If a transmitter works with a pneumatic supply, then it is called pneumatic transmitter. It uses compressed air supply as working supply. The standard input supply for a pneumatic transmitter is 20 psi (1.406 kg/cm2) and standard output of a pneumatic transmitter is 3- 15 psi (0.2 – 1 kg/cm2) of the rated span of the instrument.
A transmitter has a sensing part and a transmitting part. The sensing part of a transmitter is basically mechanical sensors depending upon the process variable. The transmitting part of a pneumatic transmitter is flapper nozzle feedback mechanism.
It contain of two ports span and zero for calibration adjustment attached with bellows. If pressure increases bellow flex allowing flapper nozzle assembly to come closer thus escaping less pressure.
As shown in figure if air supply provided is zero psi then gap between flapper and nozzle remain wide enough allow all the pressure to exit. If provide full 20 psi supply then due to gap between flapper and nozzle reduce it create back-pressure allow smaller pressure to exit and other pressure will pass through output chamber leaves with 15 psi.
All pneumatic type transmitter, convertor uses same principle for measurement.