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I recently saw "The Abyss".

Does it make sense that they do dives in these depths (700m) with soft suits?

Also - what is all the depressurization talk about? Why do divers need to depressurize long periods after they resurface? Does that mean crew members can't escape drowning submarines?

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This question is a reason why a cross-over mechanism should be in place - I bet the folks at biology.stackexchange.com would also have some great answers on this –  Tobias Kienzler May 19 '13 at 8:26
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4 Answers

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A diver's body is basically made of water, and water is incompressible to a good approximation over the relevant pressure range. So the physiological effect of the water pressure itself will be negligible: I expect there is no problem with diving at 700m in soft suits in that sense.

However, in order for the diver to inflate her lungs, she must have air supplied at the same pressure as the surrounding water. Breathing pressurised gas brings its own set of problems. The most prominent of these is a serious class of injuries called "the bends". This is a danger because the pressure increases the solubility of nitrogen gas in the blood and tissues of the body. The concentration of nitrogen dissolved in the diver's tissues increases with the time spent at depth, as the diver breathes more and more gas. If the diver then ascends too quickly, the sudden decrease in pressure causes the nitrogen gas to come out of solution and form bubbles. Potentially this results in excruciating pain or death.

Submarines have a rigid outer shell which allows the air inside to remain at atmospheric pressure even at great depth. So as long as the poor sailors can hold their breath all the way up, then there is pretty much no risk of the bends. More likely, submarine escapees will require some compressed air source if they are to have any chance of making it alive to the surface from the depths usually plumbed by submarines. In this case decompression illness does become an issue.

If I remember the Abyss well enough, the kind of deep diving they were doing would have required weeks and weeks of gradual decompression in a hyperbaric chamber before the return to normal surface life. The idea of trained divers forgetfully decompressing themselves by accident from 70 atmospheres to 1 atmosphere in a matter of seconds is so ridiculous that I burst out laughing every time I watch the end of that film. It's still an absolute classic, though :)

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Submariners would also need to pressurise to the external pressure, however briefly, simply to get the hatch open! I suspect that the time is short enough you don't dissolve significant nitrogen. –  Martin Beckett May 18 '13 at 16:36
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Hence my "pretty much no risk" as opposed to no risk whatsoever ;) But yes, this is absolutely correct, thanks for the clarification. Not only would the time have to be short, but you only have a single lungful of air. Even if you could remain at depth long enough for the gas in your lungs to chemically equilibrate with your slowest tissues, I doubt that you would get bent on ascent. But I haven't done any calculations to prove it :) –  Mark Mitchison May 18 '13 at 16:39
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The pressure itself doesn't really matter as Mark pointed out. Neither does the bends if you stay at the same pressure - it is caused by gas coming out of solution and forming bubbles as you rise (and the pressure drops)

The real challenge at very deep deaths is the chemical effects of gases at high pressure. Normal air scuba is limited at around 40m because nitrogen at higher pressure makes you drunk. You can use air with added oxygen, ie less nitrogen, (Nitrox) but oxygen itself becomes toxic at higher pressures.

You then replace some of the oxygen and nitrogen with helium, or hydrogen - making Trimix, Heliox and Hydreliox. The problem is that at each depth you have to balance having enough oxygen to keep you alive, but a low enough concentration not to be toxic - so you need to continually adjust the mixtures and types of gas as you ascend and descend. At very large depths these limits get very tight.

The different toxic effects of helium and hydrogen aren't very well researched and can vary a lot between different people under different conditions.

The record for diving use flexible suits (as opposed to hard submarine suits) is something like 700m but the systems are so complex and expensive that it's normally easier to just use ROVs and so it's not heavily researched.

