concept of I/P converter & P/I converter

What is it exactly, that you're looking for? P/I and I/P usually have standardized ratings. As for the 'P', 3-15 psig is a very common control range, and for the 'I' side, 4-20 ma, and 10-50 ma are two common ranges. There are also voltage converters that will convert a pneumatic signal to voltage, 0-10VDC and 1-5VDC are common there.

You want theory? features? build-your-own? P/I (or P/V) have at the heart of them a pressure transducer. Could be Bourdon tube with differential transformer, strain-gauge, variable-capacitance, all sorts of possibilities. Once converted to electronic form, demodulate it if necessary, and 'massage' it with any sort of signal conditioning circuitry.

For the reverse direction I/P or V/P, the central element is more limited. Usually a force balance set up with a tiny 'flapper' bleed valve and then variable back-pressure of the bleed valve is used to regulate the main air supply through mechancial advantage of wide-area piston controlling a main valve.

daestrom