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September 12, 2007

Nitrox

Today I am going to take the PADI Nitrox class. Attending the class and passing the exam will allow me to use Nitrox when I am diving. Interestingly, actual dives are not required with the class - it is all academics. You buy a "Nitrox crew pack" that consists of a textbook with questions, a DVD that illustrates what is in the textbook, and three special dive tables for Nitrox.

What is Nitrox? There are various definitions. Nitrox is something that almost no one who isn't a diver has ever heard of. And even if you explain it to a non-diver, the whole theory behind it is such that they're likely just going to shrug their shoulders. Hmmm... Nitrox. Interesting. If that.

So what is it? Well, it's a special air mix where the percentage of Oxygen versus Nitrogen is different from that of the regular air we breathe. Regular air consists of about 21% Oxygen, 78% Nitrogen, and one percent of various trace gases (mostly Argon, but also a tiny bit of Neon, Helium, Krypton, Methane and a good half dozen others). Divers generally consider Nitrogen an "inert" gas because our body does not metabolize it. A better description is that Nitrogen is generally inert. There are, after all, things like Nitrous oxide -- N2O -- which is also known as "laughing gas" because of the mild euphoria and analgesia it caused in dental patients. It's also been used as an aerosol in spray cans, and street racers use it to gain more power from their engines as it delivers more oxygen than regular air, and thus allows more fuel to more burned.

None of this matters to divers who do consider Nitrogen an inert gas. Unlike Oxygen, our body does not use it up. However, Nitrogen does play a very important part in diving anyway. That's because it is absorbed into our systems, and that absorption is pressure-related. This absorption is not an issue under atmospheric pressure. Our bodies are in a state of nitrogen saturation, our tissues absorb so and so much nitrogen and that is that. That all changes when the pressure on our bodies is increased or decreased. Decreasing pressure, as in going from sea level up to the top of a mountain where the air pressure is less is rarely an issue as pressure simply goes from one atmosphere, or roughly 14.7 pounds per square inch (psi), to maybe 10 or 12 psi at considerable altitude. It is very different when we dive. The pressure on our body increases by a full atmosphere for every 33 feet of depth. So if we dive down to 33 feet, it is already twice what we experience at sea level, and at 99 feet four times as much. And since dives don't generally last very long, we go from standard atmospheric pressure to several times that and then back within an hour or so.

What does that mean? Well, several things. When we dive, the air we breathe must counteract the water pressure pushing in on our bodies. So at 33 feet, though breathing feels no different, the pressure of the air supplied by the regulator is actually twice that on the surface. We don't really notice that due to Boyle's law that says as gas pressure increases, its volume decreases. The composition of the air, however, does not change. Even under pressure, it's still roughly 21% oxygen and 79% nitrogen. John Dalton figured that out over 200 years ago. Around the same time, another brainiac, William Henry, found that the amount of gas that will dissolve in a liquid is a function of its partial pressure and how easily the liquid absorbs gas. So what do we make of all that?

Well, if we go down to 33 feet, the compressed air we breathe still consists of 21% oxygen and 79% nitrogen, but its pressure is twice as much as on the surface. That means we now inhale twice as many oxygen molecules and twice as many nitrogen molecules. How does the body react to that? As far as oxygen goes, it simply takes what it always takes and the rest is breathed out by exhaling. Even at sea level, humans do not use all the oxygen we inhale in a breath. In fact, it's only a quarter of it or so (else doing mouth-to-mouth resuscitation wouldn't help at all). Down at our 33 feet, we still use about the same number of oxygen molecules, the rest goes to waste (a phenomenon which, of course, is exploited by rebreathers).

Nitrogen, that's another story. As a mostly inert gas it simply gets absorbed into our body tissues in compliance with Henry's law. So as we go deeper, more nitrogen gets absorbed into our body tissues. Not all absorb at the same rate -- some are "fast" tissues and others are "slow" tissues. The software program that I use with my UWATEC dive computer, for example, shows how the various tissue "groups" in my body absorb or release nitrogen during the various stages of my dives.

Now nitrogen, inert though it is considered, does two things. First, while absorbing more of it into our bodies during a descent doesn't do much, releasing the gas again as we ascend is another matter. If we ascend too quickly, the body "off-gases" the absorbed nitrogen in the form of bubbles. If all goes well, those bubbles are tiny and simply go through the blood stream into our lungs where they are safely exhaled. However, rapid pressure decrease can result in bigger bubbles and they can get stuck in the blood and block passages, or they can lodge in inopportune places like joints, the skin or elsewhere. That can have dire consequences. The dreaded "bends" is one. And the bends can range anywhere from discomfort to death.

