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The wastewater industry has relied upon the flume as the primary device for measurement of open channel wastewater flows. This method of open channel flow is achieved by placing a hydraulic structure (the flume) in the channel so as to produce a flow in the structure that is characterized by a known relationship between the liquid level, at a particular measurement location, and the flow rate of the stream. Advances in non-contact level measurement using ultrasonic level sensors (and, more recently, radar level measuring devices) have made the flume a very low-maintenance flowmeter that provides industry-acceptable accuracies of within ± 5% of reading.

In the late 1970's, a new type of open channel flowmeter was introduced into the wastewater industry. These flowmeters, called Area/Velocity flowmeters, relied upon the Continuity Equation, which states that flow (Q) is equal to the Average Velocity times the Area of the partially filled pipe. The advantage of the Area/Velocity flowmeter is that, unlike a flume, one flow device can be used on a variety of pipe shapes and sizes, without channel modification, therefore allowing the instrument to be easily transported and used between a variety of flow situations. Although Area/Velocity flowmeters have provided much needed data for control and engineering analysis throughout the wastewater industry, all of these meters tend to foul because they must be submerged in the flow to operate. Unlike a flume, these submerged sensors are subjected to error-causing buildup of grease and silt, and hence, must be carefully maintained to keep operational.

Recently, the first non-contact type of Area/Velocity flowmeter was introduced to the industry—one that uses non-contact Doppler Radar to measure velocity and ultrasonic pulse echo to measure depth. This radar sensor measures the surface velocity by reflecting microwaves off of the moving surface disturbances and converts these signals to the average velocity through an appropriate algorithm. The ultrasonic pulse echo sensor measures depth of flow and converts this depth to area. The test results on one type of these Doppler Radar open channel flowmeters shows that it has the high rangeability characteristic of Area/Velocity meters while having the low maintenance and accuracy characteristic of a flume.
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Document Type: Research Article

Publication date: 2001-01-01

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