures this voltage by means of electrodes
positioned on the surface of the probe. This gives a local velocity.
Average flow velocity is calculated using this value, measured
depth, and the shape and size of the channel.
Radar has traditionally been a means to measure level, but is now
also being used to measure flow velocity. Radar flowmeters determine
flow velocity in much the same way that police radar guns measure
the speed of an automobile. The radar flowmeter transmits a radar
signal to the flowing stream. The signal is reflected back at a
different frequency than the transmitting frequency. By analyzing
the difference in transmitted and reflected frequencies, the
flowmeter calculates flow velocity.
The other component of area velocity measurement is the depth
measurement, which is determined by a level measurement. This value,
together with values relating to the geometry of the conduit, is
used to determine area. Any of the technologies described above to
measure level with flumes and weirs can also be used to measure
level for the purpose of area velocity measurement.
method of flow measurement involves using a modification of a
formula first proposed in 1889 by Robert Manning, an Irish civil
engineer. His original formula was modified in the 1930s. It is used
to calculate flow based on values such as cross sectional area of
flow, slope of the water surface, and roughness of the conduit. Use
of this formula does not require the presence of a primary device.
However, it is less accurate than the area velocity method because
flow velocity is calculated based on assumed values rather than
being measured. Use of the Manning formula is also called the
slope-hydraulic radius method.
If much of the technology for open channel flow was developed in past
decades, what are the new developments? There are new developments on
||Increased use of area velocity
||Greater computerization of
||Increased use of radar|
Area Velocity Meters
The use of area velocity meters has
increased over the past five years as technologies have improved and
prices have become more affordable. For example, significant
improvements have been made in the Doppler technology used to measure
velocity. In addition, improvements have occurred in signal processing
Increased use of microprocessor technology
continues to drive enhancements in open channel flow technology.
While radar is a traditional level technology, it is
now being introduced into open channel flow measurement. Marsh-McBirney
of Frederick, Maryland is bringing out a new product called Flo-Dar that
uses radar to measure flow velocity. Marsh-McBirney has traditionally
used an electromagnetic sensor to measure velocity in its flowmeters.
Larry Marsh, president of Marsh McBirney, believes the non-contact
nature of Flo-Dar will make it attractive to users. "Over time, prices
will drop and non-contact will become the method of choice," he
(Editors Note: This article was originally published in
1999. Since that time Marsh-McBirney has successfully introduced
the Flo-Dar technology and it is a major component of their product
Jesse Yoder has fifteen years experience as an analyst and writer in
process control. He specializes in flowmeters and other field devices,
including level, pressure, and temperature products. He has written
over 40 market research studies in industrial automation and process
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