How does scuba diving work?
October 3, 2012 11:24 AM   Subscribe

Explain scuba diving to me like I am six years old.

I am interested in scuba diving, but don't really understand the technical aspects-- why they use breathing gas different from normal air, decompression stops, nitrogen narcosis, etc. If you were telling a small child about how scuba diving works, as well as its main risks, what would you say?
posted by karminai to Sports, Hobbies, & Recreation (14 answers total) 8 users marked this as a favorite
They do use normal air (at least in all the diving I've ever done); it's pumped into the tanks at the dock (or on the boat) using a compressor.

As to what you're looking for, I guess I'd say: people are designed to live (more or less) in one "zone" on the planet's surface. In this zone, the air has a particular density. Above this zone, the air is too thin for us to breathe--like on the top of Mt. Everest. But in addition to how dense the air is, we're also, essentially, designed to operate under the weight of that air. Imagine a balloon--at sea level it might be nice and round. Take it up to Mt. Everest, and it might expand and pop. Take it below the sea and it might get squished.

The zone we live has (more or less) one "atmosphere" of pressure. At about 33 feet below the surface of the water (just 33!) the weight of all the water equals the weight of all the air at sea level. That is, once you're 33 feet below the surface, you are under two atmospheres of pressure--one for all the air above you, and one for the 33 feet of water.

At different amounts of pressure, the air from the surface (in your scuba tank) starts to act differently in your body. The air you're breathing is under pressure, too. The pressure, in effect, squeezes "air" where it wouldn't normally go. The nitrogen you're breathing, then, can get squeezed into parts of your brainy bits, and your brain gets addled. This is nitrogen narcosis (hugely simplified).

When you start to surface, gasses can form bubbles in your body. As the pressure decreases, the bubbles expand (like the baloon on Mt. Everest). A big baloon in your body can block bloodflow and is no good (as in deadly!). To fix this, you periodically stop on your way up to the surface to allow the bubbly gas to leave the body. The number and duration of the stops depends on how deep you went and how long you were there. [On edit: coming up too fast with those bubbles in you is "the bends" or decompression sickness.]

There are also sharks. And you can wear a cool wetsuit!
posted by Admiral Haddock at 11:53 AM on October 3, 2012 [8 favorites]

For normal depths, it's normal air. No decompression for depths less than 30 feet or so, IIRC. Main risk from breathing compressed air is embolism. If you hold your breath and surface, the expanding air balloons your lungs and can explode them, netting bad results and death. True for a relatively shallow depth, too... like 4 feet.

The deeper you go, the less time a tank lasts. This is because you are breathing compressed air and the pressure has to counterbalance the water pressure outside. If you go below 1 atmosphere of pressure (about 30 feet), nitrogen builds up in your blood like carbonated beverages with CO2. When you come up, it bubbles away. The degree to which this is an issue depends on the depth you visit and the time you stay there. Up to a point, if you stay very briefly, there is no decompression. There are tables that tell you how long to dwell at certain points on the way back up depending on your depth and duration.

You need weight to sink. People have a fixed buoyancy close to neutral.

Lots of other details, but diving is like hiking. Shit can happen down there and your response times and judgment have little margin. Fear is involved, and so is chemistry and physics. If you are with partners, their experience, fear and bad judgment are as risky as their experience, calm and good judgment are positive. Who knows?

Insurance companies hate divers, pilots, rock climbers, hang gliders and racers.

It's dark down deep. in a farm pond, you can see a few inches. In clear lakes and stream pools, you can see a long way.

It's cold down deep.

It's also pretty down deep if you can see.

A good approximation is snorkelling, which has few of divings negatives, and does not suffer the same risks since your air is not-compressed. You can practice to stay down for minutes.
posted by FauxScot at 11:53 AM on October 3, 2012

My favorite analogy: a typical tank contains enough air to fill up a balloon the size of a small car*. We need the tank to be strong enough to hold all that air (squeeze an empty 2L bottle to "feel the compressibility"**).

