Eli5. Why does gas mix for divers change?

Question

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

Answer 1

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.