Diving Science: Everything You Never Wanted to Know about Decompression Sickness
On twitter, someone asked:
If you dive to 10m, do you need to worry about decompression? If so, what length of dive makes it necessary?”
The question wasn’t addressed to me, but a friend of mine brought it to my attention and since twitter is terrible for giving impromptu lectures, I thought I’d write it up here.
When I taught SCUBA Diving, this topic took about 6 hours to teach. That’s because in order to understand Decompression Sickness, you have to understand both basic human physiology and some rudimentary physics before you even get to the topic of Dive Tables. Since I don’t have a spare six hours, this is going to be the shorthand version.
Note: All measurements will use the English System: feet, pounds, etc. Sorry, Rest of the World.
Lesson the First: Pressure
Anything that has weight will exert pressure. Air has weight. The weight of a column of air that is one inch square and extends from sea level to the upper reaches of the atmosphere weighs about 14.7 pounds. Hence, one atmosphere (the unit of pressure we use in diving, and other things) can also be expressed as 14.7 PSI (Pounds Per Square Inch).
Water also has weight. One cubic foot of sea water weighs about 64 lbs (or 64.1 lbs to be a little bit more precise). So how much water would need to press down on us before we would experience another atmosphere of pressure?
The answer is 33 fsw (feet of seawater). If you want to prove it, merely take 64.1 lbs (the weight of a cubic foot of seawater), divide by the number of inches in a cubic foot, then multiply by the number of inches in 33 feet, and you’ll get 14.68… which can be rounded up to 14.7. Math is amazing. And because they’re equivalent, 33 fsw can be used as a unit of pressure (just like 14.7 PSI, or 1 atm) allowing us to convert back and forth.
Another quick principle: At the surface we experience 1 atmosphere of pressure. When we descend to 33 fsw, we experience another atmosphere. Which, if we want to include the *total* pressure being exerted on our bodies, we express as 2 ATA, or atmospheres absolute.
What does this mean for us? It means that every 33 feet we descend in the ocean is equivalent to adding an entire atmosphere of pressure on our bodies. Do we feel that pressure? Well for the most part, no. Pressure is distributed evenly on our bodies and so 10 feet deep in the ocean feels no differently than 110 feet. With one major exception: Air spaces.
The human body is mostly water and other solid things which can’t compress (well they can compress a little bit, but only at extreme pressures). Air can. And there are numerous air spaces in the human body: lungs, sinuses, the Eustachian tube in your ear… When you’re at sea level, all the air spaces in your body are equalized to one atmosphere (14.7 PSI, or 33 fsw). If you held your breath and then descended to 33 fsw, you would be experiencing 2 ATA on the outside of your body, but your air spaces would still be at 1 atm. This is uncomfortable.
Our lungs are mostly okay as they’re enclosed in our chest and thus aren’t really experiencing that force, but our ears will really hurt. The only thing separating that extra weight of water pressure, and the miniscule weight of the pressure inside our ears, is a tiny little eardrum. And that eardrum can burst.
Fortunately it’s not really difficult to equalize pressure. By simply holding your nose and blowing gently, you force more air into your Eustachian tube and sinuses which, based on physics and math I won’t get into, increases the pressure in those spaces until they equalize with the outside pressure. When you ascend and the pressure on the outside of your body is less than the pressure inside your body, those air spaces will equalize on their own. This is why your ears ‘pop’ as you ascend in an airplane. Occasionally though you’ll get reverse blocks — this happens if you have a cold and failed to take to take any decongestants, or worse, you did take some decongestants at the start of your dive, but they wore off when you were at depth. Because of the congestion, your sinuses will have difficulty allowing air to escape to relieve pressure which makes ascending very painful. And there’s really not much you can do about it except try to ascend very, very slowly.
And if you’re a SCUBA Diver, this is why you never, never, never, never, never UNDER ANY FUCKING CIRCUMSTANCES hold your breath. Because you will fucking die.
Imagine a balloon which you fill at the surface, and then tie off. The pressure inside the balloon is at 1 ATA. As you descend, the pressure outside the balloon increases and since the balloon has no way to add air to itself, the balloon has to get smaller in order to equalize (the same number of molecules, but over a smaller area, will have a greater pressure).
If you were to ascend, the process would reverse itself. The balloon would slowly expand to its original shape in order to equalize the pressure inside the balloon with the pressure outside.
