The pressure relief valve is a knob you can turn. The range is analog, as you can control how big the hole is. When you get in, this valve is closed as to let air pressure build up in the chamber.
- Flow rate air in > flow rate air out: chamber pressure decreases.
- Flow rate air in < flow rate air out: chamber pressure increases.
- Flow rate air in = flow rate air out: chamber pressure is steady.
The flow rate air in is constant, as there's no dial on the machines that control it. The flow rate air out however is relative to how large of a hole you make via the pressure relief valve.
@Goemon_ mentioned the automatic pressure relief valves at the other end of the Macy-pan chamber. There are two, and they are calibrated to not let the pressure be above 1.5 ATA (
for this model), and will adjust the flow rate of the air going out to be the same as the flow rate of air going in (see 3), keeping the pressure at 1.5 ATA. I'd imagine you can do the same thing for something like 1.2 ATA, by adjusting the size of the pressure relief valve hole to maintain a pressure equilibrium in the chamber at 1.2 ATA. This would probably take some finagling to get the right sized air out opening.