The Physics of Scuba Diving | Hacker News Reader

The Physics of Scuba Diving

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One of the challenges of (even modern) scuba diving is that failure conditions are at odds with normal reflexes.

For example, if you run out of air your first reflex as a non-diver might be to hold your breath and swim to the surface. As pointed out in the article, that’s a terrible idea at depth and can severely injure or kill you (instead, divers are trained to breath out during an emergency swimming ascent).

There are a number of other ways to die while diving and recorded mortality data proves that out. Each dive, on average, has a broadly similar risk profile (5, in micromorts) to a single jump while skydiving (8) or running a marathon (7) [1].

There’s seemingly room for technology to help make diving a bit more safe. It will be interesting to see if that does happen in the future.

[1] https://en.m.wikipedia.org/wiki/Micromort

Interesting concept… it seems diving is only about 1/4 as dangerous as the average other risks people take in a day, so it seems pretty inconsequential in overall risk unless you are a professional and dive multiple times daily for years on end, but I would guess a pro had less risk per dive due to skill.

Diving with a buddy (which you should absolutely always do) in good weather basically reduces risk to zero as long as you don't get too far away from each other. The chances of four regulators failing (two main, two octopuses) are close to nil.

Also as long as you keep a line straight to the surface. Cave diving and wreck diving are far, far more dangerous, and require special training. There are many ways to die if you can't just head straight up for oxygen in an emergency.

…as long as you don’t penetrate a wreck, dive under ice, or dive in a cave. Since a roof severy decreases the chance to survive if something happens and you need to surface.

Also, of course, tech diving, typically below 40m, can also severely limit the ability to emergency ascend to the surface without injury.

I strongly disagree that one should always dive with a buddy. But I’ll also say that one shouldn’t dive alone. Exit and entry are usually the most dangerous part of the dive.

Diving in low vis, spear fishing, or confined spaces don’t match with having a dive buddy. But they do match with redundant air supplies and other safety measures I mention in another comment.

Also unless your buddy is literally within arms reach at all times, they are not a redundant air source.

> divers are trained to breath out during an emergency swimming ascent

To add a little more detail to this. You should ascend as slow as you can given the emergency situation but gently hum out your air rather than breath out. This slows it so you avoid the panic of empty lungs when not near the surface and also allows air to escape if it’s expanding quicker than you are humming :)

Instinctually (well, once you overcome the most naive instinct of holding your breath) one might think they need to exhale fast enough that the expansion of volume in the lungs doesn't outpace the rate of exhalation, but in reality even the slightest bit of intended exhalation will keep the airway open enough to prevent lung issues, right?

That was what I was taught at least when I learned recreational diving.

Yes, the point is to keep the airways open, but that’s hard to demonstrate hence the suggestion to “make a stream of small bubbles as you ascend”

If you run out of air, ideally you're with your swim buddy and start breathing from their spare regulator and you make a safe ascent together.

I don't know why you're being down voted, but you're right.

A diver doesn't just "run out of air". A diver gets low on gas because they failed to look at their gauges, and/or didn't plan properly.

It can happen that you get a free flow on your regulator, inflator, valve or manifold, but none of these make you run out of gas immediately. You have time to swim to a team mate and breathe from their regulator (if you are on a single cylinder and can't close any valves).

If you have a valve rolling-shut (implying you're on a twinset), you _will_ run out of air until you open it again, and you would be trained to calmly open it, or just switch to your secondary regulator.

Running out of gas can be prevented by planning the dive and keeping an eye on your gauge, depth and timer.

You absolutely can just "run out of air". Equipment malfunctions happen.

Was on a dive where one of the girls hoses blew. She didn't remember her training and shot up to the surface - thankfully we were at ~5m already, but she still got the bends and they sent her to a decompression chamber.

Buddy was recently on a trip and had both his primary and secondary regs malfunctioned while deep. He was able to swim to his girlfriend, but he's been off-put from diving since then - and he's not inexperienced.

Having a buddy is nice, but honestly isn't reliable. Most of the time I'm joining a boat as a single, and while we have our group - it's really only the dive master that's paying attention to people. Everyone else is usually inexperienced or just doesn't even think to keep an eye on others because they're out on vacation.