Now imagine a balloon which you fill at 33 fsw, and then release. As the pressure decreases from 2 ATA to 1 ATA, the balloon will desperately try to decrease the pressure inside itself. Since it’s tied off and can’t release pressure by expelling air, it needs to expand (same number of molecules, over a larger area will have less pressure). If it’s a super elastic balloon, that’s fine. If it’s not… it will burst. That’s what can happen to your lungs. Your lungs are comprised of 300 million tiny little balloons. If you take a breath at 33 fsw, hold it, and then ascend to the surface you have two options: open your airway so the pressure can escape, or kill yourself as one (or more likely many) of those alveoli burst allowing air to escape and wreak havoc on your body. If you’re lucky, the air will simply escape into the pleural cavity (the space between your lungs and rib cage) and you might get away with just a simple pneuomothorax which is more or less fixable if you attend to it immediately.
If you’re unlucky, the air will escape into your circulatory system, travel up your carotid artery, block blood flow and you pass out and die. Incidentally, if you’re ever diving and you see this… string the person up by their feet. Air bubbles float and there’s anecdotal evidence of people waking up once their feet become the highest point of their body and the bubbles manage to dislodge. Obviously it’s not going to work if the bubbles are too large and/or numerous, but it’s worth a shot.
Lesson the Second: Partial Pressure
We’ll be coming back to Partial Pressure later — quite a bit actually as Dalton’s Law is my favorite — but for now lets discuss what we really mean when we say ‘1 atm.’
Our atmosphere is comprised of a bunch of different gases. Roughly, it’s 79% Nitrogen, 20% Oxygen, and 1% trace gases (Argon, CO2, and a few others). We could quibble over those percentages, but since there are only two gases that we’re really concerned with, and diving doesn’t require absolute precision, we’re going to do some rounding. Thus, for our purposes, the atmosphere consists of 79% Nitrogen, and 21% Oxygen.
We can also express percentages as Fractions, and thus we could say that 1 atmosphere consists of .79 Nitrogen + .21 Oxygen.
Or: .79 N + .21 O2 = 1 atm
Well, as we’ve already discussed… every time we descend 33 fsw, we add another atmosphere of pressure. So at 33 fsw, we’re at 2 ATA. The percentages of Nitrogen and Oxygen don’t change. All that has happened is the total pressure has increased.
79% N + 21% O2 = 2 ATA
And what’s 79% of 2 ATA? Or 21% of O2? And is there an easier way of expressing it?
(2)(.79 N) + (2)(.21 O2) = 2 ATA
Or: 1.58 N + .42 O2= 2 ATA
On the right side we have the Total Pressure exerted by the mixture of gases (aka, Air). On the left side we have the Partial Pressures exerted by the individual gases. So at sea level, we’re breathing a partial pressure of .21 Oxygen. But at 33 fsw, we’re breathing a partial pressure of .42 Oxygen. At 99 fsw, we’re breathing a partial pressure of .84 Oxygen.
And there’s the thing: at increased partial pressures, gases will have different effects on your body, sometimes really terrible effects. Like death.
We’ll get into the concept of Nitrogen later as we discuss Decompression (the whole point of this post), but for now I want to use the example of Oxygen.
Oxygen is toxic. It’s a pretty basic requirement for our ability to live, but too much of it and we get sick. At sea level, you don’t have much to worry about. You’re only breathing a partial pressure of .21 after all. But if you were to breathe 100% Oxygen, you would be breathing a partial pressure of 1. If you were to breathe 100% Oxygen for an extended period of time, eventually you would begin to experience Pulmonary Oxygen Toxicity — which causes inflammation and some other nasty effects, particularly if you’re in hyperbaric conditions.
There’s also Central Nervous System Toxicity of Oxygen which happens to divers who breathe a partial pressure of Oxygen above 1.6. These symptoms are more immediate and include Vertigo, Nausea, Dizziness, and Seizures… which often result in death because a seizure underwater is a little bit harder to manage than it is at the surface.
Oxygen is a bit of a twit.
Lesson the Third: Circulation
When you take a breath, air travels to your lungs. You might picture your lungs as two giant balloons, but this would be a little wrong. Your lungs actually comprise of millions (about 300 million) of tiny little air sacs called alveoli which is where the exchange of gases takes place.
As I’m sure you already know, your arteries carry oxygen rich blood to your organs and muscles, and then your veins carry the de-oxygenated blood back.