*it's actually more like a phone booth, but who knows what a phone booth is anymore.
**you're not actually compressing the air, but it's an analogy.
posted by disconnect at 12:09 PM on October 3, 2012

Just in case you didn't know, in order to scuba dive, you need to be certified. The Open Water I course covers your questions (and more!) because they are exactly the questions all divers need to have a sound technical understanding of. It's not a difficult course, and it does make math very immediate because suddenly you are not calculating apples and oranges, but your own air. Woot!
posted by DarlingBri at 12:40 PM on October 3, 2012 [1 favorite]

It is not always normal air. Until about 40 meters, normal air can be used. This is pretty the recreational diving range. After that depth, there are various air mixes that have a lesser percentage of nitrogen in order to avoid nitrogen narcosis, which is when the nitrogen starts to make one intoxicated. Divers are trained to recognize the onset and will swim to a swallower depth to allow it to pass.

In order to replace the nitrogen, first more oxygen is added to the mix. However, after going deeper still, oxygen toxicity becomes a problem, so then the next filler gas is helium.

The Insultingly Stupid Movie Physics website used to have a review of The Abyss posted, where they calculated that the air in the underwater habitat would have had an incredibly high hydrogen content. They did not hold it against James Cameron that the cast did not sound like chipmunks.

If you decide to scuba dive, you will learn all of these things and more in your certification classes.
posted by Tanizaki at 12:47 PM on October 3, 2012 [1 favorite]

As a beginning diver the risk is really pretty minimal, provided you follow a few simple rules that will be drilled into you when you go for certification training. You'll be doing short, shallow dives breathing normal air, and your chances of getting the bends are very low if you take even the slightest precaution (which just means not popping straight up to the surface after spending a long time underwater, and keeping track of how much total time you've been down and at what depth) and your chances of getting nitrogen narcosis are nil: that's exclusively a deep-water phenomenon.

The thing about holding your breath and surfacing: you'd have to be locked up with terror and ascending fast while holding your breath pretty hard to injure yourself that way; otherwise you'll feel pressure building in your lungs and an accompanying instinctive desire to let it out well before you pop a lung. Not say that it can't happen, it's just hard to imagine what it's like before you've experienced it and so it's easy to envision it as something you need to really concentrate on as you ascend, when actually it will come pretty naturally as long as you're not in the midst of a panic attack.

If you take a class you'll be drilled on the biophysics and precautions until you're very, very bored before you even set foot in the training pool. It's a pretty safe recreational activity as long as you pay attention during instruction and don't do stupid things that make it unsafe.

Here's an abstract with some information on the frequency and type of accidents that do happen.

SCUBA is lots of fun, and while the risks are real they are also quite manageable. There is no reason to be scared off if you can keep your wits about you and not take stupid risks. Many resort areas offer guided dives where you watch a short training video and then go down for a shallow dive with a divemaster right by your side the whole time. This is how I first experienced SCUBA and is certainly an option if you're curious but not ready to go for a real certification yet.
posted by contraption at 12:49 PM on October 3, 2012 [1 favorite]

It is not always normal air. Until about 40 meters, normal air can be used. This is pretty the recreational diving range. After that depth, there are various air mixes that have a lesser percentage of nitrogen in order to avoid nitrogen narcosis, which is when the nitrogen starts to make one intoxicated. Divers are trained to recognize the onset and will swim to a swallower depth to allow it to pass.

In order to replace the nitrogen, first more oxygen is added to the mix. However, after going deeper still, oxygen toxicity becomes a problem, so then the next filler gas is helium.

This is not quite correct, you wouldn't use a hyperoxic mixture like nitrox to go deeper than you could with air (since the MOD of those mixtures is shallower than that of air). The main reason to use nitrox is to extend your no-decompression time, I guess it also helps avoid narcosis but that doesn't usually happen at depths shallower than the MOD of standard nitrox mixtures.
posted by atrazine at 1:01 PM on October 3, 2012

"why they use breathing gas different from normal air"
99% don't. They dive with compressed air. Scuba diving is easy and fun.

Risk: The main risk is, besides diving time, rapid ascend.