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Oxygen is narcotic too (according to PADI's Encyclopedia of Recreational Scuba Diving). There are no narcotic advantages to Nitrox (indeed END isn't even taught on Nitrox courses, only Trimix). –  Gaius Jun 14 '13 at 12:01
    
@Gaius - yes phrased that badly, those two sentences weren't linked –  Martin Beckett Jun 14 '13 at 12:06
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The other answers already treat decompression sickness, but in addition to the submarine scenario:

  • How long would one have to hold one's breath? A good diver can apparently maintain something around 0.5 m/s (plus some Buoyancy due to the air in their lungs), so even for a Civil submarine at around 30 m depth, they'd have to hold their breath for a minute. At deeper depths, Buoyancy may have a more relevant influence since it's an acceleration, but still, holding your breath for a mere minute (plus you probably didn't get the chance for taking a deep breath before having to escape the submarine) is troublesome. And as Philip's answer mentions, you have to breath out since your lungs expand (the expansion would increase your buoyancy though). I don't know any details about that case, the Wikipedia article lacks detail, so I can't tell whether these two survivors were good swimmers or had luck...
  • <<My ears! Where is "up"?>> Even at 30 m the pressure is already 4 times the atmospheric pressure. This sudden change (due to your hastened escape from the submarine) might cause your eardrums to rupture, which in turn can lead to Vertigo - you might confuse up and down and accidentally swim deeper instead of towards the surface.
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Marines used to train for escape from 30m, It takes about 15 seconds with aid of lifejacket see youtube.com/watch?v=rBDlZ7EHx4E You don't worry about holding your breath, you breath out like hell or your lungs explode. –  Philip Gibbs May 19 '13 at 11:57
    
@PhilipGibbs Ah yes, a lifejacket provides significant Buoyancy, I forgot about that. That would also counter any accidental attempt at swimming downwards... –  Tobias Kienzler May 19 '13 at 14:19
    
Having said that it is not obvious that wartime submariners would have had life jackets capable of inflating under pressure, which may explain why so few survived. –  Philip Gibbs May 19 '13 at 14:23
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Apparently they had this en.wikipedia.org/wiki/Davis_Submerged_Escape_Apparatus and here is a verified story of its use bbc.co.uk/news/magazine-15959067 –  Philip Gibbs May 19 '13 at 14:29
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Remember at pressure that lungful of air had more oxygen. One of the hard things to teach scuba diving students is that they can (and must) keep exhaling on the way up - and that essentially more air is "appearing" in their lungs as they rise. –  Martin Beckett May 19 '13 at 17:30
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The compressive effect on the body itself is not a problem because we are composed of incompressible fluids and solids. All gas filled cavities in a healthy person are connected by tubes to the outside so that the internal pressure can be equalized as external pressure increases.

A deep diver is affected by a variety of other problems that can be corrected by adjusted the gas mixture.

Nitrogen is a narcotic that affects judgement as depth increases. This can become significant from 30m and is seriously dangerous from 50m down. The effect is mitigated a little by getting used to deep diving, but the sensible way to avoid nitrogen narcosis is to use a breathing mixture with less nitrogen. All gas toxicity problems are dependent on partial pressure so if you replace half the nitrogen with helium which is not toxic you can go to twice the depth without narcosis.

Oxygen is toxic with very different symptoms. It normally causes spasms and fits at 80m with probably fatal consequences. Again the remedy is to replace oxygen with helium but you need to keep enough oxygen to breath.

Despite these toxicity problems some people claim to have reached depths up to 155m on air for brief moments. It is almost suicidal to attempt anything deeper than 100m on air.

To go deeper you can use a mixture of helium and oxygen but at about 250m you start to suffer from High-pressure nervous syndrome. This is not a gas toxicity problem but a direct effect of pressure on the functioning of the nerves causing shaking. Luckily this can be corrected by adding just the right amount of nitrogen back into the gas mix. It is as if narcosis and HPNS have opposite effects that cancel each other out.

Using the right mixture of oxygen, nitrogen and helium it has been possible to go deeper. The record is unofficially 318m by Nuno Gomes in 2005. Problems are to control the gas mixture ratios as you go deeper. Off course it can take weeks to decompress allowing the gas dissolved in blood and tissue to escape slowly without blocking blood vessels or nerves. You also need to control the temperature of the gas carefully because helium at high density has a high heat capacity and hypothermia is a potential problem. The viscosity of the gas mixture also makes breathing more difficult as pressure increases.

It is likely that there are other unknown problems that would limit diving long before reaching 700m without hard suits.

Deepest escape from a submarine without scuba is probably about 65m, two survivors from HMS P32 1941. The short exposure to pressure in such circumstances means that decompression is not a serious problem but they need to breath out constantly on the way up as the gas in their lungs expands.

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