And even when you do have a good dive buddy, you're not usually attached at the hip. Very often you're looking at something that intrigues you and your buddy gets bored to swim a few meters a way, etc. It'll happen 1000x times in a single dive.

I would still argue that no one ran "out of air" in any of those situations. They panicked and/or ignored their training.

The thing about diving equipment (especially regulators), is that they fail safe, meaning that they fail in a way that allows you to keep breathing.

When a regulator fails, it lets gas go. You can always breathe from it. The cylinder won't empty immediately, so you can get to your next gas source (your buddy, or your next gas switch depth) - cave/mine/wreck excluded.

I have a huge problem with how people gauge risks from scuba diving. This stems from the fact that IMHO must divers are horribly under trained and under practiced for what they are doing and usually are not following best safety practices. For those that are, I think diver is significantly less dangerous than other accepted activities (in fact, I’d be willing to bet that my drive to the dive is far higher risk than my dive).

Some examples of best practices that are not widely followed:

1. never rely on a dive buddy for air redundancy. If you are diving deeper (or longer) than you can perform a free emergency ascent safely, then you should have a redundant air source. “Two is one and one is none”. You might think that you can rely on your buddy, but unless they are literally within arms reach at all times, they are not a reliable redundancy source.

2. Skills must be practiced. When was the last time you shot a lift bag, did a valve drill, unplugged your inflator hose, manually inflated your BCD/wing, doffed your mask and donned your back up mask?

3. Do you know how to reliably navigate? How do you get back to the boat? Do you know how to safely run a reel without getting tangled up and while minimizing risk to the reel line? How about in 0 vis?

4. I love my computers but I plan out every dive manually and have a rough idea of what to do if both my computers fail. You do have redundant computers right? And you know how to use them right? I did almost 100 dives before I got my first computer. I think this is a good path since I have a solid understanding of what to expect from my computers.

5. Entry level training generally skips a lot of these safety tools and skills. I once dove at a Padi resort and brought a pony bottle with me. They dive operator didn’t even know what a pony bottle was. Do you think they are actually teaching about redundant air supply?

I’m certain that having all the right tools in your toolbox, practicing how to operate them and being mindful about how to use them significantly reduces risk.

I think diving is very dangerous for un(der)trained divers and I think the dive industry does a disservice by not portraying these risks and the need for more training/skills practice.

Frustratingly, my insurance says I’m more of a liability because I dive to 53m regularly (at least before kids) than the yahoo who only dives once or twice on vacation every few years to 20m. I dive to 53m with focused planning, training, and the right equipment explicitly so that I reduce risks. And I apply those same techniques to my shallower dives.

There's actually another air space that you need to equalize while diving — the inside of your scuba mask. Don't forget to add air to it as you go deeper, or that thing will awkwardly squish your face.

It would be more accurate to say "suck out" rather than "squish". The pressure is lower in the mask. So the cavity acts like a vacuum trying to pull out your eyeballs.

Yes, the ridge of the mask may feel like it's crushing into your face (and cause bruising to the soft tissues it contacts), but I gather the broken capillaries and bruising in your eyes from the suction are what pose the bigger risk.

The condition is called mask squeeze (or facial barotrauma) and I experienced it once shortly after I got a Hydro Optix mask, which has a larger volume of air compared to normal masks. It was before I appreciated how much extra air you need to blow out through your nostrils while equalizing it, and occurred on a descent that went a little faster than I preferred as I was paired up with a less experienced diver who had some initial buoyancy issues and I decided to stick with her in case she needed help rather than slow my own descent.

It was scary at the time (eyes got bloodshot red like a vampire), but the good news is most cases heal up fine with time.

The Hydro Optix is a great mask if you use it right (shame they went out of business), and SCUBA is an incredibly rewarding sport if you train properly and stick within your capabilities (happy to say that diver doubled down on the sport instead of giving up, got some more training, and now has hundreds of dives under her belt and is a highly competent divemaster).

Good reminder if you feel pain while diving to listen to it and surface to figure out what's wrong. Don't be tempted to power through.

On that note, particularly if you're a newer diver, learn to avoid the temptation to surface quickly when you run into an uncomfortable situation. In most early rec diving that's absolutely an option, but as you find yourself interested in deeper dives, wreck penetrations, caves and other sites that are both an incredible experience and higher risk that's where accidents happen.

