What is a device with an edge or notch used for measuring liquid flow called?

A device with an edge or notch used for measuring liquid flow is called a weir. Weirs are structures placed in open channels to measure the flow of water. When water flows over the edge of a weir, it creates a specific head (the height of the water above the weir's edge), which can be correlated to the flow rate.

The design of a weir includes various types, such as sharp-crested, broad-crested, and V-notch weirs, each optimizing different measurement applications based on flow characteristics. The relationship between the height of the water flowing over the weir and the volumetric flow rate is well-established and documented, making weirs effective tools in water management and treatment processes.

Other choices include devices that also measure flow but operate under different principles. For example, a Parshall Flume is a specific type of flow measurement device that uses a calibrated channel with a throat section to determine flow rate, while a Venturi operates on the principle of pressure differential when fluid passes through a constriction. The Plainer Bowlus, although less common, pertains to flow measurement in specific contexts. Understanding the distinct characteristics of these devices helps clarify why a weir is uniquely suited for the function described in the question.