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dot dot Open Channel Flow: Methods, Applications, and New Developments

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Part 3 - Methods - Area/Velocity, New Developments


bullet Area Velocity
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 continuity equation:

Q = V x A

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:
bullet Doppler
bullet Transit time
bullet Electromagnetic
bullet Radar

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.

bullet Manning Formula
Another 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.

New Developments

If much of the technology for open channel flow was developed in past decades, what are the new developments? There are new developments on several fronts:

bullet Increased use of area velocity meters
bullet Greater computerization of flowmeters
bullet 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 and algorithms.

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 says.

(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 line.)

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Jump to:

bullet Part 1 - Introduction
bullet Part 2 - Methods - Dilution, Timed Gravimetric, Weirs & Flumes
bullet Part 3 - Methods - Area/Velocity, New Developments
bullet Part 4 - Comparison Table

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 control.

Visit Flow Research, Inc at http://www.flowresearch.com/




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Address: Marsh-McBirney, Inc. 4539 Metropolitan Ct. Frederick, Maryland 21704 USA
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