Venturi channels, exponential model, are designed to measure flow rate in open channels. When conditions of non-turbulent flow and totally free outlet are respected, the height of fluid (h, upstream) is directly related to the flow rate (Q). 
The width narrowing has parabolic shape to allow a great accuracy even with low flow rates.
The main advantage is a ratio of scale 1 to 100, useful when industrial process may vary often and in great proportions or to consider additional storm rains. 
They are useful either on waste water, contaminated fluid even with solids/ particles or chemically aggressive liquids.With our associated monitor BAMOPHOX 759 data logger, it is easy to record and survey the effluents. 
By construction, and adequate roughness of polyester isophtalique resin (UV protected) DEBITFLO is ideal for use outdoor. The material is resistant to hydrolysis and chemically compatible with acids; it will not afford solvents such as styrene, acetones, etc.