Take an inflated balloon for children. Diameter is about 1 ft.
Bring down the balloon in the water. It will get smaller and smaller during descend. Ascend again and it will reach 1 ft again at normal size. Are you still with me?
Ok, not take the same 1ft inflated balloon down and when it hast become small because of the pressure, let's say 1/3 feet in diameter, inflate it under water to 1 ft diameter again. Let's just assume you can do this. Now ascend again. The balloon will explode during ascend because it will inflate far beyond the original 1 ft at normal pressure.
The same things will happen with your lungs if you don't exhale when ascending rapidly. If ascending in an emergency or you made a mistake, lost you weight etc., the most important thing is to strong, forcefully and constantly only exhale during ascend.

Another problem is Henrys Law. Basically, if you increase pressure, you increase solubility of gases in your blood, especially Nitrogen. This is a function of depth and time of diving. When you ascend you want to make sure you give you body time to "exhale" the access gas in your blood and make stops of several minutes. If you stay down too long and ascend too rapidly, little bubbles will form in your blood stream. Best case, your skin gets itchy (some people call this "diver fleas"), in the worst case you would get a stroke or something. Hence obey the diving times and ascending stops suggested for hobby/Paddy divers. It is suggested you don't board a plane the same day you dived.

Enjoy. The one thing that is funny: Fish won't be sacred of you. The come and look at you.
Be careful with advanced diving, requiring special expertise skills etc. (wrack diving, trimix diving, diving under ice, at alpine seas etc.)
posted by yoyo_nyc at 1:04 PM on October 3, 2012

Like Admiral Haddock said, water is a lot denser than air. This means that even a small change in depth has a large effect on the pressure of the water around you. At the surface, the pressure is one atmosphere (1 atm) and you breathe normally. The way you breathe is by using the muscles of your chest and diaphragm to create a small pressure difference between the air outside your lungs and the air inside, this cause air to enter or leave the lungs.

If you had a metre long snorkel, could you breathe underwater? The answer is no. The pressure of the water outside your lungs would be higher than the pressure at the end of the snorkel and you wouldn't be able to inhale. The solution to this is to breathe pressurised air, the pressure of the air is always matched to the pressure of the water around you. As you get deeper, the pressure of the air you breathe increases to match and you can always breathe normally.

Breathing compressed gases brings with it certain challenges, including decompression requirements, narcotic effects, oxygen toxicity, and others.

First of all, decompression: as you breathe compressed air which is 79% nitrogen, nitrogen dissolves into your blood. As you come up and start breathing lower pressure air again, this nitrogen comes out of solution as little bubbles. If you come up slowly, this is not a problem - the little bubbles are filtered out by your lungs and all is well. If you come up from a great depth suddenly, then large bubbles can form in places they don't belong such as your brain or your joints. Divers follow decompression schedules from tables or computers to avoid this. They may also breathe gasses that have less nitrogen in them to increase the amount of time they can stay underwater without having to spend a lot of time decompressing.

Most recreational divers follow a system where they stay below their "No Decompression Limit" which is the deepest and longest that you can dive without having to do decompression stops on the way up. Note that most of those divers still do a "just in case" decompression stop just below the surface.

Another risk is over expansion injuries caused by the air in your lungs expanding if you come up without exhaling. The compressed air expands when you ascend, so you always need to be exhaling. As long as you blow a constant stream of bubbles, you'll be fine.

why they use breathing gas different from normal air

As others have said, for most diving SCUBA divers actually use compressed air. The reason that some divers use different gases has to do with an idea called partial pressure. Partial pressure is just the pressure of the gas multiplied by the percentage of the gas component. So at the surface, the pressure is 1 atm (=1 bar) and the air is more or less 21% oxygen and 79% nitrogen. The oxygen partial pressure is then 0.21 bar and the nitrogen partial pressure is 0.79 bar. Now I mentioned decompression earlier, and in general to make decompression easier you want to keep the nitrogen partial pressure as low as possible.

So why not dive pure oxygen? That would solve decompression issues. Unfortunately at oxygen partial pressures above 2 bar or so, the nervous system has a tendency to go bananas and you can have a seizure. To stay safe, most civilian divers try and stay below 1.4 bar oxygen partial pressure at all times. Since total pressure goes up with depth, so does partial pressure. In fact, 100% oxygen would be a 1.4 bar partial pressure at a depth of just 4 metres! Not useful for going very deep then.

