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80m is moderately serious. I've personally done 75m in the Atlantic and I wouldn't say I'm superhuman or anything (or military). It requires helium mixes and p

80m is moderately serious. I've personally done 75m in the Atlantic and I wouldn't say I'm superhuman or anything (or military). It requires helium mixes and personally I'd only want to do it with a rebreather, not normal "blowing-bubbles" type SCUBA gear (too wasteful on the breathing gas). 20-30 minutes at the bottom will mean around 90-120 minutes decompressing. You don't want to be doing anything taking too long, or being too laborious. Could be done from a fishing boat without any problem.

rich_sasha 2y ago
Yes, it's very doable given the right training. But it's not really an extension of what a typical "PADI diver" might do, is what I meant. Having had equipment failures no deeper than 30m, I decided any dives that require decompression are not for me. I'll leave deep dives and sabotage to others :)
wkat4242 2y ago
You know, I never dove but I always wonder, why do deep divers not breathe a nitrogen-free atmosphere before they dive to flush out the nitrogen? I understand astronauts do this too (they breathe pure oxygen but at a lower pressure so it's not dangerous). And because this a sudden pressure drop won't give them the bends. It would seem simple, being able to deep dive then without decompressing. I'm sure there's a good reason it's not being done, I just wonder what.
- rich_sasha 2y ago
  I think what you breathe pre-dive is almost irrelevant. The real kicker is what you breathe during the dive, and that's always going to be gas under pressure. The levels of gas in your blood go up very quickly. Recreational divers can only afford ~10-20 minutes at 30m depths before they need to decompress. And that doesn't even count as "deep". You can't breathe pure oxygen under water, as it becomes very toxic very quickly as pressure goes up. So you have to dilute it with something. Nitrogen is easy to get hold of (actually you just compress atmospheric air). I'm also not sure cutting nitrogen out helps much with decompression sickness, you get it with other gases too - maybe more slowly, I'm not sure. You can, and do, replace nitrogen with other gases, mostly helium (and in rare circumstances, hydrogen), but for other reasons. Nitrogen too becomes toxic under pressure, around the 30-40m mark, with symptoms similar to drunkenness - not good under water. Funnily enough, helium does the same deeper down, except the symptoms there are the opposite - nervousness, tension etc. So to some extent perhaps these two balance out. Hydrogen has very low toxicity, but for some reason people feel queasy about mixing pure hydrogen and oxygen. Crucially, too, a lot of deep diving is commercial in nature, and cost-sensitive. Anything that isn't just compressed and bottled air is getting very expensive very quickly. It all gets incredibly complicated. Deep enough, you need to breathe gas that contains so little oxygen that you would suffocate on it at surface pressure. At pressure, it's fine - the partial pressure of oxygen is sufficient, and having less of it makes it less toxic. But you have to be careful to only use this gas deep enough, else you'll run out of oxygen. Or at other times, when decompressing, you might use an oxygen-rich mix, which while more toxic, helps you purge some of the other gases from your bloodstream, cutting decompression times. It's a fascinating topic to read about from the comfort of my sofa.
- echoangle 2y ago
  Might work for technical divers but regular leisure divers use compressed air which contains nitrogen anyways. If you prebreathe pure oxygen and then begin the dive using regular air, you will reaccumulate the nitrogen pretty quickly, I think, so you will need to decompress like normal. And you can't dive with pure oxygen because it becomes toxic when going lower than about 6m due to the high partial pressure. Also, you would need to have pure oxygen available and breathe that for maybe an hour before each dive, which would be expensive and annoying. For touristic dives, especially along the shore, it's easier to just plan the dive so you can still see something interesting while waiting for decompression.

rich_sasha 2y ago

Yes, it's very doable given the right training. But it's not really an extension of what a typical "PADI diver" might do, is what I meant. Having had equipment failures no deeper than 30m, I decided any dives that require decompression are not for me. I'll leave deep dives and sabotage to others :)

wkat4242 2y ago

You know, I never dove but I always wonder, why do deep divers not breathe a nitrogen-free atmosphere before they dive to flush out the nitrogen? I understand astronauts do this too (they breathe pure oxygen but at a lower pressure so it's not dangerous). And because this a sudden pressure drop won't give them the bends. It would seem simple, being able to deep dive then without decompressing. I'm sure there's a good reason it's not being done, I just wonder what.

rich_sasha 2y ago
I think what you breathe pre-dive is almost irrelevant. The real kicker is what you breathe during the dive, and that's always going to be gas under pressure. The levels of gas in your blood go up very quickly. Recreational divers can only afford ~10-20 minutes at 30m depths before they need to decompress. And that doesn't even count as "deep". You can't breathe pure oxygen under water, as it becomes very toxic very quickly as pressure goes up. So you have to dilute it with something. Nitrogen is easy to get hold of (actually you just compress atmospheric air). I'm also not sure cutting nitrogen out helps much with decompression sickness, you get it with other gases too - maybe more slowly, I'm not sure. You can, and do, replace nitrogen with other gases, mostly helium (and in rare circumstances, hydrogen), but for other reasons. Nitrogen too becomes toxic under pressure, around the 30-40m mark, with symptoms similar to drunkenness - not good under water. Funnily enough, helium does the same deeper down, except the symptoms there are the opposite - nervousness, tension etc. So to some extent perhaps these two balance out. Hydrogen has very low toxicity, but for some reason people feel queasy about mixing pure hydrogen and oxygen. Crucially, too, a lot of deep diving is commercial in nature, and cost-sensitive. Anything that isn't just compressed and bottled air is getting very expensive very quickly. It all gets incredibly complicated. Deep enough, you need to breathe gas that contains so little oxygen that you would suffocate on it at surface pressure. At pressure, it's fine - the partial pressure of oxygen is sufficient, and having less of it makes it less toxic. But you have to be careful to only use this gas deep enough, else you'll run out of oxygen. Or at other times, when decompressing, you might use an oxygen-rich mix, which while more toxic, helps you purge some of the other gases from your bloodstream, cutting decompression times. It's a fascinating topic to read about from the comfort of my sofa.
echoangle 2y ago
Might work for technical divers but regular leisure divers use compressed air which contains nitrogen anyways. If you prebreathe pure oxygen and then begin the dive using regular air, you will reaccumulate the nitrogen pretty quickly, I think, so you will need to decompress like normal. And you can't dive with pure oxygen because it becomes toxic when going lower than about 6m due to the high partial pressure. Also, you would need to have pure oxygen available and breathe that for maybe an hour before each dive, which would be expensive and annoying. For touristic dives, especially along the shore, it's easier to just plan the dive so you can still see something interesting while waiting for decompression.