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. Flumes have been around since the 1920's. The most common flume is the Parshall Flume, named for Dr. R. L. Parshall, who developed it for use in irrigation. A Parshall Flume installed in a pre-fabricated manhole.