We want to keep oxygen below 1.4 bar at all depths we'll be diving while still reducing nitrogen content. That's where nitrox comes in. Nitrox is like normal air but with a bit more oxygen and a bit less nitrogen. Standard mixes have 32% oxygen and 68% nitrogen. This is usable down to 30 or so metres and reduces your decompression issues significantly.

What if we want a gas mix that we can take deep while still reducing decompression? Then we add helium to the mix and we now have trimix (nitrogen, oxygen, and helium). Helium dissolves into your blood like nitrogen, but it comes out much more quickly and means you have to spend less time waiting at decompression stops on the way up. Some trimixes are hypoxic, which means that they're less than 21% oxygen. This is good for going very deep, but it can be dangerous because breathing them in shallow water can cause you to lose consciousness.

The reason you'll sometimes see divers switch gasses mid-dive is that different gasses are good for different purposes. A diver may use a bottle of normal air or nitrox until they get to 30 metres or so, switch to a trimix tank with less oxygen in it to go deeper to 60m, switch back on the way up, and then use a bottle of 50% oxygen while they're decompressing at 6 metres. I should note though, that the vast majority of divers never do gas switches and that the recreational diving depth limit is 40m. Most of the above you will not need to know as part of your normal training, I just like talking about breathing gases and I have trouble restraining myself once I get started.

Nitrogen narcosis feels like breathing nitrous oxide. You feel very relaxed and you decision making abilities are impaired.

A beginner will be trained (with PADI anyway) to dive on normal air to a max depth of 18 metres and won't need to know most of the above.
posted by atrazine at 1:41 PM on October 3, 2012 [2 favorites]

If you were telling a small child about how scuba diving works, as well as its main risks, what would you say?

You breathe air underwater from a tank. It's the best fun in the world. Always do what the grown up tells you to do, and don't mess around. It's about as dangerous as crossing the street, and the bad things that can happen are just as bad.
posted by cromagnon at 2:11 PM on October 3, 2012 [3 favorites]

No decompression for depths less than 30 feet or so, IIRC.

Atrazine touched on this, but just to be clear, what people often refer to as "decompression" happens when you stay at depth for too long and go beyond the limit of nitrogen in your blood. You should never enter decompression for recreational diving. It's dangerous stuff, and requires a series of "decompression stops" (which may lead to running out of air) and/or time spent in a recompression chamber.

The good news is that if you use dive tables or a dive computer, it should never happen. And it's not the case that you automatically enter decompression below a certain depth. But the deeper you go, the less time you have before entering decompression. Depending on how much nitrogen is already in your blood from previous dives, you can spend close to an hour at 30 feet and be fine, but you might only have 15 or 20 minutes at 90 or 100 feet. (If you're using Nitrox instead of air, bottom times are generally longer, though it can cause some other problems once you get below 100 feet or so.) The job of the dive computer is to tell you when it's time to go up.
posted by eugenen at 2:17 PM on October 3, 2012

One thing to keep in mind about all the multiple gas mix diving stuff is to remember that you're unlikely to ever do it and very likely be happier for it. The reason the vast majority of people go scuba diving is to see stuff. Most of the stuff anyone wants to see is near enough the surface you'll never need to use anything but regular pressurized air.

Several of the deep "technical" divers I've spoken to have gone deep down and found themselves thinking, "Oh what" upon seeing a small mound of nondescript rocks.
posted by bswinburn at 2:26 PM on October 3, 2012

The other answers cover decompression and breathing gas pretty thoroughly, but I thought of a couple of other things that you might want to know. There is a fair amount of equipment involved in diving and you will be told about it in class, but in case you are curious here is a rundown. The basic stuff most people buy first (although you can rent pretty much everything you need) are mask, fins and snorkel. You said to explain like I am talking to a child, so if any of this is too obvious I apologize in advance. The mask allows you to see underwater by holding a pocket of air in front of your eyes; there are any number of styles and if you wear glasses you can get a prescription version. Water magnifies things so you may find that even if you wear glasses you can still see fine without them underwater. Fins help you swim with less effort (especially useful if there is any current where you are diving) and like masks come in a variety of styles. The snorkel may seem redundant when you have a tank of air on your back but it is good for breathing at the surface without having to use your tank. (If you pursue this as a hobby you will find that conserving air is a big topic of discussion among divers). Unless you are diving in tropical waters you will need a wetsuit, which holds a layer of water next to you that acts as insulation. They come in different thickness for different degrees of protection from the cold. For really cold water there are dry suits, but they are much more complicated and are considered more technical than recreational diving. Even in warm water you may want to wear a thin dive skin to protect you from stinging creatures underwater and sunburn on the surface.

