> The atmosphere is about 20 percent oxygen. People breathe this in, and breathe 15 percent oxygen out
That's standard air pressure though. As you dive and pressure increases, the metabolized oxygen stays roughly constant (metabolic rate is independent of pressure) but the "amount" of oxygen breathed in shoots up because the inspired volume does not change either (and there's more oxygen in the same volume under pressure). Thus the "extraction efficiency" goes down (you proportionally breathe out more and more oxygen which wasn't metabolized).
This is why closed-circuits rebreathers are so much more efficient than open-circuit cylinders, and the o2 cylinder is so much smaller (excluding safety cylinders): at atmospheric pressure an open circuit may "waste" 75% of the oxygen, at depth it may be >90%. A rebreather or a closed environment (under pressure) will allow much more efficient oxygen use.
> Even at sea level the exhaled air is about 16% oxygen (down from 21%). Giving the body access to more oxygen per breath won't make the air supply last longer[*].
-Incidentally, in the survival suits we wear when helicommuting in the North Sea contain a rebreather device - which is simply a small bag in the suit's lining with a mouthpiece and a valve.
The idea is that when getting underwater is imminent, you take a few deep breaths. When you exhale into the bag, you can then draw the spent air back in for another gulp or two of air.
The idea being that in a couple of breaths you'll either be in the clear or dead - so a couple of extra breaths is all you realistically need. (Ever the cynic, I suspect part of the reasoning behind it is to keep you busy and un-panicked in your last few seconds alive, but anyway...)
Interesting. I always assumed divers had 100% oxygen tanks, but I guess percentage and pressure are different things. But with that in mind, wouldn't a great percent of oxygen just mean there is less of the other gases, but the relative pressure would still be the same as gravity didn't change?
I'm not a diver, but the overall logistics of how to get exactly the right amount of oxygen into your system is fairly interesting. Rebreather technology has been pushed and developed by the same people who use it.
this is a good point, there is a lot more to diving really deep than storing a sufficient amount of oxygen. Basic physiological processes work differently at high pressures.
> The volume would decrease rapidly with depth but the available oxygen per breath would increase to compensate.
> Unless you have something specific to correct?
Firstly, how would you bring the highly buoyant bubble of air underwater, in a controller manner?
But let's say, for the sake of argument, that you could.
Even though the volume of the bubble would indeed decrease with depth (e.g. it would halve at 10m), the volume of each breath would remain roughly unaffected by depth. This is the reason scuba divers deplete their air supply faster the deeper they go.
The amount of available oxygen per breath is irrelevant. Even at sea level the exhaled air is about 16% oxygen (down from 21%). Giving the body access to more oxygen per breath won't make the air supply last longer[*].
If the air supply wouldn't be at a slightly higher pressure than that, the air would get sucked out of your lungs as far as I understand.
This is why dive tanks run out way way faster the deeper you go (around 1 hour at 18 meters deep and around 10 minutes at 30+ meters out the top of my head).
These seem to talk mostly about diving, where you breathe from a regulated air supply. I assume that air supply is not at 30-100x atmospheric pressure?
Scuba divers are at ambient pressure and breathe through demand valve regulators. Pressures inside their lungs are almost exactly the same as outside so there's no extra stress.
Increasing pressure with depth does increase work of breathing due to higher gas viscosity. Beyond a certain depth, divers can no longer ventilate enough to take in O2 and remove CO2. This problem can be mitigated to an extent by breathing lower density gasses such as helium. Occasionally hospitals even have patients with severely impaired lung function breathe heliox just to get better gas flow.
In theory it's possible to use a 2nd-stage scuba regulator as an improvised mechanical ventilator by partially blocking the exhaust valve with your hand and pressing the purge button. In practice it's unlikely to work, but if I was trying to rescue someone who had stopped breathing in the water and had no way to get them back to shore or a boat quickly then I might try it as a last-ditch effort.
The only point I was trying to convey with that statement was that there might be much less oxygen by volume in deep-diving heliox but the partial pressure of oxygen is still maintained so that cellular respiration can continue normally. Was that the only thing you thought was outrageously wrong with my comment?
