Pneumatic controller

1. If what you need to control is just constant air pressure in a supply line or enclosure, couldn't you use a pressure regulator and maybe your transducer to read it, instead of setting an electronic control?

2. If you need accuracy in pressure regulated, and need for some reason to use such control system (maybe because you have another control loop that requires the regulated pressure to vary automatically according process conditions...) you'll need a proportional pressure regulating valve.

3. I attached what I think is what you'd like to build. It's not what you asked, but it's to call for your attention that a pressure regulator is needed, that they need control loops usually (and preferable) driven by 4-20mA.

4. If your process calls for changes in regulated pressure, you should include in the control system another input to compare with pressure signal.