Focus on being able to calm yourself, trust your dive buddy, recognise signs of impaired cognition in both yourself and them, and work through whatever situation you find yourself in as a team. It's not a solo activity and any action you take impacts both of you.

Good advice - I wish diving leaned a bit more into managing buoyancy early on. I used to sometimes dive tail for a dive instructor doing group instruction if there was a shaky diver - some stressful moments. My ears hated it because I naturally am a slow decend/ ascend diver

Have you ever tried taking Sudafed before a dive to see if it helps your ears? I've had minor MEBT (and plenty of discomfort) without, but so far I can shoot right down after taking it.

From the research I did before making the decision to try it, results are very mixed (there are plenty of anecdotes, but studies are few) [0] but it seems like as long as you try it on land first (on a non-diving day) to ensure no side effects, there's not much reason not to give it a go [1].

[0] https://pubmed.ncbi.nlm.nih.gov/1610044/

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3931412/

This is interesting. I've never given it much thought (and didn't know MEBT was a thing), but I've experienced the same discomfort while: SCUBA diving, skydiving, ordinary air flight descent, and even some hilly car driving.

Doc says there's nothing wrong or clogged etc about my eustachian tubes. It's easy enough to manage (except skydiving, where the pressure change is too fast and not under any meaningful control of mine!), but if pseudoephedrine would make things less distracting during SCUBA, I'm interested.

Thanks for the links!

It’s very important to note that pressure plays a role in how quickly the body processes out the medicine. A dose might last 6 hours on land may last much less under pressure. One significant danger to this is a decongestant wearing off unexpectedly early while underwater, resulting in congestion a depth and preventing equalization upon ascent. This could result in a barotrauma often called “reverse squeeze”.

Definitely worth noting, but personally I've never had any discomfort during ascent even prior to using the decongestant, and based on some other comments here, that seems to be because our ears are better equipped to readily equalize when over pressurized than under... approximating a check valve, I suppose?

I use the generic (CVS brand) 12-hour extended release 120mg about an hour before diving, then two dives with a surface interval between, so I'm only relying on about 4 of those claimed 12 hours. I'm not sure the magnitude of the effect you warn of regarding accelerated processing, but there's a good margin to work with. There's also a 24hr formulation available.

I’ve done the same. But my point is the “specs” are not well documented and a novice may not have considered this factor.

If you live in northern California, like I know a lot of the nerd herd here does, one of the greatest dives in the world is a two hour car ride away in Monterey. The kelp forest there is like no other place on earth; it looks like a 3D set from a sci-fi movie. The bad news is visibility is a coin toss, and five times out of six it's like diving in pea soup. And the water is cold, you need double wetsuits at least. But if you ever get a high-visibility dive there, you'll be hooked for life.

A lot of the dives are shore dives, you can just walk into the ocean from next to the aquarium and see some amazing things (or cold murk). Point Lobos state park may be the greatest diving site in North America.

U.S. Navy Diving Manual. A 'slightly' more comprehensive take on everything SCUBA:

https://www.navsea.navy.mil/Portals/103/Documents/SUPSALV/Di...

What a fantastic resource for the curious.

Thank you!

When equalizing during scuba, you're upright and can draw in a full lung of air before blowing your nose. So the air already tries to go "up" to your head, and with a fully expanded lung it's easy to "blow" the air out.

When freediving, however, this is more complicated. Your body is upside down, so the air floating up goes towards your stomach, not your ears where you want it. And since you only have the same air you started with at the surface, your lungs get compressed. So at 10m your lungs are only "half full". Then it becomes quite hard to actually blow any air out of them to equalize.

To avoid the mask squeeze, a freediver also have to let some air out of the nose to fill the mask. However, that air is much more valuable to a freediver. So often one use a mask with much less volume. Or even smaller goggles filled with water to avoid having to waste the air. Latter mostly for competitions, though. I personally freedive to see stuff underwater, heh.

You can get back that precious air on the ascent (you probably know this already). Just sniff it back in, instead of letting it escape the mask.

