Weirplate

Why a difference in temperature between the water and the weirplate should cause the water to spring clear at low heads cannot very well be explained.

Source: wiktionary

The final solution adopted was to measure flow indirectly using an ultrasonic level sensor to record depths of discharge over a weirplate set at a level to exclude tidal ingress into the sewerage system.

Source: wiktionary

The liquid is forced to flow over the restriction, and the effluent flow rate is a function of the measured height of liquid above this sharp-edged obstruction, or weirplate.

Source: wiktionary

The weirplate at the bottom, inside the drum at F, serves the same purpose as in the machine you have seen — namely, to provide an inner wall of water of the desired thickness within the drum.

Source: wiktionary