What is it really, why do we use it and what are some of the most important things to remember?
Nitrox is a contraction of Nitrogen and Oxygen, indicating the two main gases of which the mix is composed. As mentioned in a previous article, this isn’t actually what sets the mix apart since normal atmospheric air consists of these same two gasses, too. Nitrox is actually more accurately called enriched air since it is atmospheric air that is further enriched with additional oxygen. But what’s the purpose of using Nitrox?
The reason Nitrox is useful stems from the basic physics of diving and decompression. As we dive, the gas we breathe comes under pressure and we absorb more of it with every dive. As the dive progresses, our tissues absorb more and more nitrogen – an inert gas that our bodies cannot process – so it just gets absorbed and stays in our tissues until we start to ascend. Then, because the pressure of the nitrogen in our tissues becomes higher than the surrounding pressure, it starts to move out of our bodies and get expelled when we breathe. If we ascend too quickly compared to how much nitrogen is in our tissues, however, the gas cannot be released fast enough and bubbles begin to form in our blood and other tissues – a condition that leads to decompression illness.
By adding more oxygen into the same total mix, the result is that there is less room for nitrogen. By having less nitrogen in the mix, there’s less of the gas to be absorbed, meaning we get longer bottom time without the risk of decompression illness. So in truth, the added oxygen in Nitrox is less about getting more oxygen and more about getting less nitrogen.
The overall benefit to divers using Nitrox is all about bottom time and safety. By enriching atmospheric air from 21 percent oxygen to 32 percent, our maximum bottom time at a depth of 20 meters or 60 feet increases from 45 minutes to 75 minutes.
Nitrox is particularly good if you’re looking to do a long and shallow dive, perhaps on a shallow reef. It has additional benefits for repetitive diving, too, such as is experienced on a liveaboard or other dive holiday. This holds particularly true if you maintain the dive time limitations for air, as the smaller percentage of nitrogen in your breathing gas means that you have less risk of accumulated decompression illness from your repeated dives. Some divers also claim that they feel more fresh, alert and less tired when diving on Nitrox, which is supposedly due to the extra shot of oxygen. So far, there isn’t much science to back this up, however. So unless the divers were oxygen-starved to begin with, it is speculated that this is mostly a psychosomatic effect.
But if Nitrox is so great, you may wonder, why don’t we just all dive using Nitrox all the time?
Well, first of all, not all dive centers can supply the mix. In fact, in some areas of the world you cannot get Nitrox at all. More and more places are getting it, however, so this is becoming less of a problem. Making a Nitrox mix also requires a more advanced compressor system as well as training for those using the system. As a result, a tank full of Nitrox is typically slightly more expensive than an equivalent tank full of air.
Far and away, the most important thing to know about Nitrox has to do with the fact that we’ve added more oxygen to the mix.
Every gas has something that is called its partial pressure. This is the pressure of the gas in relation to the amount of that gas in a given mix. You find it by timing the percentage of a gas in a mix with the pressure under which you’re using it. So in normal air (21 percent oxygen) at 20 meters/60 feet (3 atmospheres of pressure), the partial pressure of the oxygen is 0.21 x 3 = 0.63.
Oxygen becomes toxic to humans at a partial pressure somewhere beyond 1.6, and therein lies the problem. As we increase the percentage of oxygen in a mix, the depth at which that oxygen reaches a partial pressure of 1.6 – and thus toxicity – becomes shallower. And as the first symptom of oxygen poisoning is extremely violent and dangerous – convulsions in the abdomen cause the diver to drop his or her regulator, violently exhale and then equally violently inhale water, usually followed by unconsciousness – the importance of using Nitrox with extreme care is obvious. The result is that the maximum recommended depth when using a mix of 36 percent Nitrox is 34 meters compared to 40 meters (for recreational divers) using surface air.
When diving on Nitrox, it is important that you are sure of the mix in your tank (you’ll measure it before every dive) and the maximum depth allowed when using that mix. Always keep a close eye on your depth gauge during the dive.
For this exact reason, Nitrox diving requires a specialty certification within most organizations.
Once you’ve taken this course and are successfully able to apply what you’ve been taught, Nitrox can greatly enhance the enjoyment and safety of your overall diving experience.