To expand further, oxygen is toxic at 1.6ATA. For those unfamiliar, pressure doubles every 10m (33ft). So if I tried to breath 100% oxygen at 33ft (aka 2.0ATA), I’d die pretty quickly. At 66ft, that caps you at about 40% oxygen. At 99ft, you essentially can’t breath any higher oxygen concentration than natural air.

As such, nitrox’s purpose isn’t to increase depth. It’s actually used to increase bottom time (at a loss of max depth) without needing decompression. You get this increased bottom time due to lower amounts of nitrogen.

I’ve logged hundreds of nitrox dives from 28% - 60% oxygen levels, but never bit the bullet and gotten into more advanced air mixtures like Trimix. Trimix and other specialty blends are hard to get air fills for at the places I most commonly dive, so I keep pushing it off.

> For those unfamiliar, pressure doubles every 10m (33ft).

Does it? Or does it increase by one atmosphere every 10m?

Increase by one atmosphere. If it doubled then at the bottom of the Marianas Trench it'd be 10^330 atmospheres, or approximately 10^318 times greater than the pressure at the center of the Sun.

It doesn't double every 10m. Each 10m of salt water roughly adds 1 atm. So it only doubles at the first 10m. So the oxygen becomes toxic ~66 meters when diving with air.

When you add helium to your tank on a dive, the resulting mix is usually called "Trimix".

When you get to technical dives requiring Trimix, one often dives with several different gasses. The gas you breathe at the deepest point of the dive would be toxic if you were to breathe it at a shallower depth.