Yup, my club looks at it as kinda a "badge of honor" if you don't release any bubbles while ascending, heh.

I'm a certified scuba diver, but to me freediving seems totally insane haha, props!

I'm both, but freediving feels much safer to me, heh. No gear issues, and if anything is wrong just swim to the surface.

I did SCUBA a few times. My negative take is that it's like equipment heavy bird watching. If you're only visiting reefs, and you're a strong swimmer it may be best to stick with snorkeling.

I never got advanced enough to do wreck diving. That stuff looks very cool, but I've become more personal risk averse since parenthood. Don't let this dissuade you. There are plenty of wrecks that are a safe as reefs--and more that are not.

And if you're into nerding out over equipment, then you'll like SCUBA. Traveling with SCUBA gear is a burden similar to traveling with golf clubs.

Yeah, I agree. I have done skydiving, bungie jumping, and snorkeling. There is nothing like SCUBA diving. It is absolutely the most amazing experience of them all. Peaceful and thrilling at the same time.

But yeah too many gadgets to buy and since becoming parent, I also decided to stick with snorkeling.

Gotta strongly disagree that you may as well snorkel if you’re just seeing reefs. It’s a totally different immersive experience while diving. There’s vastly more to see diving than the limited plane you see top-down in very shallow water.

I have done a lot of diving and snorkeling. In the proper environment you can have the full-on "Jacques Cousteau" experience with just a mask, snorkel, fins and some patience.

The two sports are quite different -- but the difference in risk profile, costs and equipment required understandably rules out scuba for some.

On non-dive days in the tropics I will typically get up at 6am and start snorkeling right away usually until 9 or 9:30a. My wife says I am first to turn snorkeling into a endurance sport.

Yes, I hear you on that. Unless you can hold your breath for while, it's tough to replicate that immersion. That being said, I do like the increased physical rigor of holding your breath and diving deep. I find I spend too much time on vacation just floating around--SCUBA only adds to that. chacun à son goût

When diving, I have a sense of flying/floating and when I am under it almost feels like I’m wrapped in a big fluffy blanket. When I dive, I feel like I am part of the environment, rather than just observing like you would at an aquarium. Snorkeling, while nice, doesn’t have the same feeling.

I wish someone would invent scuba lite.

Say limited to less than 10ft of depth. Maybe three minutes of breathing - who cares. Just enjoy being underwater for a little bit and seeing things you can’t normally get to.

But way less expensive and dangerous.

I think they call this snorkeling

In all seriousness though, you could buy one of these emergency air tanks that would let you breath for 6-12 minutes https://a.co/d/fDZXXhv

I wouldn't trust Amazon with my life(-support equipment), especially not a product with 3.5 stars and obvious misspellings. Things like shoddy air quality, masks designed for aesthetics over function, etc. can create unintuitive risks that extend your stay underway from "6-12 minutes" to "the rest of your life."

At that point just hold your breath, with a little practice 3 minutes underwater isn’t stressful. Fins dramatically improve underwater mobility and a Snorkel + mask let’s you stay at the surface while watching what’s below.

There is sea trek, https://www.sea-trek.com/ which is for people to walk underwater in swallow depths. Not tried it, but looks interesting.

The thing you are looking for is https://en.m.wikipedia.org/wiki/Snuba

bilsbie says:>"I wish someone would invent scuba lite."<

"Oxygen rebreather" I was going to say, but the first image at https://www.divein.com/diving/rebreather/ set me back a ways. The diver looks like a giant version of a wolf spider.

The WWII oxygen re-breather was a smaller-than-SCUBA unit for shallow dives used primarily by special forces (it recycled your exhalations removing the CO2 and produced fewer bubbles at the surface to reveal your presence). It has been decades since I looked at this tech and at first glance it seems oxygen re-breathers have only grown bulkier.

So-called "mini-SCUBA" tanks are available priced in the low hundreds of USD:

https://duckduckgo.com/?q=a+smaller+scuba+setup&ia=web

Finally there's a DIY version based on the James Bond movie "Thunderball":

"Working 007 Mini Scuba Tank! - Breath Underwater With This Spy Gadget"

https://www.youtube.com/watch?v=LqLt_G2dhCI

A safer experience is an underwater drone:

https://bestreviews.com/electronics/drones/best-underwater-d...?

