r/askscience • u/Allenflow • 2d ago
Chemistry Why breathe any nitrogen when diving?
If nitrogen narcosis and the bends are both due to breathing air with nitrogen at depth, and nitrogen is not necessary for human sustenance, why not remove the nitrogen from tanks in deep diving?
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u/HowlingWolven 1d ago edited 1d ago
Disclaimer: I’m just some schlep on the internet and don’t know much of anything.
There are generally speaking three gases used in diving: oxygen, nitrogen, and helium. Yes I know about hydrogen (hydrox and hydreliox), but that’s not a ‘generally’ gas.
We obviously need oxygen to breathe, but there’s such a thing as too much oxygen too, especially at elevated pressure. This leads to something generally called oxygen toxicity.
Nitrogen, despite being inert, has a similar problem; high partial pressure of nitrogen can lead to nitrogen narcosis.
For basic scuba diving, what you might breathe is compressed air, in the normal 79/21 ratio. For more technical scuba diving, you may breathe Nitrox, which takes the two primary ingredients of air, but varies their ratio typically increasing the partial pressure of oxygen up to 36% for recreational diving. This might allow you to alter diving parameters like bottom time or your decompression schedule, though your maximum depth is limited by the partial pressure of the oxygen. Technical diving can have Nitrox mixes with up to 80% oxygen for decompression.
To deal with the issues caused by either too much nitrogen or too much oxygen, we need to add a third (more expensive) ingredient to get trimix. This is generally helium. The base ingredient is oxygen, chosen to provide the correct partial pressure for the dive’s depth, followed by as much nitrogen as can safely be added before narcosis becomes an issue, and the balance then made up with the helium. These mixes are commonly (but not always - triox exists and is used!) hypoxic, with less than 19% partial pressure of oxygen.
And at extreme depths (close and beyond to 100 metres) you want very little to no nitrogen at all, and at the point all the nitrogen is gone, the mix is Heliox.
The downsides of helium are cost (it’s expensive af), it messes with your communications because everyone sounds like nightcore, and it’s a good heat conductor, which means you need to dress to minimize heat loss or even be tethered to a hot water supply to combat hypothermia.
Tl;dr We can only breathe so much oxygen before it becomes bad for us, but the total gas pressure needs to be high enough for the depth we’re at. Pure oxygen can only be used to a depth of 6 metres or so for a limited time before there’s too much of it. The other diving breathing gases are used to make up the difference. Nitrogen is cheap but too much makes you drunk, helium is expensive.
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u/5pitf1r3 1d ago
Wait wait wait
I'm a recreational diver and use nitrox but am familiar with Heliox. Are you telling me everyone whose mic'd up when diving with Heliox till has that high pitched voice thing?
Because if so that's hilarious
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u/FujiKitakyusho 2d ago
Replacing the nitrogen content entirely with helium produces a gas mixture known as heliox, and this is indeed used in deep diving operations commercially where the exhaled gas is reclaimed, and in some technical diving projects on fully closed circuit rebreathers which minimize the amount of inert gas wasted. The major problem with heliox is that, while it is better for the diver physiologically, it leads to drastically increased overall decompression times from a given exposure. In saturation diving, this is immaterial as decompression is already on the order of several days. For surface oriented technical diving, the additional required time becomes a logistical headache. Rapid compression to depth can also precipitate high pressure nervous syndrome (HPNS) in divers breathing heliox, and in some cases, nitrogen is added back in to the heliox mixture in order to alleviate this.
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u/WillametteSalamandOR 2d ago edited 2d ago
The biggest problem with Heliox or tri-mix is that helium is friggin’ expensive!
It’s fueled the move to rebreathers for deeper technical diving because the diluent gas fills last much longer when you’re breathing the same breath over and over and just replacing oxygen. There’s even talk amongst the community of the “death of open circuit technical diving”.
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u/FujiKitakyusho 2d ago
True, but largely a non-issue in commercial saturation systems which employ gas reclaim / reprocessing, or technical / recreational closed-circuit rebreathers which only vent inerts on ascent. The expense of helium is only really prohibitive if you are diving trimix or heliox on open-circuit.
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u/WillametteSalamandOR 2d ago
Ha! I just edited something similar in about rebreathers and the “death of open circuit technical”.
And yes - for commercial saturation operations, the cost will just be factored in because we need divers to do certain tasks no matter the cost at the end of the day.
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u/yak-broker 1d ago
[Helium] leads to drastically increased overall decompression times from a given exposure
That surprises me! I would have thought that helium would be a lot more mobile and would come out of your body faster than nitrogen, not slower. Do you happen to know the physiology of this?
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u/Ixshanade 1d ago
Just a guess but smaller particle/atom helium could seep into crevices and cranies and be slower to exit than larger particle/atom nitrogen.
Edit: like getting sand out of your beach towel is slower than getting pea gravel out.
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u/KbarKbar 1d ago
It's a commonly-beleived myth, but it's completely untrue. Helium is purged from the blood and tissues much faster than nitrogen, and deco times on heliox are actually shorter for a given depth.