The part that usually gets missed in those 5th grade science classes is that’s not all that’s being carried back and forth. You might know that carbon dioxide is a waste product of metabolism, and so when you exhale, you’re actually exhaling more CO2 than you breathed in. But what about Nitrogen? Did you think that your lungs have magical Nitrogen filters which ensure that Oxygen is the only gas which is transferred to your red blood cells and ferried all over your body?
Of course not. Your cells carry Nitrogen around too. And it carries Nitrogen back, along with the CO2. And back and forth all the live long day.
And it doesn’t matter because Nitrogen is inert and doesn’t do anything to us so who cares if it hitches a ride? Well, it doesn’t matter so long as we stay at sea level where the partial pressure of Nitrogen stays at a steady .79.
See, there’s this thing that Nitrogen does at increased partial pressures. It becomes Narcotic (as anyone who is familiar with laughing gas — which is an oxide of Nitrogen — can attest). Everyone experiences it differently, and has different sensitivity, but most people will definitely experience some Nitrogen Narcosis symptoms as the Partial Pressure of Nitrogen reaches 3.58 to 4. And the symptoms are very similar to being intoxicated, hence there’s a joke rule in Diving known as Martini’s Law which states that every 50 fsw you descend is like drinking one dry martini on an empty stomach.
Or, if we think cutesy rules are below us, we can simply use Dalton’s Law to figure out at which depth a partial pressure of 3.58 occurs.
At 1 ATA, the partial pressure of Nitrogen is .79.
At 2 ATA the partial pressure of Nitrogen is (2)*(.79).
So Total Pressure, times the Fraction of the Gas (percentage) = Partial Pressure.
Here, we have the Partial Pressure of the gas (3.58) and we have the Fraction of the Gas (.79) and we’re looking for the Total Pressure.
TP * (.79) = 3.58 Or, TP = 3.58 / .79 = 4.5 ATA
To convert to depth, we merely subtract 1 (because one of those atmospheres is the actual atmosphere), and multiply by 33 fsw. Which gives us… 116 fsw. Easy peasy.
And if Narcosis was all we had to worry about… we’d be fine. Except it’s not.
See, by virtue of living on this planet and breathing our atmosphere, your tissues have Nitrogen in them, at a partial pressure of .79. But what happens when we descend underwater and that partial pressure increases to 1.58? The partial pressure of Nitrogen in our blood will be greater than the partial pressure in our tissues… and as we’ve seen before: Pressure wants to equalize.
Over time, the partial pressure of Nitrogen in your tissues increases, and if you stayed long enough (maybe 24 hours), the partial pressure of Nitrogen in your tissues and in your blood would eventually even out.
And unless you fell in love with a mermaid and decided to live in the ocean forever, eventually you would want to ascend. Now the partial pressure of Nitrogen that you breathe, and that is transferred to your blood, would be less than the partial pressure of Nitrogen in your tissues. And once again, that pressure LONGS, YEARNS, DESPERATELY SEEKS to equalize. And so the Nitrogen comes out of your tissues…And this is where we have problems.
When Nitrogen comes out of our tissues, it has a tendency to come out in the form of bubbles. Tiny bubbles, big bubbles, many, many bubbles. These bubbles clog our circulatory system, get stuck, and basically wreak havoc. The symptoms caused by this are collectively known as Decompression Sickness. Otherwise known as DCS, otherwise known as The Bends. And it only took me about 2000 words words to get there.
Lesson the Fourth: A Brief History of Dive Tables
You might think that the history of Decompression Sickness begins and ends with the history of SCUBA Diving. But you’d be wrong. You see, the first people to experience the effect I briefly described above, were bridge workers. Caissons were these giant things that allowed people to work under water. They’d spend the entire day excavating, come up to the surface… and experience pain and sometimes death. It became known as Caisson’s Disease, and was a mystery until the work of Haldane… aka the goat sadist.
To make a long story short, Haldane would take goats, put them in a chamber which he then pressurized, take them out after a specified point of time… and see what happened. He eventually came up with a theory and a set of tables regarding decompression sickness. The point of the tables was to determine how long one could dive (or work) at depth and not suffer ill effects upon surfacing. Caisson workers who only worked at 2 ATA (33 fsw) typically didn’t have any symptoms no matter how long they stayed down there. But if you went to 3 ATA, or 4 ATA, the story was different. The deeper you went, the less time you could stay. Furthermore, and this is really key, you needed to come up slowly.
Apparently around the time that Haldane was working (around 1905), the standard advice to divers (I could get into the history of SCUBA equipment, but I won’t… just know that the modern regulator wasn’t invented until the mid 20th century) was to ascend slowly at first, and then go faster and faster as you got closer to the surface.