> I wish someone would invent scuba lite.

People are constantly inventing new products to fill this niche. Snuba being one of the least objectionable ones. Most are just flat out dangerous and marketed to uncertified divers, you can see these on places like kickstarter.

Freediving is always an option, but fatality data indicates it’s more dangerous on average than scuba. Lack of training is a problem since anyone can buy a mask and fins and start doing it. Shallow water blackout is no joke.

Someone already mentioned Snuba - which is an interesting choice. I enjoyed that, as you're not bogged down by weight of equipment - but you're also tied to a boat and the setup I had didn't give me access to see the air left up top so I was always a bit paranoid.

Most dive shops have "Discover Scuba" which is basically they give you some brief training, but then they go on a scuba dive with you to a shallow depth (6m I think?) and basically hold your hand the whole time.

> Most dive shops have "Discover Scuba" which is basically they give you some brief training, but then they go on a scuba dive with you to a shallow depth (6m I think?)

My ex-girlfriend got into scuba that way. But they never would take you down to 6 meters the first time.

She thought that she mastered buoyancy control instantly.

Little wonder when you have a dive instructor holding and operating your inflator hose :)

It's a great way to figure out if scuba is for you.

There is a thing called a "Resort Dive" that most places in the Caribbean offer to non-certified.

It is better than Snuba.

Really interesting article, thanks for sharing, OP!

> If you push air into your middle ear cavity by pinching your nose closed while attempting to blow air out of it, air will be forced into this cavity. With more air in the inner ear, the pressure on both sides of the membrane will be equal and you will feel normal.

I know SCUBA divers don't have to do this constantly, only when descending. What's the physical reason behind this? Why doesn't the inner ear pressure go back to normal levels when the diver starts breathing normally again without pinching her nose?

It's because when you're under water ALL air is pressurized by the crush of the water around you.

When the scuba tank releases air into your lungs (and therefore your sinuses, which are connected to your inner ears), it's still partly pressurized, meaning it's at a higher density than it would be up at the surface. As long as you stay down, your inner ear matches the pressure level of the air at that depth. When you start to surface, the air in your lungs expands gradually. Same for the air in your inner ear, but your ears naturally bleed the over pressure back into your sinuses (unless you have a cold or something, in which case resurfacing can be really painful and must be done more slowly). It's just when you're going deeper that you need to force more air into the inner ear because what's already there gets compressed and your sinuses function like a check valve preventing more from getting in without intervention.

It's like in the airplane, but reverse. In ascent, the air volume can naturally squeeze out. On descent however the air won't normally squeeze in easily.

I can open my eustachean tubes voluntarily with a yawning-like maneuver. Otherwise you have to pinch your nose and force the air either with your tongue and throat, or lungs.

It depends on individual physiology, some people have eustachian tubes like railroad tunnels and have no trouble equalizing, while others have tight ones and struggle. You eventually learn your own personal best technique; I know that if all else fails, I can tilt my head back and look directly up while equalizing.

The sounds you hear during this process are pretty entertaining, too. Think of an elderly cat with a head cold, snoring.

There is a YouTube channel that sometimes talks about various technical aspects of diving too. I have been watching it for almost a year, it is sometimes interesting, I discovered many things about diving that I didn't know before; https://youtube.com/c/DIVETALK

That is a golden nugget. Thank you!

Very interesting article, Thanks!

However, since this is a technical article with a fair amount of math, I feel compelled to be pedantic and point out a math/language problem in the article.

The article says,

"For every 10 meters of depth, the pressure increases by 1 bar, or 14.5 psi. That means that on a dive 20 meters (around 60 feet) below sea level, there would be a water pressure of 43.5 psi, three times greater than the air pressure at Earth’s surface."

While the numerical statement is true - at 20 meters, the water pressure would be 43.5 psi, this not "three times greater" than the air pressure at the Earth's surface (14.5 psi). Rather, it is "three times as much". "Three times greater than" would be 49 psi.

I know that this is common usage, seen often in TV commercials, "...new - now 3 times bigger than before...", but I don't expect to see it in a technical article.

