[kashunstva]

I’m not a diver; but the physiology of these depth effects is fascinating! TIL that these hypoxic mixes are needed because of a combination of two effects: nitrogen narcosis[0] and oxygen toxicity[1]. The former is due to diffusion of any gas, apart from helium, into neurons in the CNS under high pressure, while the latter is due to the raised partial pressure of O2 at depth. 100% O2 is cumulatively toxic at sea level, but at depth its toxicity is enhanced at what would be a normal 21%. At least that’s my layman’s understanding.

[0] https://en.m.wikipedia.org/wiki/Nitrogen_narcosis [1] https://en.m.wikipedia.org/wiki/Oxygen_toxicity

[sfeng]

In diving we think in terms of partial pressures. At sea-level, 21% of air is oxygen, and .21 is also your partial pressure of O2. In order to breath at depth, though, we need to be supplied with a higher and higher pressure of gas to fight the pressure of the surrounding environment opposing our lung expansion. At 10 meters, the pressure doubles sea-level, and you are breathing gas at twice atmospheric pressure (meaning there are twice as many O2 and Nitrogen molecules in every liter of gas).

Consequently, the partial pressure of oxygen you are breathing is doubled, to .42. This continues, until you reach a partial pressure which is incompatible with life. That danger-zone is generally considered to be between 1.4 and 1.6, so air cannot be safely breathed below about 60 meters (1.6/.21 - 1 = about 6 atmospheres). Similarly, pure O2 cannot be breathed safely below about 6 meters (1.6/1 - 1 = about .6 atmospheres).

Your description of nitrogen narcosis is correct. To dive deeper than your nitrogen narcosis limits you start replacing some of the nitrogen with helium in the gas you breathe. This works up to the limits of even the most technical recreational diving, after which you will get symptoms of something called HPNS which could mean replacing the helium itself with hydrogen.

Both helium and nitrogen will get stored in your inert tissues and will bubble out catastrophically if you ascend too quickly. With either inert gas you must carefully plan your ascent to avoid the bends.

[Gordonjcp]

> Both helium and nitrogen will get stored in your inert tissues and will bubble out catastrophically if you ascend too quickly.

I'm not going to link to the Byford Dolphin incident, because it would need a [TW] and I hate that shit, so you can just google it yourself. The 30-second rundown is that on the surface divers live in a very strong steel tank pressurised to the same degree as they would be on the seabed, so maybe make sure you close the door properly.

[Maursault]

> so maybe make sure you close the door properly.

Incidental, and didn't get the chance.

      Crammond opened the clamp that was keeping the trunk sealed before Diver 4 (Hellevik) could close the door to the chamber.

The better advice is to make sure your crew is well-rested and your equipment has failsafes in place.

[Gordonjcp]

> The better advice is to make sure your crew is well-rested and your equipment has failsafes in place.

The equipment did have interlocks to prevent it being opened incorrectly but nobody used them because they were annoying and fiddly.

[_qua]

If you're interested in the physiology of respiration at the limits this lecture by John West is fantastic https://www.youtube.com/watch?v=QRN124iuqZ8 . It has been some time since I watched it in med school but if I recall correctly, I think it is pretty accessible to anyone who is technically/scientifically minded.