Article published on December 13, 2003. Layout and editorial structure updated on July 16, 2026, with no changes to the original text.
As always with the space available on a sheet, it's impossible to both explain everything and have acquired all the necessary data. Once again, we aim to create some points of reference.
From compressed air to breathing mixtures
The idea of using gas mixtures with a composition different from that of air is very old. What would effectively be a rebreather today already existed around 1880, and like today's ARO, it used pure oxygen, meaning a mixture different from air and—a special case of the general definition—consisting of only one gas. Already in the early stages of diving's history, it became clear that nitrogen and oxygen become dangerous to the body above certain partial pressures, and alternatives to compressed air were sought.
Nitrogen narcosis , for example, soon began to claim victims and as early as 1919 engineer Thompson put forward the idea that helium could be used to avoid narcosis.
There are some good diving stories online, but I'll just point out that in 1937, Max Nohl set the then world record in Lake Michigan, a remarkable 140 meters deep, while breathing helium-oxygen, of course. The real revolution for recreational divers occurred many years before the introduction of trimix in our industry, when Albert Bühlmann decided to make public the values needed to calculate trimix tables. Bühlmann's name is rightly inseparable from that of Hannes Keller, with whom he developed the ideas—still the basis of today's decompression procedures—that led to the practical use of helium, nitrogen, and oxygen mixes.
But there is not only 'trimix' on the contrary, an equally rich and important sector is that of nitrox, that is, mixtures of nitrogen and oxygen but in proportions different from those in which they are present in the air.
Overall, artificial breathing mixes attempt to address the limitations of traditional compressed air, both in terms of depth and bottom time.
Let's see.
Nitrox: principles, advantages and limitations
Mixed gas diving, even if twenty additional gases are discovered tomorrow that can be used by divers, will always begin with a solid understanding of nitrox, and it could never be otherwise. The reason is that the gases are the same ones every diver is accustomed to (and therefore there's already a familiar starting point), but the fact that the percentages can be varied opens up a whole new world. All, I repeat, all the problems that can arise with any mix arise with nitrox.
Obviously, this statement needs to be taken with a grain of salt: the technical challenges of diving itself are clearly different. With nitrox, you can't go down to, say, 65 meters, while with trimix, you can. Therefore, diving techniques, equipment use, and problem and emergency management are generally more complex with trimix, or even some issues are unique to trimix.
This, however, is because the mix is inseparable from certain dives, and not so much for its effects on physiology. But from a practical perspective, the approach that views the mix as a piece of equipment to be chosen and configured like the rest, depending on the intended purpose—well, that's something you can learn very well with nitrox, and only with it.
Why nitrox reduces nitrogen absorption
Very briefly, since all divers now know what we're talking about, the idea is that since decompression requirements depend only on the amount of inert gas, eliminating some of it allows for less decompression or longer no-decompression periods. The key concept of nitrox is to perform deco calculations at a so-called fictitious depth, which is shallower than the actual dive depth. This depth is determined by keeping the partial pressure of nitrogen equal in the two situations: air at the fictitious depth and nitrox at the actual depth.
Let's take a little example, just to get the idea straight, but first we need to know that the mixture is usually indicated with the acronym EAN followed by the percentage of oxygen.
EAN32 and air equivalent depth
Let's take a dive to 30 meters using a 32% oxygen mix, or EAN32. The nitrogen percentage will be (100-32), or 68%, and the partial pressure of nitrogen at 30 meters will be equal to the 4 bar present at the depth multiplied by 68 and divided by 100, as I'm sure you know from your courses; if you don't, it doesn't matter. Doing the math gives us 2.72 bar. Now consider a depth of 24.43 meters, which corresponds to a hydrostatic pressure of 3.443 bar. If we were breathing air at that depth, the partial pressure of nitrogen would be equal to 3.443 multiplied by 79 and divided by 100. If you do the math, you'll find that the value is equal to 2.72 bar, as before.
What does that mean? It means we're going to 30 meters with EAN32, but the nitrogen absorption is exactly the same as if we were descending to about 24 meters. I've obviously constructed the example, but in reality, that value of 24.43 meters can be determined using procedures learned in nitrox courses. Obviously, from a decompression standpoint, everything is exactly the same as if we were descending to 24 meters instead of 30, which, needless to say, shortens the decompression or, if you prefer, lengthens the no-decompression period. Cool? Too much, I'd say.
