Oxygen weathering is a primary constraint on life on Earth, and every carbon-hydrogen based organism in the past 2.5 billion years has had to develop biochemical coping mechanisms for this toxic gas that wants to react with carbon and with hydrogen; It is harnessing this reaction ("respiration") with biologically mediated processes and modulating it to specific rates that permits us life.
For humans, acute breathing gas toxicity only happens in a high pressure environment.
Air approximates an 80/20 nitrogen-oxygen mix. Atmospheric pressure is 14.7psi.
The 120psi air compressor in your auto body shop is equivalent to a dive only 81 meters deep. SCUBA divers and later saturation divers have probed the various limits of the human cardiopulmonary system using very specialized gas blends all the way down to 700 meters. Too much oxygen partial pressure causes all the symptoms you see listed, and higher partial pressures cause symptoms to appear faster.
> The curves show typical decrement in lung vital capacity when breathing oxygen. Lambertsen concluded in 1987 that 0.5 bar (50 kPa) could be tolerated indefinitely.
This means you could breath 80/20 nitrox at 2.5 bar, or 37 psi, or 25 meters depth, "indefinitely" in the sense of hours or days.
PS: Chronic use of 100% oxygen at atmospheric pressure causes other types of toxicity. Some of the oxidative damage therein, accumulated over the years at a normal 20%, probably directly analogizes parts of the human aging process. Other types of oxidative damage probably work faster than proportional exposure. We only start to notice damage like this in people with impaired lung function who rely on an artificial supply of oxygen boosted to beyond an 80/20 ratio, to breath.
To add to this, when diving with compressed air most people get woozy and otherwise intoxicated from oxygen around 30 meters from the surface. For some people 25 meters is enough for such symptoms to occur.
Diving on normal air, oxygen toxicity occurs around 60m.
Are you sure you aren't talking about Nitrogen narcosis ('raptures of the deep')?
Yes, he is. Oxygen toxicity causes seizures, not narcosis, and kicks in at around 1.6 bar of partial pressure (just below 65 m when breathing 21% oxygen as in regular air). PADI uses 1.4 bar to add an extra safety margin.
Oxygen toxicity is really the one thing in recreational diving that will kill you if you do it wrong, though for recreational divers the risk only exists when using enriched air(*).
Fortunately it's trivial to avoid it by only using enriched air where the sea floor is at a safe depth, but you should know the math nevertheless. For example if the sea floor is at 35 m (4.5 bar) you won't enrich air above 1.4/4.5=31% oxygen, probably more like 28%.
Oxygen toxicity is also the (or the main) reason why enriched air must never be stored in white or yellow bottles. If you see yellow you can assume it's 21%, while for any other color you must use an oxymeter before using it. Not doing so can be literally the difference between life and death.
Scuba diving is safe but a lot of the safety is about procedures, as you can see.
(*) Enriching air above 21% oxygen is done to avoid the other issue with nitrogen, which is decompression sickness. It lets you stay longer on the bottom. In other words, enriched air improves the trade-off between bottom time (limited by nitrogen) and maximum depth (limited by oxygen toxicity).
Got the symptoms wrong, but oxygen toxicity is also present from 25 meters down.
Not with 21% oxygen. 25 meters is 3.5*0.21=0.73 bar of O2 partial pressure, which is within even the strictest limits that apply to rebreathers (1.3 bar).
If you're breathing 100% oxygen for decompression, that's a completely different story and not something a recreational divers will do.
Have you got a reference for that?
E.g. https://dan.org/health-medicine/health-resources/diseases-co....
I don't see any thing that supports "when diving with compressed air most people get woozy and otherwise intoxicated from oxygen around 30 meters from the surface".
It does say:
"Be aware that oxygen toxicity is unpredictable. Divers have experienced convulsions at shallow depths under conditions where most experts would not have expected them to occur."
But that doesn't say what depth, what oxygen % and how often it happens.
It also says that oxygen toxicity is a possibility only above 21% oxygen. In fact, with regular air there are way too many things that have already gone wrong if you are at 65 m depth (or even 50 m).
It's clear from reading the document that convulsions at "shallow depths" refers to the case of breathing 100% oxygen, where 1.5 m difference is the difference between <1.6 bar (safe) and >1.7 bar (absolutely not safe).
> all the way down to 700 meters.
700m! That's wild, I mean nuclear submarine crush depths are at like 400-500m? I get that it's not like you can compare a hard steel tube with a human body but regardless, it's wild.
The published data for military submarines is the nominal test depth, not the actual design limit. The operational depth may be much deeper but that will be classified.
Recent US submarines all have test depths described by as being in excess of the same few hundred meters. In all likelihood that is a throwaway value. It seems unlikely that they produced generations of submarines that were less capable than their older ones.
> The published data for military submarines is the nominal test depth, not the actual design limit. The operational depth may be much deeper but that will be classified.
We know the actual collapse depth for an older sub: 730m for the USS Thresher (test depth: 400m), in 1963.
Test depths of current generation subs are ~20% higher; pushing them to 700m or so might be plausible, but not much more. Radical hidden capabilities would either require substantial advances in material science or drastically different hull thickness, neither of which is really feasible to hide from adversaries anyway, especially considering how little utility you get from hiding this (compared to e.g. exact capabilities of anti-air interceptors or radar characteristics for bombers/fighters).
https://en.wikipedia.org/wiki/Russian_submarine_Losharik says test depht of 2,000–2,500 meters (6,600–8,200 ft), allegedly happened in 2012 somewhere in the Arctic Ocean.
