Nozzle_and_flapper
The nozzle and flapper mechanism is a displacement type detector which converts mechanical movement into a pressure signal by covering the opening of a nozzle with a flat plate called the flapper.[1] This restricts fluid flow through the nozzle and generates a pressure signal.
It is a widely used mechanical means of creating a high gain fluid amplifier. In industrial control systems, they played an important part in the development of pneumatic PID controllers and are still widely used today in pneumatic and hydraulic control and instrumentation systems.