Nitrox is a label used to describe a variety of gas mixtures used in SCUBA diving. The standard gas mixture used in recreational diving is plain ol' compressed air. Nitrox differs from compressed air in that the natural balance between nitrogen and oxygen (79% and 21% respectively) has been altered.
Nitrox may have either more or less oxygen than compressed air. Nitrox that has greater than 21% oxygen is referred to as enriched air. When you hear someone refer to nitrox you can be almost certain that they are referring to enriched air. Technically speaking:
Enriched Air is Nitrox. Nitrox is not necessarily Enriched Air
Having made a big to-do about that, I will now use the word nitrox to refer to enriched air as almost everyone else does.
So what? What is it for? The purpose of using nitrox is to increase your "no decompression" time limits (or NDT/NDL) during SCUBA diving. The NDT describes how long you may stay at depth while diving and not be forced to perform decompression stops prior to returning to the surface. Exceeding this limit can lead to severe decompression sickness. Decompression sickness is directly related to absorption of dissolved nitrogen by your body tissues and the speed of your ascent. The lower levels of nitrogen in nitrox causes the body to absorb less nitrogen. The ultimate goal:
More Diving, Less Death
Using nitrox allows for greater bottom time, but there is a catch. The greater concentration of oxygen make the dangers of oxygen central nervous system toxicity appear at much shallower depths than with compressed air. O2 CNS toxicity is some seriously bad mojo and unlike nitrogen narcosis doesn't come on slowly. There is usually little or no warning that your CNS is about seize up.
Generally speaking, nitrox is used by commercial divers who spend a lot of time at relatively shallow depths and recreational divers who want to cram a whole lot of diving into one day. I fall into the second category. On a dive trip where I might make four or five dives, I start out with the deep dives (100ft - 150ft, aka narcville) on compressed air and then switch to nitrox for the remaining, more shallow (less than 100ft) dives.
Miscellany:
- Nitrox requires specially cleaned cylinders, or tanks due to the combustible/corrosive nature of oxygen.
- Nitrox does NOT directly reduce the risk of nitrogen narcosis as some have stated. Information to this effect is largely considered to be purely anecdotal and there are no research data to substantiate this. Nitrox forces people to remain more shallow than they might on compressed air so they may never venture into their nitrogren narcosis zone.
- The nitrox/enriched air certification course which I took from PADI was the most technically interesting diving course I have ever experienced. Cool stuff.