I typically avoid analogies, as they’re imperfect, but there’s a classic one in Diving. Imagine a coke bottle. Then shake it. Now unscrew the cap. What happens?
Well, soda is carbonated… which is a fancy way of saying that it’s pressurized and carbon dioxide is forced to dissolve into the liquid — which only works at extreme pressures. When that pressure is released (by unscrewing the cap), the CO2 comes out in the form of bubbles. The faster you relieve the pressure, the more explodey the Carbon Dioxide tends to become.
That’s your body when you ascend from depth. The faster you ascend, the more rapidly you’re causing a pressure differential between your blood and tissues. The Nitrogen comes out even *more* rapidly, which means more bubbles. In the most extreme form of DCS, bubbles (or one large bubble) will block blood flow to your brain and you’ll pass out (and typically die) within a few minutes of reaching the surface (this is essentially the same effect, but for a slightly different reason, as the extreme forms of lung barotrauma that we discussed up above).
Fast forward a few decades or so, and the next great advancement of Dive Tables was achieved. This time, instead of Haldane we have the US Navy. And instead of goats, we have Navy Divers.
The way I always told the story in my classes was this:
The Navy would send 10 divers down to 60 feet, have them stay for 70 minutes, and bring them back up. 5 of the 10 divers were okay, the rest of them had decompression sickness and had to go into a chamber for treatment. Thus, 60 feet for 70 minutes was deemed unsafe.
They took 10 more divers, sent them down to 60 feet for 50 minutes, brought them back up, and all the divers were perfectly fine. So it was safe, but maybe they could stay even longer.
10 new divers: 60 feet, 60 minutes. 8 of the 10 divers were okay, 2 had to go into the chamber. So the Navy wrote in their table that going to 60 feet for 60 minutes was an acceptable depth/time ratio.
In reality it probably wasn’t quite as cavalier as that. At least, I hope it wasn’t.
But the basics are true: the Navy came up with a comprehensive set of Dive Tables filled with acceptable depth/time ratios for avoiding Decompression Sickness but allowed an error of 2 Navy divers.
The other really important thing they did was come up with a set of decompression tables. You see, if you know that if you go down to 60 feet and stay for 3 hours that you will experience DCS when you ascend… what are you supposed to do about it?
Well there are two ways, both premised on the same principle. If DCS is caused by rapid offgassing (release of Nitrogen), which is exacerbated by higher pressure differentials… then all you need to do is control one of those two things: come up really, really slowly… or come up to a safe depth (a depth where you won’t experience DCS symptoms, but will allow for offgassing), and stay for a certain amount of time until you’ve released enough Nitrogen to ascend to the surface safely. You can do this either by coming to the surface and then jumping into a hyperbaric chamber, which will then pressurize you to 33 fsw, or whatever the schedule dictates, or you can ascend to a specified depth on your way up (such as 33 fsw) and wait. The dive tables included these times.
So, for example, if you dove to 60 fsw and stayed for 65 minutes, you would have exceeded the specified depth/time ratio. So the tables would tell you to ascend to 10 fsw, wait for 5–10 minutes, and then ascend. These became known as Decompression Stops. They can get really complicated depending on the type of diving you’re doing. For instance if you were to dive to 900 feet, then you would have to do many, many decompression stops on your ascent. If you’re curious, my cave diving Instructor once dove to 937 feet. He said it took him about 12 minutes to descend to that depth, and then about 12 hours to come back up.
As another example, my father participated in the Atlantis Dives at Duke University in the late 70s/early 80s. He and the other divers set a world record by ‘descending’ in a chamber to 2,132 feet. It took about 10 days for them to reach that depth, and then about 3 weeks to ascend. Of course since no one had ever done that before, there weren’t tables in existence for how to come up. So by ‘ascent’ I pretty much mean that they would ascend, get the Bends, and then have to descend to treat… then start over. There were also issues with Oxygen Toxicity (keeping the partial pressure of Oxygen high helps with treating DCS, but allowing it to get too high leads to its own issues) and even with all the back and forth, within a few hours of reaching the ‘surface’ they were all back in the chamber again getting treated.
Fast forward again: If you’re a certified SCUBA Diver, chances are you don’t use the Navy Dive Tables. Or you might, you just don’t know it.