Oxygen partial pressure and toxicity
The flipside of this coin is oxygen toxicity for the central nervous system (CNS). To establish a clear idea, let's say as a first approximation that 1.5 bar should not be exceeded, otherwise the associated symptoms will develop, which are very difficult to manage (at least while diving). Now consider an EAN100, a specific nitrox made from pure oxygen. The calculation is straightforward: breathing pure oxygen, the partial pressure (and also total pressure in this case) of 1.5 bar occurs at just 5 meters. Obviously, this example is also easy to follow. In reality, this 1.5 bar limit is reached at a depth dependent on the oxygen content. Roughly speaking, EAN21 (i.e., air) becomes toxic at about 60 meters, while EAN100, as we've seen, only at 5 meters. The oxygen-related toxicity limits of other "EAN" divers will fall somewhere in between these two values. But this is just the beginning. Oxygen toxicity issues are complex, involve exposure time, and I won't even dream of mentioning them.
It's very important to know that CNS poisoning is a very, very serious matter, and its management during diving shouldn't even be considered unless you're diving at stratospheric levels. For us, the only thing to do is to make sure it doesn't happen. Again, I don't mean to scare anyone. Thousands of nitrox dives are done, and accidents are rare . It's all about proper information beforehand, proper training during, and attention afterward. And this brings us to the next series of considerations.
Training and buoyancy control with nitrox
How to learn?
Unfortunately, with mixed gases, we're moving away from the established "three-level" framework. While experience has shown that the traditional three certification levels are adequate for a complete training program for a recreational diver, with mixed gases, we're entering a world that, let's put it euphemistically, everyone can see differently. From a technical standpoint, with a well-designed nitrox course, you can cover the entire course in a long, moderately intensive weekend, if you can handle it. Splitting it into different levels might be an option (perhaps due to a lack of time constraints for the learner), but I can't help but think it could also be very cost-effective, from the perspective of the opposite of the student. I don't know, you decide; a lot depends on the instructor and the agency.
The two possible scenarios
What I do want to emphasize, however, is the practical aspect of nitrox diving; I cannot stress enough that you should never expose yourself to a CNS poisoning. In short, if you go on a wall dive with an EAN36 and there's a 100-meter depth below you (but was that really necessary?), the possible scenarios fall somewhere between the following two:
- 1) Your instructor was a conscientious person, he explained everything to you thoroughly. theory, and has insisted ad nauseam on taking care of your trim and the equipment details that ensure it. You, in turn, are a conscientious person who has studied and assimilated the theory thoroughly, you have understood the seriousness of potential problems and – through constant, targeted training – you now have full and complete control of your trim.
- 2) The second scenario is obviously the opposite, and I don't even think it's necessary to explicitly outline it. In any case, it's always good to have a seabed beneath us that ensures, even if we were to sink halfway down because our dive buddy punctured our BCD as a joke, the resulting partial pressure of oxygen would never give rise to toxic episodes. Afterward, of course, we'll execute our buddy. With time and attention, we can also relax the strictness of this rule, but always with great respect for oxygen, by far the most complex gas a diver is forced to breathe. Good buoyancy will also be useful for trimix, but for a different problem, as we'll soon see.
Trimix: helium, nitrogen and oxygen
As you may know, helium is not a narcotic, unlike nitrogen, and therefore must be used to overcome the narcotic limit of nitrogen in the air, a limit which the recreational world very conveniently finds at about 40 meters.
The first experiments with helium saw it as a complete replacement for nitrogen in the mixture; this is still the practice in the professional field, with the exception of stratospheric dives at several hundred meters, where such strange things happen to the cell membranes of nerve cells that a small percentage of inert narcotic calibrated to the nearest tenth of a percent must be introduced (to limit the so-called HPNS).
In sports, nothing of the sort is done; on the contrary, efforts are made to exploit the advantages of each inert gas while limiting its disadvantages by appropriately determining the percentages. In any case, recreational diving in heliox (helium-oxygen) is extremely rare; as we mentioned, trimix (helium-nitrogen-oxygen) is preferred.
Cost and characteristics of helium
Helium is very expensive, even for those hardcore enthusiasts for whom diving is everything. It also dissipates body heat more easily than nitrogen, although recent experience among serious technical divers has shown that this issue is far less significant than it is in the professional field. Another problem with helium is its much faster exchange rate than nitrogen, about 2.65 times faster.
Exchange speed and ascent control
Simply put, if we accept the classic 24 hours for nitrogen saturation (in reality, it would take much longer), with helium it takes just over nine. Obviously, desaturation is just as rapid. One consequence of this is that a ballooning, theoretically "forgiven" by air, would not be so if breathing helium as an inert gas. This argument—never convincingly proven—has been used to justify certain schools of thought regarding deep-sea diving. If this is your first encounter with gas mixtures, well, know that in the past there have been furious arguments that dragged on for months and years, with mutual accusations, even vicious manipulations, and so on.
