It depends on the tables and physiology, because you need to safely oxygenate all the body's tissues with regular breathing at depth. He could probably dive [edit: said Nitrox, I'd learned non-air Nitrox increases maximum depth but it increases time-at-depth] Trimix until about 100m, deeper might need a different mix.
Nitrox’s maximum operating depth is ~32m at 32%. Oxygen toxicity is a quick death. To go deeper you need to add helium and actually reduce the amount of oxygen in the gas.
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.
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.
I you were referring to me, I was using a 36% oxygen Nitrox mix. We were working at less than 10 m, so this increases working time not depth, but you need to be careful with oxygen toxicity as mentioned above. For deeper dives, air and then Trimix for up to 76 m.
Also for anyone interested, you are dealing with partial pressures of oxygen (PO2) with Nitrox. With 36% Nitrox mix (36% O2, 64% N, EAN36)you shouldn't be going below 28 m, since you don't want to exceed a PO2 of 1.4. (36% * 4 ATA = 144% or 1.44 PO2). Oxygen toxicity comes on without warning in most cases, and you convulse and typically drown. You want to stay between 0.16 and 1.6 (max!, hence the 1.4 PO2 guideline). There are some subtleties you can find out by researching Nitrox diving, or taking the course and certifying for Nitrox diving.