As for the actual SCUBA (Self-Contained Underwater Breathing Apparatus) gear, you start with a tank of compressed air. In class you will learn you to hook it up to the rest of your gear, which starts out with a first stage regulator, a device that reduces the pressure in the tank to a pressure usable by your second stage regulator, which is the part you breathe through. Also coming off the tank are hoses for a pressure gauge (to tell you how much air is left; a depth gauge is often paired with it), your bouyancy compensator (more about that in a second), and a backup second stage regulator (or octopus) that can be used by you or another diver in the rare event a regulator fails. The second stage regulator is a clever device that automatically adjusts the pressure of your breathing gas to the depth of the water as well as only supplying gas when you breathe in. The buoyancy compensator is a vest that you can inflate with air from your tank to float and vent air out to sink. Typically you only need to fill it with air when you are on the surface. Most divers need weights to maintain neutral buoyancy (which feels like weightlessness and is one of the coolest things about diving) but more experienced divers can achieve this through controlling their breathing, which means less weight to swim around with and less work (and less air use) Other things such as the amount of body fat on the diver and whether they are in fresh or salt water affect the amount of weight you need as well.

Finally, you need at the very least a watch to tell you how long you have been at a given depth. Nowdays most people use dive computers to automatically calculate how long they can stay at a given depth or depths without signficantly risking decompression sickness as well as calculate how long to wait between dives (the surface interval). During training you will also learn to do those calculations by hand, the old fashioned way, using dive tables. It may sound complicated but it really isn't that hard for most people; in many instances you will dive with a group where the leader has come up with a safe dive profile for the group, but it never hurts and is good practice to run the numbers yourself. You will also keep a log book of each dive which in theory you may be asked to show when you get air (the only thing you need certification for is air; all the other stuff is useless without it) but all I have ever been asked for is my certification card (and some places seem pretty lax about that, which always worries me).

Finally, there are any number of gadgets that you can buy to take with you underwater, from dive knives to dive lights to writing tablets, but they tend to be extra stuff to lug around unless you are certain you will need them on a given dive. As I mentioned above the most important things to buy for yourself are a mask, snorkel, and fins. They are relatively cheap and are the things where personal preference makes a big difference; they can also be used for snorkeling around when you arent scuba diving. The other stuff can be bought piecemeal as you decide if they are worth the cost of renting vs. owning, and most people tend to leave tanks for their last purchase as they are widely available to rent and are the biggest and heaviest item to lug around when you travel.

Diving is a blast and worth pursuing if you are interested. With some common sense precautions it is very safe (although the more extreme forms of technical diving are much more dangerous) and you can see some amazing sights even as an open water diver (in fact, because light drops off pretty quickly as you get deep, most of the brightly colored corals and fishes you see on TV and in films are not that deep). Unfortunately I have not had time to go diving much in the past few years, but I am hoping my daughter shows an interest in it and when she is old enough (10 for most classes) I hope we can take a class together and go on some good trips.

And one last bit of advice for the novice diver: as the consensus in this AskMe shows, it is OK to pee in your wetsuit.
posted by TedW at 3:04 PM on October 3, 2012 [1 favorite]

If you're interested, many dive shops do "try-out" dives in pools, where you'll get maybe 30-60 minutes in a shallow swimming pool, with a divemaster instructing you. It's usually free or very cheap, or at least was back in the '90s. You can ask your divemaster for more detail on any or all of these questions (although the answers you've got here so far are all really good). I encourage you to give it a shot, and pursue training if you like it! I've got Open Water Diver, which is the really basic PADI certification, and I would love to go back and do Advanced Open Water and some of the more complex/technical certs too. It's a blast!
posted by Alterscape at 4:00 PM on October 3, 2012

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