ABSTRACT
The basic pneumatic control mechanism is the flapper-nozzle unit. This unit, with amplifying relay and feedback bellows, is a simple, rapidacting control mechanism. The nozzle pressure is applied to the pneumatic relay, which contains a diaphragm operating a small ball valve. Because the diaphragm has a large area, small pressure changes on its surface result in a significant force to move the ball valve. Flapper-nozzle detectors employed in the set-point transmitter and derivative sections of the Model 130 controller use a different type of pneumatic amplifier. If the flow or volume output requirements are small, an aspirating relay may be employed. The integral function takes place within the automatic control unit itself. The integral bellows opposes the feedback bellows; thus, if the feedback bellows introduces negative feedback, the force created by the feedback bellows will act like positive feedback; that is, it tends to move the flapper in the same direction as error.