Can confirm - I went on a bunch of dives with a former Navy Seal and he used less than half the air on any given dive than the rest of us (who were all only regular or advanced certified) did. His explanation was that he'd just mastered the meditative mindset/breathing technique to use as little air and energy underwater as possible.
Mindset and breathing technique is certainly important in minimizing open circuit gas consumption, but it's not the only factor. Some people just have a naturally higher tolerance for CO2 loading (the instinct to breathe is driven more by increase in CO2 levels rather than lack of O2). Perfect buoyancy control helps a lot since you're not wasting energy on depth keeping. Equipment configuration should be streamlined to minimize drag. A high level of physical fitness also allows you to keep your breathing under control during periods of exertion, like finning against a current.
But I've also seen former military divers who had relatively high gas consumption. Some units mainly use closed circuit rebreathers or surface-supplied gas where breathing rate doesn't matter.
> A cylinder that lasts for an hour near the surface will only last for a couple of minutes at 100m
And unfortunately for saturation divers, you can't pressurize cylinders on-site to make up for the difference.
Pressurizing your tank to 210 bar (which puts the same stress on the cylinder walls as 200 bar at the surface) doesn't change the factor-of-10 increase on the other side of your P1V1=P2V2 balance.
Not to mention that you need even more air for decompression time...
Working from inside a hard-shelled pressure vessel would definitely be inconvenient. You could also build your home office 5000 m above sea level. Or work in a high-altitude balloon.
Either way, the rebreather option seems a lot more practical.
Thanks for sharing. Following the comments on youtube and the wikipedia page [1] I found this scientific analysis [2] about probable causes of the accident. My layman summary: rising pressure of respired gas at high depth (about 26 atmospheres at the bottom) increases the respiratory resistance both in the diver (lungs, larynx) and the diving equipment. There is a limit to exhaled volume flow-rate called "effort independent flow" where compression of air-ways due to exhalation keeps air-flow from increasing further. This limit lies at lower flow-rates when breathing high-pressure gas. Physical exertion causes increased CO2 production by the diver which needs to be matched by correspondingly increased air-flow for CO2 removal.
Once point of effort-independence is reached, lots of respiratory effort is wasted, contributing to further CO2 production. This causes a downward spiral of increased CO2 accumulation in the diver that could only be broken by switching to alternative breathing equipment that reduces respiratory effort (i.e. open-circuit gear).
Seems that close-circuit diving equipment is generally more risky to use [3] due to difficult to detect and non-correctible failure modes.
Scuba diving oxygen exposures are much more limited. Most divers still breathe compressed air, so a typical dive to, let's say, 66 ft / 20 m would only cause an oxygen partial pressure of 0.63 ATA.
Some of us do go deeper and use mixed gasses with more oxygen, but we carefully limit oxygen exposure to ensure safety. Acute oxygen toxicity can cause a seizure which is usually fatal underwater. So we never go above 1.6 on the oxygen, and then only for brief periods.
If it was 20% that would be survivable with just an oxygen mask fwiw. Humans can go down to 16% atm pressure safely if they breathe 100% oxygen. The rest of the body can easily handle the pressure difference (1 full atm of pressure is equivalent to being 10meters underwater as a guide, human orifices have no issue with it contrary to pop sci beliefs).
So yes it's not 20% and it would be nice if it was.
That's standard air pressure though. As you dive and pressure increases, the metabolized oxygen stays roughly constant (metabolic rate is independent of pressure) but the "amount" of oxygen breathed in shoots up because the inspired volume does not change either (and there's more oxygen in the same volume under pressure). Thus the "extraction efficiency" goes down (you proportionally breathe out more and more oxygen which wasn't metabolized).
This is why closed-circuits rebreathers are so much more efficient than open-circuit cylinders, and the o2 cylinder is so much smaller (excluding safety cylinders): at atmospheric pressure an open circuit may "waste" 75% of the oxygen, at depth it may be >90%. A rebreather or a closed environment (under pressure) will allow much more efficient oxygen use.
reply