Instrumentation pneumatic signals are maintained at the 3-15psi range. Air compressor system sends air at a much higher pressure which is then reduced using restrictions and 3-15psi is regulated further.
Flapper Nozzle system:
A flapper-nozzle system is used to convert mechanical motion into pressure change and vice-versa. Thus which can regulate the airflow pressure in the system.
A Flapper nozzle system is shown in the above picture. There will be a small gap between the flapper and the nozzle which maintain the pressure to the output, the air escape through this gap. The air comes out through the restriction. If the flapper moves down and closes off the nozzle opening so that no air leaks, the signal pressure will rise to the supply pressure. As the flapper moves away, the pressure will stabilize at some value determined by the maximum leakage through the nozzle.
Current to Pressure converter using flapper nozzle:
The current-to-pressure converter simply I/P converter, is very important in process control. The I/P converter gives a linear way of translating the 4-20mA current into 3-15 psi signal. A basic design for I/P converter is shown below.
The flapper in this system is attached to spring and a magnet metal at the flapper. The current through the coil at the end of the flapper produce a force that will tend to pull the flapper down and close off the gap. A high current produces a high pressure so that the device is direct acting. Adjusting of the spring and perhaps the position relative to the pivot to which they are attached allows the unit to b calibrated so that 4-20 correspond to 3-15psi.