Eli5. Why does gas mix for divers change?

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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
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The deeper you go, the higher pressure the gas needs to be. Up to about 100′ (more like 130′) nitrogen is annoying but not dangerous…you just have to deal with decompression.

However, as the nitrogen pressure starts going up you start to be exposed to narcotic effects of nitrogen, called nitrogen narcosis. You also start to get issues with oxygen toxicity. And decompression to remove dissolved nitrogen becomes a bigger and bigger issue. To counteract all that, deeper gases are made up of mixtures that won’t poison you, put you on a trip, and can be decompressed more easily.

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

Diver here with nitrox certification (though that’s not exactly a very difficult thing to get).
Your body responds to the partial pressure of gasses.
As you go down the pressure increases and so do the partial pressures of all the gasses you breath.

The air we breathe is mainly nitrogen, with a large chunk of oxygen.

As the partial pressure of nitrogen gas increases nitrogen gas starts building up in the blood stream.
If they go up too quickly nitrogen bubbles could form and kill them, so if they’ve been down for too long, they’ll hang out at a particular depth for the nitrogen to go away.
Divers calculate how long they could stay at a particular depth before they risk these bubbles, now-a-days many divers wear a computer to calculate and help them keep track.

That’s the problem with nitrogen, a simple solution was, just take gas with less nitrogen in it, hence nitrox. Less nitrogen, more oxygen, means more time at lower depths without having to wait and decompress.

But why not all the time? Because at higher partial pressures, oxygen will kill you. Oxygen starts doing weird chemical reactions when there’s enough of it, you OD on 02 and die. So when using nitrox, you cannot go below a certain depth.

If the partial pressure for nitrogen gets too high, you start acting drunk. Why this happens isn’t known,but when it has been charted by trial and error.
This happens with oxygen too, so it’s not really a feature with nitrox.

This is where helium comes in.
Helium, doesn’t get you drunk like nitrogen, and it doesn’t kill you like oxygen (though you can’t breathe it).
So why not just use helium always? Because it’s really really expensive, that’s pretty much it.
It doesn’t give you any benefits for normal diving, but it doesn’t have a penalty beyond price either.