What device is commonly used to measure the flow rate of a liquid in an open channel?

A weir is specifically designed to measure the flow rate of liquids in open channels. It functions by creating a barrier across the channel, which causes the liquid to flow over it at a controlled height. The height of the water flowing over the weir is directly related to the flow rate, allowing precise calculations of how much liquid is passing through the channel.

Weirs operate on the principle of fluid mechanics, where the flow rate can be calculated based on the height of the water above the weir. This measurement is consistent and reliable, making weirs a standard choice for applications involving open channel flow, such as rivers, streams, and wastewater treatment facilities.

In contrast, while flow meters can measure the flow rate in various systems, they are usually designed for closed pipe systems rather than open channels. Valves control the flow of a liquid but do not measure flow rates. Manometers measure fluid pressure rather than flow, which is unrelated to the question about measuring flow rate.