The oldest SCUBA Certification agency is the YMCA. The second oldest is NAUI (National Association of Underwater Instructors). NAUI was formed due to an increase in dive accidents which was itself caused by a combination of increased popularity of the sport due to shows like Sea Hunt, but a lack of qualified instructors. NAUI eventually developed its own set of dive tables… sortof. See, the Navy Dive Tables were really good. They were just also a little dangerous for recreational divers who may have forgotten that according to the dive tables, 20% of the divers may experience DCS by following the depth/time ratios.
So NAUI simply took the Navy Tables… and made them a little bit more conservative. Rather than a Depth/Time ratio of 60 fsw for 60 minutes… NAUI made it 60 fsw for 55 minutes… and made 60 fsw for 60 minutes the first required DCS stop.
90 feet for 30 minutes became 90 feet for 25 minutes… and 120 feet for 15 minutes became 120 feet for 12 minutes. And in each case, the original Navy Depth/Time ‘safe’ limit became the first Decompression stop. Only instead of 10 feet, NAUI recommended 20 fsw.
If you happen to be a PADI Diver, then they also have their own tables… but they look nothing like the NAUI or Navy tables. There’s some similarity of course — it’s not like PADI believes you could go to 120 feet for an hour and be okay — but the specific numbers are quite different.
What everyone agrees on though is the tables are a guideline, and not law. Everyone is different and thus you may be more susceptible to DCS than someone else. As such all organizations recommend a ‘Safety Stop’ which is when you stop at 20 or 30 fsw for 3 minutes before coming back up to the surface. Of course, chances are that if you’re diving today… you haven’t looked at a set of tables since your Open Water class. Dive computers are the superior method for one simple reason:
They keep track of multi-depth profiles.
As you may have guessed, no one goes down to 60 fsw and stays there. More likely you dive down to 60 fsw (or 80 or 90 or 100), and then work your way back up, spending most of your time at shallower depths. According to the tables, you need to use your maximum depth in order to find your maximum allowable dive time. So if you went down to 120 feet for just a fraction of a second, you still can only dive for 12 minutes in order to be ‘safe’. Dive computers, however, are constantly recalculating your maximum allowable dive time based on your dive profile (max depth, current depth, etc) which extends the amount of time you’re allowed to dive. When you add in surface intervals (which you can track with a dive table, but again, it’s quite conservative), and alternative dive mixtures like Nitrox (a gas mixture with an increased percentage of Oxygen which will thus increase the maximum allowable dive time), computers become essential if you want to maximize your dive time.
Lesson the Fifth: To Answer the Original Question
Well, it took me almost 4,000 words to get here… and believe me, this is the short version, but to answer the original question:
If you dive to 10m, do you need to worry about decompression? If so, what length of dive makes it necessary?
10m is approximately 33 fsw. If memory serves, the maximum allowable dive time for 30 fsw is about 150 minutes.
So according to the tables, yes. You need to worry about decompression.
But practically… umm. No. Not really. I would almost bet money that if you went to 33 fsw, stayed there for 24 hours, and then ascended (slowly to prevent lung barotrauma), then you’d be fine (but don’t test me on that — you’d probably still have some form of The Bends just not severe enough to cause you major issues… doesn’t mean it’s a good idea).
As for ‘what length of dive makes it necessary’, a lot depends on what it is you want to play with. The easiest thing to do to make it a ‘concern’ is to increase the depth dramatically… but if you were to increase it beyond 130 fsw, then you’re going to have more problems than just DCS. And speed of ascent is the easiest way to cause an issue, but the symptoms may be much more dramatic (death) than what you’re interested in.
And of course I say ‘almost’ because the fact is… everyone is different. If this were the long version of my lecture, I would move into things like Henry’s Law and things which affect the solubility of a gas in a liquid. For now, just know that you have an increased likelihood of DCS depending on things like temperature, the physical difficulty of the dive, your general health and level of exertion, and all sorts of other little fudge factors.
Lesson the Sixth: DCS Symptoms
I’ve alluded to this before, but haven’t really explained it. There are two types of Decompression Sickness, and the primary difference is the time frame. As I mentioned before, if you ascend rapidly then you’re likely to run into problems with Arterial Gas Embolism (a bubble which blocks blood flow… particularly dangerous if it blocks blood flow to something really important… like your brain).
