of the most practical ways to measure or control flows
in lines is to generate an accidental head loss, by means
of a narrowing in the section of the pipe.
This can be made by inserting a plate
with a hole with a diameter smaller than the inner one
of the tube between two flanges.
The measurement of the flow is determined
by the difference of pressure (differential pressure)
between the entry and exit side of the plate.
There are three types of narrowing