This type of flowmeter allows measurement without process interruption, however, and allow portability. In open channel flow measurement, weirs and flumes are used to produce known and repeatable characteristics in the process stream. Weirs, flumes and other devices develop a liquid head that is used to measure flow rate. These devices are particularly useful when large flows are to be measured or where liquids are handled in open channels or in pipelines that are not ordinarily full. Flows from 100,00 GPM to the millions of gallons per day are measured using these devices.
Weirs are the simplest and most economical flow-measuring devices available.
Basically, weirs are obstructions or dams placed across the channel containing openings or apertures of known geometry through which flows the liquid to be monitored. Head is measured as the rise in level of liquid in the pool upstream of the weir. V notch weirs are used for smaller flow rates than those handled by rectangular or trapezoidal weirs. All produce known characteristics in the process stream that can be measured to indicate flow rate. Weirs use staff gauges mounted on the side of the weir box to provide a measurement of head.