So for most diving like 100 ft you breath normal compressed air but as you go farther down and for longer they start replacing nitrogen with helium or in shallow water like 30 ft they replace it with oxygen gas. Why not just use normal air for everything?
In: Biology
As pressure increases, nitrogen starts to act like a narcotic, so if you inhale it at that depth you’ll feel drunk and the situation will be dangerous—many have lost their life due to thinking they can breathe the water. So some of the nitrogen needs to be replaced with helium, which doesn’t cause that problem.
At shallower depths, people typically inhale compressed air so there’s no special mix. But sometimes at shallow depths such as above 30 feet, you can inhale pure oxygen using a rebreather instead of breathing air with SCUBA. It isn’t typical to do this but an advantage of it is that you won’t have bubbles. Air is about 79% nitrogen so when breathing air you have a lot of nitrogen to exhale and it makes a lot of bubbles. If you inhale all oxygen using a rebreather then you won’t have many bubbles except for some CO2. That has military applications because if you’re, say, spying on someone underwater it will be less detectable.
But the reason you don’t do this at deeper depths than 30 feet or so (I’m not sure what the depth limit is) is that oxygen becomes toxic at higher pressure and concentrations. So it’s dangerous to breathe pure oxygen deeper than that. The amount of oxygen in air is still OK for the most part at deeper depths but when you go quite deep (not sure, definitely over 100 feet) then there could be too high a concentration of oxygen so in addition to replacing some of the nitrogen with helium, they have to lower the oxygen concentration as well
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