In general, it is an object of the present invention to provide an apparatus for metering forward and reverse flow in a sewer that can operate under both free flow and submerged flow, open channel conditions when the sewer is flowing at less than full depth and under surcharged conditions when the sewer is flowing full and the metering manhole is surcharged. It is another object of the present invention to provide a portable flow metering apparatus for quick installation in a sewer adjacent to a manhole. It is another object of the present invention to allow floatable sewage solids to be conveyed through the device when it operates under free flow and submerged, open channel conditions. It is another object of the present invention to provide a flow metering apparatus requiring a relatively small head loss in creating the static pressure head differences used to calculate flow rates.
For the purposes of this patent, the term tubular venturi metering device means an apparatus that fulfills the function of a modified venturi flume when the sewer in which the invention is installed is flowing less than full and the function of a venturi tube when the sewer in which the invention is installed is flowing full. The term tubular venturi metering device also means an apparatus wherein the constriction is configured so as (1) to cause the device to maintain its metering function even at low liquid flow rates and (2) to cause the constriction to fill with liquid at approximately the same flow rate that the upstream pipe fills with liquid and thereby maintain its metering function during the transition from open channel to full pipe flow.