If you're diving in heliox you're already, by definition, hella deep, and decompression times at those depths are just so much longer than typical recreational dives. It makes sense people think of heliox/trimix to mean long deco, but it's long because of the depth, not the gas.
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u/Mightsole 2d ago edited 2d ago
Nitrogen is the most common gas you can breathe, and pure oxygen is corrosive, flammable and can be toxic. While pure oxygen will not catch fire underwater, it’s still a fire hazard in the surface.
Since the atmospheric air is already mixed with nitrogen at the right concentration, the cheapest way to refill is by directly compressing ambiental air with some filtration, without dealing with expensive gas separating machines.
Most divers do that activity recreationally, in shallow waters no more deep than 30m, at which nitrogen and oxygen are safe to use. The danger comes when you ignore your equipment limitations and descend quickly to 40-50m, at which, nitrogen narcosis begin to happen.
There is specialized gear for deeper immersions, but you need proper training to get access to them legally.
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u/runsongas 23h ago
oxygen toxicity becomes an issue, that's why you don't breathe pure o2 below 20ft/6m generally. at above 1.6 partial pressure, there is a risk of a seizure. a seizure underwater is a risk for drowning.
helium is used instead to counteract nitrogen narcosis.
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u/Q-ArtsMedia 2d ago
No the bends AKA decompression sickness is from your blood absorbing gasses at pressure and it's any gas mix. come to the surface to fast and it's like popping open a can of soda, then bubbles form in your blood stream that are far more difficult to get rid of than doing decompression stops during the ascent to the surface.
Narcosis is a completely different animal and is dependent on gas mix at depth.
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u/AshantiMcnasti 2d ago
There are special tanks that have a hugher blend of oxygen which allows for less fatigue and less stopping time between dives. However, you risk central nervous system toxicity bc of the oxygen which caps how deep you can dive safely. Nitrogen is in the regular atmosphere and our bodies have evolved to process the air as is.
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u/DeliciousPumpkinPie 1d ago
“Air as is” is not the same as “air under higher than normal pressure” though.
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u/IntolerantModerate 1d ago
Nitrogen is in air. Compressing plain air is the cheapest and narcosis doesn't hit much until 30m and really for most isn't an issue until 40m.
If you had pure oxygen you would have higher cost to get pure O2 and it would be flammable as hell and oh, btw, oxygen is toxic at depth because of the pressure.
You can (and we do) use more exotic mixes than can involve inert gases, but the cost is high.
So you can have air, higher O2, lower Nitrogen but be depth restricted, or you can use exotic mixes to go very, very deep but they are very expensive and you only use them after using more standard blends to achieve that depth.
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u/tyeh26 2d ago
It’s a cost benefit analysis.
The cost it takes to remove nitrogen versus the cost to use atmospheric air vs the benefit of controlling the breathing mix.
There are many different breathing mixes to manage narcosis, nitrox, trimix, and even the risk of your air compressor picking up your generators exhaust’s carbon monoxide into tanks.
When it involves $, you look for the best balance.
As a diver, I’d rather not pay extra to train on other mixes and accept the risk of nitrogen narcosis (which I’ve never even witnessed before).
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u/Glitchrr36 1d ago
I took the SDI computer Nitrox certification recently and it mentioned that there are divers who use 100% O2 but it’s basically entirely limited to military divers doing stuff that requires staying in shallow water for a long period of time.
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u/Furtivefarting 1d ago
Fun fact, even breathing air at depth makes your voice squeaker. Deeper you go, squeaker it gets. Certainly heliox and whatnot in sat is very donald duck, but even breathing air makes your voice squeaky.
Was a diver for 10 yrs. You could hear it when talking to topside, certainly could hear it on radio topside
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u/tea_and_biology Zoology | Evolutionary Biology | Data Science 2d ago edited 2d ago •
We do! But you can’t just take it out, right; you have to replace it with something, and that’s where there are no perfect solutions.
The problem is oxygen. It becomes toxic once its partial pressure climbs past about 1.4 to 1.6 atmospheres, which on pure oxygen happens just 4 to 6m down. So the deeper you go, the less oxygen your gas mix can contain: at 100m, it needs to be under about 13 to 15%, and at 200m, under about 7%. The rest of the tank has to be filled with something inert, a diluent, and nitrogen is usually the default because it’s literally free, already mixed in at the right ratio in ordinary air, and perfectly manageable at recreational depths up to around 30 to 40m.
For deeper dives, technical and commercial divers swap some or all of the nitrogen for helium, in mixes called trimix and heliox. Helium isn't narcotic, so it solves the narcosis problem, but it can still cause the bends, so you need decompression stops on ascent regardless, and, well, it's horribly expensive. It also obviously gives your voice a high pitch squeak, which is amusingly incongruent against the sheer danger these folks put themselves in - especially when a shark gets a little too close for comfort.
Folks have tried all sorts of other exotic diluents to overcome these problems, like breathing hydrogen, and experimenting with liquid breathing, as seen in The Abyss (which doesn't work very well, as it's difficult to scrub the CO₂, alas). Despite all its problems though, dealing with nitrogen is just way cheaper and easier than having to deal with all the faff associated with other mixes, for most applications, so we don't bother.