For a number of years commercial and military divers have used Oxygen Enriched Air (Nitrox, or EANx) in many of their activities. Nitrox has only recently been accepted for recreational diving use. Along with this acceptance comes controversy. Many divers claim that Nitrox adds safety, and others consider it dangerous. Well, both can be correct, but it is highly dependent on diver behavior. Lets explore the issues a little. To fully understand Nitrox and its use, you will want to take a Nitrox course or read more about it. I'll try to hit the high points and explain its potential benefits.
First, what is Nitrox? Air is about 21% Oxygen and 79% Nitrogen. Nitrox is Oxygen enriched air. Common mixtures are 32% O2 / 68% N2 (Nitrox I, EANx 32), and 36% O2 / 64% N2 (Nitrox II, EANx 2). Some resources claim that air is Nitrox to demonstrate its similarity to air. This is not really accurate. Both are mixtures of primarily oxygen and nitrogen, but air is the specific mix that occurs naturally in our atmosphere. You can fill a tank from any air compressor on the planet and your dive tables will work for your breathing gas. Nitrox requires special measurement of the oxygen content in your tank to safely plan the dive. And, if you mention Nitrox in a dive shop they will know that you are referring to oxygen enriched air, and you will likely be required to have special certification to buy it.
Why is it used? Because Nitrox has less nitrogen due to the increased oxygen content, the diver does not absorb nitrogen as quickly on Nitrox as a diver breathing air. This can offer two significant benefits. These are:
Less nitrogen absorbed than would have been absorbed using air.
The lesser nitrogen loading allows the diver to make adjustments to the planned profile to handle emergencies WITH LESS RISK simply due to the lesser N2 content at the time of the emergency. That adds to safety in my opinion. Bottom line...early in the dive, emergencies may be easier to handle safely due to lower N2 loading than the diver might have if breathing air. Later in the dive, we rely more heavily on slow ascent rates and safety stops to off-gas N2, especially at high pressure groups. Since breathing Nitrox decreases the N2 content vs. what would have been absorbed breathing air, we can more safely hurry to the surface later in the dive (please, ONLY in an emergency), or possibly lessen the severity of the DCS hit due to lesser N2 at the time of the emergency.
Lesser nitrogen loading keeps you further away from no-deco limits than you would be if diving on air. Consider 2 diving buddies....Diver A is diving on air and Diver N is using Nitrox with 36% O (EANx 36). They plan to dive together at 70 feet for 38 minutes. Unfortunately, they aren't watching their time well, and suddenly realize they have been down for 44 minutes and are low on air (no surprise!). The air diver has a decompression obligation due to their oversight and air is limited. Diver N has absorbed less N2 because of the oxygen that displaced part of the N2 in the mix. He/She is still nine (9) pressure groups away from the no-deco limit. Clearly, Nitrox has provided a margin of safety in this situation.
I like to use Nitrox on days where I'll have multiple dives in a single day, or expect to get near a no-deco limit on any dive. I dive a lot with air breathers so I get an extra margin of safety as we reach the air based limits.
Longer bottom time to reach the no-deco limits.
With a lower rate of N2 absorption, divers can extend their bottom times to the no-deco limits. This can be helpful if a diver has a specific task to perform at depth. Remember the example above? We had an extra nine (9) pressure groups through which we could go to get to the no-deco limits AFTER overshooting the air limit. The no-deco limit at 70 feet for air is 40 minutes, while for Nitrox II (EANx 36) it is 75 minutes. That 25 minutes can be important to a working diver.
Why the controversy? There are three major considerations.
|Most divers dive a very safe profile anyway. If the profile is inherently safe, you really don't add safety by using Nitrox. It is the profile that approaches a no-deco limit on air that can be made safer using Nitrox.|
|Second, many divers like to extend their bottom time by any method available. It really doesn't matter how long it takes to get to a no-deco limit. Once you get there, you are there.|
|There is also a problem with oxygen toxicity if the diver dives too deep with an oxygen content that is too high, or if the accumulated oxygen exposure is too great over an extended period. The same problem exists with air, but nitrogen narcosis limits usually keep us well away from the 216 foot O2 exposure limit with air, and with air, the N2 accumulation concerns usually keep us from too much O2 exposure.|
Is Nitrox really safer? Special training is required, and available to prepare the diver to properly plan and execute their dives. With this training, diver behavior becomes much more significant in determining the safety of a dive. If a diver avoids no-deco limits with either gas, their should be no problem, assuming the diver used his/her training to verify they will avoid oxygen exposure limits.
Do I need special equipment? Normally, if you use Nitrox with less than 40% O2 concentration you should not need a special regulator. But, you will want to check with experts regarding any particular product. Many new regulators are built to be either Nitrox or air compatible without internal modification.
You will always need your tank converted to be oxygen compatible and have a Nitrox band added to it in the USA. Many Nitrox blenders put pure O2 in the tank first, then add highly filtered air to achieve the target mixture. This requires special valve components and tank O-ring. You can always use air in a Nitrox tank, but ONLY if it is filtered to Nitrox blending standards. Otherwise, you have to have the tank re-cleaned before the next Nitrox fill.
How can I learn more? I have a few links on my Technical Diving page that address Nitrox diving. Your dive shop can probably provide books on the subject.
|These are my opinions and are not those of any particular agency.|
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Randall C. Allen
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