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Part 2 - Methods - Dilution, Timed Gravimetric, Weirs & Flumes

Methods Used to Measure Open Channel Flow

A variety of methods are used to measure flow in open channels. Many different factors help determine which method is best for a given application. These factors include the nature of the application, budget, accuracy needs, and reporting requirements. This section discusses different methods for measuring open channel flow.

bullet Dilution
Using the dilution method, a tracer solution is added to the flowing water. Somewhere downstream, a measurement is made of the amount of solution contained in the flowing water. This amount is used to calculate flow rate, based on a theoretical formula. Examples of tracers used in the dilution method are radioactive and fluorescent dyes. Two techniques used in dilution flow measurement are the total recovery method and the constant rate injection method.
bullet Timed-Gravimetric
In the timed-gravimetric method, the liquid is captured in a container for a specified period of time. The liquid is then weighed, using a mass or force measuring device such as a beam scale or load cell. A gravimetric meter, such as a weigher, works somewhat like a positive displacement meter used to measure closed pipe flow. Positive displacement meters collect fluid in a small cavity or chamber, and count the number of times the container is filled with fluid. The timed-gravimetric method gives high accuracy, but is not suited to continuous flow measurement. A variation of this method uses a container with a known volume and a stopwatch to capture the flow and spot-check the flow rate.
bullet Use of Weirs and Flumes
A very common method of open channel flow involves the use of a hydraulic structure such as a weir or flume. These hydraulic structures are called primary devices. A primary device is a restriction placed in an open channel that has a known depth-to-flow relationship. Once a weir or flume is installed, a measurement of the depth of the water is used to calculate flow rate. Charts are available that correlate various water depths with flow rates, taking into account different types and sizes of weirs and flumes.

In some respects, a weir or flume in an open channel operates like an orifice plate used to measure flow rate in a closed pipe. An orifice plate is also a restriction placed in a pipe that causes a pressure drop across the plate. By measuring the pressure differential across the orifice plate, flow rate can be determined, using an equation. The difference is that for open channel flow, water level is the variable measured instead of difference in pressure.

A weir resembles a dam placed across an open channel. It is positioned in such a way that the liquid can flow over it. Weirs are classified according to the shapes of their openings. Types of weirs include:
bullet V-Notch
bullet Rectangular
bullet Trapezoidal

Water depth is measured at a specific place upstream from the weir. An equation for determining flow rate is associated with each type of weir.

A flume is a specially shaped portion of the open channel with an area or slope that is different from the channelís slope or area. The velocity of the liquid increases and its level rises at it passes through the flume. Types of flumes include:
bullet Parshall
bullet Palmer-Bowlus
bullet Leopold-Lagco
bullet H-type Flumes
bullet Trapezoidal
bullet Cutthroat

To determine flow rate, liquid depth is measured at specified points in the flume. An equation is associated with each kind of flume, taking flume size into account.

The choice in type of flume depends on the application. The Parshall flume was first introduced in the 1920s to measure the flow of irrigation water. Today, Parshall flumes are used in sewers and in sewer treatment plants. Palmer-Bowlus flumes were developed in the 1930s to measure wastewater flow. The U.S. Department of Agricultureís Soil Conservation Service developed H-type flumes in the same decade. The purpose of H-type flumes was to measure runoff from small agricultural watersheds. Today, H-type flumes are used to measure low flow of streams, feedlot runoff, and sewage flow.

Flow measurement with weirs and flumes also requires a secondary device to measure level. There are a number of technologies used to measure level. These include:
bullet Submerged pressure transducer
bullet Float
bullet Ultrasonic
bullet Bubbler
bullet Electrical

Pressure transducers submerged in the flow stream measure the hydrostatic pressure of the liquid above the transducer. This pressure is proportional to liquid level. These transducers are not affected by wind turbulence, or floating debris. However, their accuracy may be affected by temperature changes.

A float is used in combination with a pivoting arm or a cable and pulley. The float is used to measure level. This level value is converted into the position of a shaft, where the level value can be read. Floats are mechanical devices and require periodic maintenance.

Ultrasonic level meters work by transmitting a pulse to the surface of the liquid. By measuring the time it takes for the pulse to be reflected back to the meter, the ultrasonic meter determines the level of the stream.

Bubblers measure the hydrostatic pressure of the liquid. A bubbler with a bubble tube is located at the bottom of the flow stream. Bubbles or pressurized air or other gas are emitted from the end of the bubble tube at a steady rate. A transducer in the bubbler measures the amount of pressure required to maintain a steady bubble rate. Liquid level is proportional to this pressure.

Electrical methods measure depth by making use of the change in an electrical circuit that results from changing levels. These methods often involve the use of a capacitance probe. The accuracy of these probes can be affected by changing liquid characteristics, or by coatings of solid or grease.



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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