Its fun to see my favorite hobby show up on HN once in a while, I went through the trouble a long time ago to become a Divemaster while helping teach a lot of these concepts in classrooms. I have around 800+ dives. Scuba diving is like entering a alien world and I wish I still had the availability to go as often as I did when studying for my current job.

I still work as an underwater guide on the weekends sometimes, I love taking people out for the first time but I also value the time I spent in the ocean more than ever.

I know a lot of people here live in or frequent the CA Bay Area, if you are interested I cannot encourage you enough to take a SCUBA class and get certified so you can dive in Monterey, CA. It had a massively positive influence on my life and I made many friends and memories through it.

The classroom theoretical courses we had parallel with practical exercises were more advanced than what 12 year-old me was learning in school. I'm glad: immersion in applied science made me a more interested student. Education should have more of that !

For the people who might be tempted to skip scuba and just hold their breath: beware of the so-called free diving blackout, which occurs at the surface AFTER the dive, and is usually lethal if you don’t have surface support.

If you like the water but don’t want to deal with gear, do get free diving certified to be educated about the risks and mitigation strategies.

Personally, I’m hooked to Scuba, I find the focus on being calm, breathing and the feeling of weightlessness and flying are one of the most relaxing experiences in life. I surface after every dive just grinning like an idiot.

How dangerous is scuba diving in shallow waters, say less than 15ft/5meters? I'm planning on learning basic scuba with my wife and kids, and my feeling (born out of ignorance, masked as common sense, I know) was that it was very safe if you don't go deep, however some comments here are making me question that. So how safe is it, in general lines, if you don't go more than a couple of meters? And how much better/more fun is it than snorkeling? Thanks in advance for any answers!

It is far more fun than snorkeling and (in my opinion) anything less than 10/12 meters is very safe, if something goes wrong you just slowly kick up and don't need a safety stop. A couple of caveats: you should get certified with an instructor who takes the time to teach you safety techniques (like how to recover if your mask gets pulled off); the certification will involve at least one deep dive. I recommend proceeding directly to 'advanced' certification (the next level after open water diver) right away, because the basic open water course is targeted at minting as many new scuba divers as possible, and is not enough to adequately practice what you learned.

As far as shallow water diving goes, the best places are coral reefs (like Hawaii), you can spend all day in 30 feet of water and never get bored. The great advantage over snorkeling is that you can get very close to things (like tiny fish that look like designer handbags defending their little patch of coral) and see things in 3D, sea turtles slowly drifting past you, small sharks warming themselves on sunlit patches of sand like cats.

For maximum safety, avoid diving sites that have currents or boat traffic, and shore dives with strong surf, those are probably the biggest risks to a beginner in shallow water. It's also a good idea if diving with your family for the most proficient one or two of you to eventually take a rescue diver course, which will teach you how to help a panicked or incapacitated diver. Good luck and have fun—good safety habits go a long way in this activity, and you are in for years of enjoyment diving together.

Funny... I'd have said that snorkeling is among the best ways to spend time in the water. The cool life is near the surface anyway, and requires zero training.

Scuba lets you get closer and it's wonderful to feel the true 3D freedom of movement. But in terms of sight seeing snorkels get got 90% of the joy for 1% of the effort.

> How dangerous is scuba diving in shallow waters, say less than 15ft/5meters?

Bear in mind that the rate of change in pressure is higher at shallow depths. At the surface pressure is 1 atmosphere, it will be 2atm at ~33ft, 3atm at ~66ft and so on. This means that if you have a momentary loss of buoyance control and shoot up, say, 10ft, it is a much larger sudden change in pressure (i.e. more dangerous) to go from 20ft to 10ft than from 100ft to 90ft. So having expert buoyancy control is actually more important at shallower depths.

It just takes practice, but it is a risk for new divers who go in thinking they'll just stay shallow but do so without having mastered boyancy yet.

For shallow dives I use nitrox at 36% oxygen to reduce nitrogen absorption. It's easy to get nitrox certified after completing basic & advanced training so I'd recommend it if shallow diving is a primary goal.

Although at 15ft or less I generally just snorkel if the visibility is any good. At such depths you can see the same by snorkeling, with less risk and hassle.

https://archive.ph/nZVRe

Interesting article, I would have thought principles regarding fluid mechanics would be used such as Bernoulli's principle.