The other type, the more common symptom associated with the term ‘The Bends’ is joint pain. These symptoms typically don’t occur until an hour or more after surfacing. The tricky thing is since diving is a physical activity, some people ignore the stiff, achy feeling in their joints as they think it’s related to being sore from diving. This is problematic as the longer you wait to treat DCS, the more likely you are to experience permanent damage (including paralysis). Less common is ‘skin bends’ which looks like a rash. In any event, the way you treat DCS is to administer 100% oxygen. This helps you offgas Nitrogen more efficiently and quickly, and for whatever reason, people feel better while breathing it.
Well, until it turns toxic.
The other option, if available, is to get thyself to a hyperbaric chamber. A technician will administer Oxygen and pressurize you to 2 ATA. Now instead of a partial pressure of 1 of Oxygen, you get a partial pressure of 2! This increases your chances of CNS Toxicity, but as you’re usually lying down, and you’re being observed closely, this isn’t as much a concern. Increased O2 gets rid of the Nitrogen, and the increased pressure will ‘crush’ existing bubbles, and ensure that any new bubbles aren’t of troublesome size.
Lesson the Seventh: I am not a Physician
I taught diving for about 5 years, and I’m also a certified Cave Diver and Nitrox Instructor. Still, a lot of the information I’ve presented today relies heavily on analogies (which are never perfect), and if memory serves, Decompression science is still a bit… fuzzy on the details. The tables are based on experimental data, and then finding algorithms, equations, and theories to ‘match’ the results… For example, Haldane concentrated on the concept of half times… In effect that at 33 fsw, the half time of Nitrogen was x. So after x minutes, half the tissue was saturated with Nitrogen. After x minutes again, another half, and so on and so forth. And the same thing on ascent. He also concentrated on ratios and said (I believe) that you could handle a 2:1 ascent after any amount of time… So if you went to 2 ATA for 24 hours, you could ascend to the surface without issue. The Navy, NAUI, and PADI somewhat disagree. But again, their data is also based on experimentation, equations, and conjecture.
In other words, Decompression Theory is slightly imprecise, but it’s based on really important physics and physiology principles. If you’re a SCUBA Diver and you don’t have a rudimentary understanding of these things, you’re putting yourself in danger.
Lesson the Eighth: James Cameron’s Film, The Abyss, is So Scientifically Implausible that It Gives Me Fits
A. They’re in an underwater habitat at about 1000 fsw. This means they have to be breathing Trimix which is Air + Helium (due to Nitrogen Narcosis and Oxygen toxicity, in order to dive below 130 fsw, you need to add Helium — there are decompression tables for it as well, as it behaves similarly to Nitrogen in terms of offgassing/DCS). My father relates from his personal experience that communication becomes absolutely unintelligible at this depth. In fact I think it starts getting unintelligible at much shallower depths.
B. Running around? Please.
1000 fsw is about 31 ATA. Do you know what the density of your gas mixture is at that depth??? Dense enough you can’t breathe through your nose, and dense enough that it takes a piece of paper a long-ass time to fall.
C. “They must have done something to us.”
In the film’s ending, the habitat and Ed Harris are transported to the surface in a matter of minutes. Ed Harris has just been down to some ridiculous depth… I can’t even remember how deep — over 2,000 fsw I think. But regardless, the habitat divers have all been living at 1000 fsw for days, weeks, months. Aliens can’t just “do something” to you to prevent the fact that in minutes, they would all fucking be deader than dead upon ascent.
Forget the shaken soda bottle… those poor souls would be the world’s most insane Diet Coke and Mentos experiment in the history of the universe.
D. That said… the film did get a few things right:
HPNS is a serious thing and was a significant problem for commercial divers who worked on Oil and Natural Gas rigs. Due to Nitrogen Narcosis, a lot of these divers used Heliox (Helium and Oxygen). But they experienced some severe symptoms (twitching, paranoia) which made work difficult. Dr. Peter Bennett theorized that the cell membrane may be compressed just the tiniest bit at depth, and perhaps by introducing a small amount of Nitrogen into the gas mixture (and creating Trimix), you could counteract the effect. That was the Atlantis Dive Series in which my father took part. They found that it *did* help with HPNS, but the corresponding Nitrogen Narcosis made the whole thing a little moot.
E. If you pretend that The Abyss is a Fantasy film instead of Science Fiction, it almost works again.
Lesson the Ninth: Fuck Typos and Grammar
This ended up being way longer than I intended… so I’m just going to go ahead and post it without any careful proofreading. If something was horribly mangled to the point of incomprehensibility, please post in the comments and I’ll correct it.
NOTE: This post was originally on my blog which has since been removed. Since this was one of my more popular posts, I’ve decided to post it on Medium so the information is still available.