>It seems unlikely that they produced generations of submarines that were less capable than their older ones.
i wouldn't be sure. There seems to be no military advantage to deeper so why spend the money. A sub needs to hide, but it can't do any other job when too deep. sinking ships can only be done when near the surface. If the sub can get under a couple thermo layers that is good enough, any deeper is more a party trick than useful.
i'm not in the navy but that is how I read the unclassified information I have access to.
> sinking ships can only be done when near the surface.
Isn't this itself a huge assumption?
Sinking ships via upward facing torpedoes would be a huge tactical advantage at first glance. Less time to detection and deploy countermeasures or evasive maneuvers.
Perhaps, I am wrong in assuming they cannot be fired below a certain depth?
just watched: https://www.youtube.com/watch?v=V-mzZXwCn68&t=2s
while these torpedoes are going in the other direction seems like the technology for surviving, functioning and navigating depths definitely exist.
Found the SCUBA diver
Indeed, the grandparent post is a pretty good summary of the takeaways you get from taking PADI’s enriched air nitrox course (which is a requirement if you ever want to dive with enriched air).
In the olden days this was tracked manually (the ratio of your depth to percentage of air and time under water) via so called “dive tables”. The purpose and output of the dive table is to determine the safe amount of time you could dive at a certain depth without risking narcosis.
As this is a sliding window based on multiple variables - and you are very rarely maintaining a constant depth as you dive - it’s of course annoying and less accurate to hand calculate this. Modern dive computers just seamlessly calculate it all for you nowadays.
Fun fact: those dive tables were created by the US Navy conducting experiments on its own divers, there was a real human cost to acquire that information.
Numerous other training agencies also teach how to use nitrox safely. PADI training isn't specifically required (or even particularly good).
I don't see where the person you replied to claimed there was anything special about PADI.
Never done it, never gonna.
Thank _Neoshade_'s legendary story in a Reddit comment for that - https://www.reddit.com/r/todayilearned/comments/dv99nf/til_t...
With a side helping of Neal Stephenson's Cryptonomicon, and the submarine / treasure hunting arc which describes decompression sickness.
Scuba diving is great. You don't have to do deep or risky dives to enjoy it. There's a ton of fun in diving around reefs 10m down.
That story is pretty wild. And relatable.
I got myself into a little trouble when I dove the Blue Hole 16 years ago. We were warned pretty heavily how many people have died doing it, so I went in with a healthy level of anxiety. It was my second dive where the dive plan was to go to 40m, which is the limit on regular air.
The descent was surreal. You have the wall of the crater on your side, but everything else is different shades of blue. Past about 10m, there's not really any wildlife to look at, just blue. We descended straight down, going in slow motion. As we went down the blues got gradually darker and deeper.
At probably the high 20s, I started to notice I could really see the surface clearly anymore, and I started to panic. My breath started racing and I started being annoyed by my regulator in my mouth, which is an unnatural feeling to being with. For maybe a minute, I debated whether I should try to get myself under control, or signal my dive instructor I wanted to ascend. Meanwhile, we were still drifting downward. I worried whether nitrogen narcosis might affect my judgment or ability to control my panic.
In the end, I decided not to be a hero. I gave my instructor the thumbs up to ascend, and we went through the orderly process of safety stops. When we got to the top I told her I explained I was feeling panicky (you can't really communicate anything nuanced below the surface), and then I spent the rest of my tank diving the first 10m, which was relaxing, and let me finish the day on a high note.
I have over 5000 logged dives and have only suffered DCS once, with the right training SCUBA can be very safe.
that comment is a classic and certainly entertaining, but there are multiple levels of safety to prevent something like this from happening, the first of which is the wall of tombstones that greets you when you arrive at that specific dive site. To end up in that situation means to have already made a number of big, big errors.
I remember the Blue Hole as one the best dives I made, and not even the scariest: that prize goes to the time I was in calm waters at 20 meters, and the pressure regulator just failed, leaving me without air from both mouthpieces. And that's why you have a buddy...
I don't think I've ever had anything fail on me diving, but I've been with people who have run out of air (my buddy was constantly using all his up), so having to breathe off someone else's tank isn't uncommon.
As I mentioned in my sibling comment, I did have a scary time on the Blue Hole. I think my other most nervous dives were:
- Pacific dive in Costa Rica in rough seas and surge. We suddenly had visibility drop to near zero when we hit the outflow current of a river. Definitely a lesson in how quick conditions can change.
- Cavern diving in a cenote in Mexico. Nothing weird happened, but we went kinda far in, and I get nervous in overhead environments.
Running out of gas or having to breathe off of someone else's tank is uncommon. Gas planning and monitoring is a fundamental skill that every diver needs to master just to get a basic open water certification. If your buddies aren't able to do this reliably then they need remedial training.
It's good to practice gas sharing as a contingency in case of equipment failure but actually running out is not acceptable.
Yeah this is no laughing matter, i would refuse to dive with someone who lacks the basic skills to monitor their air responsibly.
same, blue hole is notorious because inexperienced divers get pressured into deep dives they haven't trained for by local guides looking to make a quick buck.
my scariest dive was when a 14 year old got separated from the group and thought it would be a good idea to continue his dive for 30 minutes.