Traditionally, differential pressure flowmeters have been used to measure flows in full closed conduit. Two exceptions to this rule are the U.S. Geological Survey and University of Illinois sewer meters. Although these function as differential meters in the pressure flow region, they are also fully functional in the open-channel flow region, where they act as Venturi flumes. This dual mode of operation represents the main advantage of these flowmeters.
The U.S. Geological Survey (USGS) flowmeter is similar to flumes with a U-shaped throat. The flume does not obstruct the part of the pipe immediately below the crown, thus transition from open-channel flow to pressure flow is fairly smooth and head losses are reduced. Rating curves for the USGS flowmeter are available (Smoot 1975).
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