For most divers, Nitrox diving is the first taste of diving with exotic air and opens up a world of longer, safer dives
Nitrox is the common name used for breathing gasses with a higher percentage of oxygen than in normal atmospheric air.
Actually Nitrox is somewhat of a misnomer as it’s an abbreviation of Nitrogen and Oxygen, which are the main components of both atmospheric air and Nitrox. The proper term for the gas is Enriched Air Nitrox (EANx), pointing to the extra oxygen that the mix is enriched with.
The good of Diving Nitrox
The reason for diving with Nitrox is to use the added oxygen to push out some of the nitrogen in the atmospheric air. By lowering the amount of nitrogen in the mix, and through that, the amount of nitrogen our tissues absorb during the dive, the risk of decompression sickness is decreased. This allows for longer dive times spent at greater depths.
For repetitive dives, the nitrogen load carried from one dive to the next is lower than with atmospheric air, again decreasing the risk of decompression illness.
Many divers also report feeling more alert, and less tired after diving with Nitrox.
The bad of diving Nitrox
However, there is a downside. Oxygen is actually toxic for the human body if breathed under pressure.
How much pressure depends on the oxygen content of the gas mix breathed.
100% of oxygen becomes toxic at only 20 feet or so. The less oxygen in a mix (the more “lean” it is) the deeper you can go before it becomes toxic.
As the first symptoms of oxygen toxicity are violent convulsions, followed by powerful inhalations and loss of consciousness, it can be fatal during a dive.
What is the Enriched Air Nitrox Partial Pressure
Oxygen toxicity happens when the oxygen reaches a critical level of what is known as partial pressure.
Partial pressure is the pressure of any given gas in a mix and the recommended partial pressure for oxygen is 1.4, with 1.6 used as a margin of safety.
As Nitrox comes in varying mixes, the partial pressure of 1.4 is reached at varying depths, depending on the mix.
Therefore, one of the things a Nitrox course teaches you is to measure the mix in your tank and calculate the maximum safe depth it can be used on.
The Different Nitrox mixes
The most common mixes used are EANx32 and EANx36, with 32 % and 36 % oxygen, respectively, compared to 21 % oxygen in atmospheric air.
Other mixes are also used but a mix richer than 40 % is quite rare in recreational diving. For a dive shop to supply Nitrox to divers, they need to clearly specify what mixes their tanks have.
Maximum Depths with Nitrox
Needless to say, once you’ve measured the mix in your tank, which is done with an O2 meter and you have calculated the maximum safe depth that mix can be used at, it is imperative that you stay above that depth.
Keeping a close watch on your depth gauge is critical, especially in situations where the maximum possible depth of the dive exceeds your mix’s maximum depth such as wall dives.
Most modern dive computers allow you to set a depth alarm, where it will give you an audible warning when a certain depth is reached. This is highly recommended if you dive with Nitrox.
Diving Nitrox with your Dive Computer
Some dive computers have a Nitrox setting built into them. These come in two versions, either where the dive computer has a selection of preset mixes that you can choose from. Or where you enter the mix yourself and the computer subsequently determines the maximum depth and dive-time based on that. The latter is typically found in more advanced computers.
If your computer has presets and your mix doesn’t match any of them exactly. The general advice is to always err on the side of caution and round up, not down, especially for deeper dives.
Say your computer has a preset for Nitrox mixes of 32 %, 34 %, and 36 %, yet the mix in your tank reads as 33 % when you measure it on the O2 meter. In that case, set your dive computer to the preset for 34 %, which will reduce your depth compared to the mix you actually have, allowing for a margin of safety.
If, however, your dive is a particularly long one, where you’ll be coming up on the maximum dive time of your mix, and with little or no risk of exceeding the depth limit (say you’re diving in an area where the bottom is quite a bit shallower than your maximum depth). It’s safer to set the preset to 32 %, which will reduce your maximum bottom time, again, giving you a margin of safety.
Always know what you’re doing
Nitrox diving is for many the first taste of non-atmospheric gasses, and with that, an early intro to technical diving.
For this reason, it should be taken seriously and no diver should attempt diving with Nitrox without having taken a Nitrox course with an accredited dive center.
With this certification, though, diving can become safer and more enjoyable.
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