PADI IDC and IE Exam regulator questions often focus on either the first stage or the second stage
How it works
The job of the first stage is to bring the high cylinder pressure down to intermediate pressure, which is the pressure that’s in all of the hoses, apart from the high pressure hose that leads to your submersible pressure gauge.
This the part that you breathe from. It’s job is to bring the intermediate pressure down to ambient pressure.