Today, all of this has been tempered over time; we can enjoy a perspective to which many missing pieces have been added, and as such, it's much more balanced and realistic. But this mention also allows me to anticipate that if entry into the world of gas mixes begins with nitrox and a good understanding of oxygen—basically, technical matters—the main obstacle to the rest of the journey is orienting yourself correctly and maintaining the right mental perspective. To be clear, if you believe in the wrong things, you'll also put them into practice; and if you put something into practice, it's because you believe in it, but unfortunately, this doesn't allow you to decide on its objective correctness. Let's return to the technical aspect, which is undoubtedly better.
How to make a trimix blend
With helium involved, things change very little for those familiar with nitrox; you simply "build" the mix starting with the amount of oxygen that isn't toxic for the planned dive, you "add" nitrogen taking into account the narcotic effect you decide to accept, and finally, the rest of the mix will be that absolutely inert helium from the standpoint of the effects important to the recreational diver. I think it's pointless to provide calculation examples because all of this, which on the surface appears to be the cornerstone of trimix diving (along with deco procedures, the subject of considerable study and discussion in the advanced and tech diving community), actually represents almost nothing in the overall picture.
If, in fact, it's decided that a helium mix is needed, it means the objective is serious, whether because it's deep, or because of the bottom time involved in the need for absolute mental clarity (e.g., archeosub), or both, or something else entirely. In any case, it has little or nothing to do with the 40-meter safety limit. This would open up an extremely broad and complex discussion, one on which there are profound differences of opinion among those in the field.
Technique, problem management and team organization
I can only tell you that the technical aspects (mixing, etc.) and decompression may seem like a lot, but they're actually a small part of the whole. Almost everything essential concerns diving technique, problem management, team organization, and so on. However, these days, the so-called tech-rec sector is gaining ground, that is, the type of diving that's no longer strictly recreational and isn't yet the advanced, heavily decompression-delayed type.
On the one hand, it's certainly a good thing that agencies are officially taking into consideration what is essentially a tradition, especially in our area; it would be foolish to deny that forty meters of no-decompression diving are often too shallow for divers in our seas, while, on the contrary, decompression dives are commonplace. On the other hand, the very realization that this situation more or less doesn't create major global problems may well justify doubts about the appropriateness of introducing the relevant certifications. Whichever way you look at it, there's no doubt that helium gas is highly desirable beyond 40-45 meters, both for reasons of safety and enjoyment.
Trimix and deep mental clarity
Everyone who's tried trimix diving has appreciated the benefits and enjoyment of a clear mind. I confess I still have a vivid memory of my first trimix dive. Narcosis sets in earlier than we suspect (around 30 meters), but with no way to compare different situations, we "grow up" with a kind of blinders on that we no longer even notice, having always had them; when they're removed, it's truly a new and wonderful experience.
At this point, I imagine you're wondering what to do if this interests you. To answer, I'll add the final piece to the overall picture: that very favorable property of helium that actually influences—or rather, doesn't influence—diving technique, choices, and so on. Helium is indeed faster at exchanging gases than nitrogen; but it also has less affinity for liquids. In other words, it dissolves and leaves the body faster than nitrogen, but all other things being equal, about half as many helium molecules dissolve as nitrogen.
All things considered, the two factors almost balance each other out, and—even more so given that not all of the nitrogen is replaced with helium in trimix—it follows that the difference is almost identical compared to an air dive. Perhaps a little more deco and careful attention to buoyancy: but the first factor has very little impact except for relatively long bottom times, while the buoyancy issue is no different from that of air dives. Of the two, unlike nitrox, it's necessary to be careful not to become uncontrolled; I repeat, this is essentially because you're outside the decompression limits, not because helium is dangerous.
Training and access to trimix
The consequence of all this is that while I have no hesitation in strongly recommending a complete training program with nitrox, I wouldn't be so sure about trimix. Those familiar with nitrox can learn the additional theoretical knowledge on their own—especially since the Internet has existed—and being part of a group of experienced divers can be quite sufficient for the practical aspects.
The issue of refilling remains. If your group doesn't have its own equipment, and you don't have a trimix certification, a refill center could formally refuse to refill you. However, given our current state of knowledge, I can't see any strictly physiological or "technical" reasons for doing so. Here too, what matters most is the diving technique, and this, especially if you don't stray too far from the safety curve, has very little to do with the gas used.
Synthesis
Here it is. With nitrox, be careful not to sink; with trimix, be careful not to become unsteady . I hope you'll take this summary essentially as a useful joke to help you establish a general mood; on the other hand, if you examine it carefully, it 'contains,' albeit at a latent level, many of the themes of this type of dive. Have fun in the water.
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