Choosing The Right Technology
Choosing The Right Technology
As published in WATERWORLD Magazine, September 2005
By Patrick L. Stevens, P.E.
Choosing which technology is right for your agency can be difficult and there is little objective information available to assist in the selection. There are several issues that a user should consider when selecting a flow meter including: the size of pipe to be measured; the minimum depth of flow in the pipe; whether the pipe surcharges or not; the uniformity of flow in the pipe; required meter uptime; required accuracy of measurement; and frequency of battery replacement. The chart below provides some helpful information in evaluating the different technologies and manufacturers. This chart contains information from the manufacturers as well as subjective information obtained from users.
Every technology is designed around the assumption that uniform flow will exist at the metering site. Uniform flow means that the depth of flow, the velocity of flow and the direction of flow will be constant at least through the section of pipe where the meter is located. For that reason nearly all flow meters are installed on the incoming line to a manhole to measure in the most uniform flow possible. Flow passing though a manhole is normally disturbed, making measurement in the manhole channel a risky undertaking. In deeper or surcharged flows, the depth-velocity relationship in a manhole has little relationship to that in the pipe.
Click here to view a PDF document of the Sewer Flow Applicability Chart.
Fortunately, there is a simple two-step process that can be used to help sort through the manufacturer’s claims. This method is described in detail in the paper Peeling the Onion of Meter Accuracy published in the WEFTEC 2001 proceedings. Copies of this paper can be obtained by contacting Patrick Stevens at 256-430-3366 or at www.adsenv.com/peeling.
Peak Velocity Doppler technology is the only wastewater measuring technology that is verified by the ETV Program and the only one that is successful with measurements in a wide range of flow rates and pipe sizes including both low flow and large pipes. Some manufacturers promote their technology as being best in large diameter pipes, but the true test of which meter technology could be appropriate is based on a combination of pipe size and the range of flow depths expected.
Technologies that are marketed for large diameter pipes (greater than 48") may not perform well when depths are low. This can be a problem in sanitary sewers where dry day flows are less than 30% of pipe height, but is especially an issue in combined sewers where the average depth of flow can be 5% to 10% of pipe height.
Regardless of the meter used, the scattergraph is the true test of whether the meter recorded hydraulically valid data. Information on scattergraphs and a scattergraph poster can be obtained from the ADS Environmental Services website at www.adsenv.com/scattergraphs. You can also visit the ADS website for more information on flow monitoring technologies: http://www.adsenv.com/.
Click here for additional information on Sensor Technologies.
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