The writer just had to mix SI units and... Whatever the US uses. Why?

My first time SCUBA diving was at Boy Scouts. I was 12 years old. The pool was only 3 or 4 feet deep, but they still gave a safety talk before turning us loose with tanks, regulators and buoyancy control devices: "never stop breathing". They told us that if we inhale at 3 feet below the water surface then stood up with our mouth closed, the air will expand in our lungs and cause a lot of damage to our lungs: barotrauma. All it takes is a constant stream of little bubbles to save your lungs from the changes in pressure.

The tragedy of our last 3 years is that if all our critical care doctors were trained in basic SCUBA diving, they would have known better than to have slapped people on ventilators for 'pneumonia', and would have known better than to treat patients' low blood oxygen saturation levels with supplemental oxygen.

This article covers the physics, but curiously doesn't use the terms: barotrauma and oxygen toxicity. Nitrogen washout is another problem experienced by people who breathe oxygen-enriched atmospheres. Not a problem for brief trips underwater, but a huge problem for patients kept on oxygen-enriched atmospheres for long period of time.

tl/dr: Breathing higher-than-atmospheric levels of oxygen will destroy your lungs (#OxygenToxicity), as will pressure differentials (#Barotrauma).

Medical Hyperventilation [0] is a medical tragedy. If only our doctors were all trained to be SCUBA divers.

After the first few months of using high-pressure differential ventilation on their pneumonia patients, doctors switched to using different ventilator management strategies. But they never issued a Mea Culpa [1].

The other problem experienced by patients treated by SCUBA-naive doctors is #OxygenToxicity. From the article:

>> But underwater, the problem is likely to be too much pressure. If the partial pressure of oxygen gets around 1.6 ATM, it can cause people to have convulsions.

Healthy people can tolerate a pure oxygen atmosphere for about 24 hours...

The third problem is nitrogen washout, where the nitrogen in patients' lungs was replaced with oxygen, leading to collapse of the air sacks.

I wrote a blog post about "Medical Hyperventilation", but it was too late, and I've no credentials to speak of. Oh well.

[0] https://www.taxiwars.org/2021/06/folly-medical-hyperventilat...

[1] https://en.wikipedia.org/wiki/Mea_culpa

Correction:

>How about 40 percent oxygen and 60 percent nitrogen? (Note: This is real stuff, >it's called Nitrox.)

As a nitrox certified diver I can offer that most common nitrox mixes do not exceed 34% oxygen and often are 32% oxygen. For safety, a diver with 32% oxygen is typically limited to about 112 feet water depth, before potential oxygen toxicity sets in to the nervous system.

For safety a diver with 36% oxygen is limited to about 95 feet water depth.

A large majority of underwater reefs around the world are above these maximum nitrox depths.

Furthermore, nitrox is quite popular, especially among older divers as many seem to agree they have more energy near the end of a 2 tank or multiple tank dive day.

Using plain air and 2 tanks or more dives per day, it's been common to come back to the hotel and fall asleep to get a rest (or similar on liveaboards).

But using nitrox on all dives it's common for even the older crowd to have enough energy to be ready to go downtown to the bars and have some drinks after a day of diving.

I think thermal insulation plays a FAR bigger role in how tired one feels. I wear 7mm wetsuit even in tropical waters. All the “tough guys” make fun of me for it, but I’ll do five 1-hour dives a day on a live aboard trip while they get in two or three 1/2-hour dives. Even in 85 degree water, your body gets cold fast.

>Furthermore, nitrox is quite popular, especially among older divers as many seem to agree they have more energy near the end of a 2 tank or multiple tank dive day.

Anecdotally, Nitrox is supposedly good for hangovers.

> As a nitrox certified diver I can offer that most common nitrox mixes do not exceed 34%

You're right. I have never practically seen a Nitrox mixture with 40% oxygen. 28% would be far more typical. Mostly I dived with mixtures around 32%.

For wild and wonderful reasons, however, my SSI Nitrox certification allows mixtures of up to 40%. This would allow a maximum operating depth of 25 meters (30 meters if you stretch the safety margin), which is not very much when you go - for example - wreck diving at a lot of places.

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