As liquid begins to flow through the channel, it must spill over the crest of the weir in order to get past the weir and continue downstream in the channel. The following photograph shows water flowing through a Cippoletti weir made of 1/4 inch steel plate:Īt a condition of zero flow through the channel, the liquid level will be at or below the crest (lowest point on the opening) of the weir. A V-notch weir has a triangular notch, customarily measuring either 60 or 90 degrees. A Cippoletti weir is much like a rectangular weir, except that the vertical sides of the notch have a 4:1 slope (rise of 4, run of 1 approximately a 14 degree angle from vertical). Three styles of weir are shown in the following illustration the rectangular, Cippoletti, and V-notch:Ī rectangular weir has a notch of simple rectangular shape, as the name implies. The first form of open-channel flowmeter is the weir, which is nothing more than a dam obstructing passage of liquid through the channel. By measuring this liquid level rise, it is possible to infer the rate of liquid flow past the obstruction. If an obstruction is placed within a channel, any liquid flowing through the channel must rise on the upstream side of the obstruction. A very different style of variable-area flow meter is used extensively to measure flow rate through open channels, such as irrigation ditches.

Flume flow sensor