Yesterday night I finally completed my Recreational Nitrox class. This allows me to get fills and use Nitrox mixes of up to 40% oxygen when I go diving. I figured since maybe one person that reads my blog is a diver I would explain the basic principles behind this and gas management in general.
When you first get certified as an open water diver, almost everyone learns while on compressed air. This consists of 21% oxygen, 78% nitrogen, and <1% other stuff (argon, CO2, hydrogen, etc.). We are taught to dive within what are called No-Decompression Limits (NDL). This name is in fact misleading, as all dives are decompression dives to some extent. The point of the NDL is that you can ascend safely to the surface without mandatory decompression stops with minimum risk of decompression sickness (DCS), more commonly known as the Bends.
The basic premise behind DCS is a law of physics called Henry's Law. Henry's Law states that as the partial pressure of a gas increases, it's solubility in liquid, in this case blood, increases proportionately. This becomes an issue with nitrogen as you spend time at depth and the amount of nitrogen absorbed into your tissue increases. As you ascend, the pressure decreases again and the nitrogen comes out of solution. When diving within NDL and ascending slowly, less than about 15 feet/min, the size of the bubbles coming out of solution are sufficiently small to usually avoid causing any problems. However, beyond NDL or with too rapid an ascent, larger bubbles can collect in joints, under the skin, or worst case scenario, in the brain, subsequently causing the symptoms of DCS.
Where Enriched Air Nitrox (Nitrox) comes in is increasing these no-decompression limits. With a higher percentage of oxygen in the mix, there is also less nitrogen. The lower nitrogen content of the mix means there is a lower partial pressure at depth, and slower absorption into the tissue. Therefore you can spend more time down with the same effect, or the same amount of time down with less effect from nitrogen than if you were on air. Non-harmful effects of nitrogen include narcosis, which is similar to mild anaesthesia at depth, as well as a nitrogen "hangover" afterwards.
Contrary to what many people think, Nitrox does not allow you to dive deeper. It is actually classified as a mid-level gas, most useful from around 60 feet down to around 110 feet based on what mix you use. Beyond those limits you actually run the risk of oxygen toxicity. This is caused when the partial pressure of oxygen in the mix exceeds around 1.6 atmospheres, or 1.6 times the pressure exerted by air at sea level. Oxygen at high enough pressures is actually extremely dangerous and can even cause convulsions. For this reason, when diving Nitrox, you must be aware of the maximum operating depth of your mix. This is a concern not often covered with air, because the MOD of that is around 190 feet, well beyond the scope of recreational diving. However, when used with care and proper planning, Nitrox can make for a very enjoyable and safe dive.
Just remember, friends don't let friends dive air.
-Z
