Introduction 

The flapper nozzle is the primary device for all pneumatic instruments which convert a measurement to a pneumatic signal.  Figure shows the layout of the device.

 Principle of Operation:

The air supply input (20 psi (1.4 bar)) passes through a restrictor (small hole).  It then goes out of the nozzle or down the air signal output line.  If the flapper is placed against the nozzle, no air can escape through it.  So, the air signal output shows full pressure.  If the flapper is pulled away from the nozzle, most of the air flows out of the nozzle, so the air signal output pressure is very small. The back pressure output signal depends on how near the flapper is to the nozzle.  A simple graph of the output pressure (P) against flapper distance (X) is shown below.

The graph is linear (straight) over the distance A B.  This reflects only a few millimetres of travel of the flapper.  This part of the curve is used to convert a change in a measured value connected to the flapper into an output signal.  The restrictor increases the speed of operation.  The small volume (V) can change pressure quickly before the air supply can pass through the small hole in the restrictor.