Smart Control valve positioner has three pressure gauges mounted on it.
The purpose of these pressure gauges are:
The first pressure gauge indicates the total input instrument air supply to the valve positioner.
Second Pressure Gauge indicates the pressure supplied to the control valve from the positioner.
The third pressure gauge mainly useful when we have a double-acting control valve where it needs another port connection of instrument air for its operation from positioner. In this case, it indicates the amount of instrument air going to valve.