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.
Ultrasonic level instruments are commonly used to measure the rise in head. The use of a stilling well should be considered if foaming or excessive turbulence is encountered. Flumes are slightly more complex devices. Flumes use complex geometries to form the process stream into a controlled flow that allows accurate and repeatable measurement over the range of the flume. The loss of head through a flume is about one-quarter that of a weir of equal capacity.