The second thing nitrogen does is exhibit a narcotic effect if its partial pressure becomes too great. Partial pressure means the pressure the gas represents of the total pressure in a gas mix. Nitrogen's partial pressure is about 71% of the total pressure of air at sea level. As we dive, nitrogen's pressure stays the same in terms of percent, but at 33 feet that 71% is now twice as much in terms of absolute pressure, so we breathe in twice as many nitrogen molecules with each breath. As we go deeper yet, that partial pressure of nitrogen somehow interacts with our nervous system and causes a narcotic effect called "nitrogen narcosis" or, by more poetic souls, things like "rapture of the deep." While that may have its pleasant aspects, being what really equates to being kind of drunk while diving at, say 120 feet, isn't a very good thing.

So this is where Nitrox comes in. When most divers say "Nitrox," they mean air that has a larger percentage of Oxygen and a smaller percentage of Nitrogen. What does that do? Well, with less nitrogen in the breathing mixture, less will be absorbed into the body. If you dive with Nitrox to a certain depth, it's really like diving with air to a lesser depth. Anyone who took the basic open water diving certification classes is familiar with the dive tables. They are about how long you can stay under without having to make decompression stops so that the absorbed nitrogen can be released safely. With less nitrogen, less is absorbed. And that means you can stay down longer. If you stay down the same amount of time as with air, and at the same depth, and then come up, you have absorbed less nitrogen and need less of a surface interval, if that matters to you.

So it's all good, that Nitrox stuff. Well, yes, mostly. But everything comes at a price. With Nitrox it is not only the higher price of a tank refill (they need to create that different kind of air somehow; it cannot just be run through a standard air compressor), but also what another gas can potentially do to us. And that gas is Oxygen. Yes, life-giving Oxygen can also be toxic. How so?

As we go deeper and the partial pressure of Oxygen increases, it can have its own effects on the body. It can make you experience a bunch of syndromes such as tunnel vision, ear ringing, nausea, euphoria or anxiety, dizziness, and twitching or muscle spasms. Those are all warning signs of impending CNS, or central nervous system toxicity. That generally leads to convulsions and you drown. Very bad.

So we have to watch oxygen as well. The partial pressure of oxygen in air on the surface is 0.21 atmospheres. At 33 feet it's 0.42. And at 66 feet 0.63. It's been determined that the maximum safe partial pressure of oxygen is about 1.4 atmospheres, with brief "contingency" exposures of 1.6 acceptable. With compressed air, we get there well past the recreational diving depth limit of 133 feet, so it is not generally an issue when you dive with compressed air. With Nitrox, it's different. Two common Nitrox mixes contain 32 and 36% oxygen. With a 36% oxygen mix, usually called EANx36, we reach that 1.4 atmosphere barrier at just under 100 feet. You can see the problem here, and you can see why divers who want to use Nitrox and take advantage of the longer dive times it affords need to know about all this and the theory behind it. Sure, dive computers take care of it, but you need to understand how it all fits together.

And there's more. While air is air is air, with Nitrox there can be errors. If you think you're diving with 32% oxygen in your tank but, in fact, it's 42%, things can go real bad quickly if you go deeper than you should. So one of the cardinal rules of Nitrox diving is that the diver MUST test the Nitrox mix personally, with an oxygen tester. There are also mandatory records and labels to provide extra safety and accountability.

But even that is not all. Oxygen can be dangerous. It can ignite in pure form, or even when it is present in high concentration. The crew of Apollo 1 perished in a flash fire when a spark ignited the pure oxygen in their capsule. Some filling methods use pure oxygen which is then diluted until the proper mix is reached. So tanks used for Nitrox must be "oxygen-safe." Depending on the oxygen mix, other parts of the equipment, such as regulators, may also need to be oxygen-safe, though the kind of Nitrox recreational divers use does not require anything other than a dedicated Nitrox tank.

So I will soon find out about Nitrox and how it feels. Carol swears by it and uses it almost exclusively. Like many Nitrox divers, she feels less fatigue after a Nitrox dive, and she generally feels better. Me, I have only done a few dives and have not been subjected to to the rigors of lots of repeat dives, so I guess I'll find out.

Posted by conradb212 at September 12, 2007 11:19 PM

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