Recent water pollution control legislation and the need for a closer accounting of sources of water flowing within municipal sewage and storm conduits have produced a need for a more accurate measurement of water flow. Presently, various schemes are employed to give an indication of the flow rate contained in open channel flows such as found in municipal and industrial conduits. An open channel flow measurement differs from a closed conduit flow in that the area of a partially filled pipe is constantly changing whereas the area of a closed, filled conduit is a known fact.
These known measuring systems have various inherent drawbacks which affect the accuracy of flow measurement. For example, in a partially filled conduit or channel it is difficult to measure the average velocity of fluid flow, since the velocity varies at different cross-sectional locations as a function of the configuration of the conduit as well as the varying heights of the liquid (in accordance with tides, sewage production, weather conditions and the like). While it is conventional to mount the probe devices in the bottom of the conduit in order to maintain them free from debris in the fluid, the velocity which is sensed at this location is not accurately related to the true velocity of the fluid flowing in the conduit.