The Dive Otter

Nitrox / EANx

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Definition: Nitrox refers to any gas mixture composed of nitrogen and oxygen. This includes atmospheric air, which is approximately 78% nitrogen, 21% oxygen, and 1% other gases. The most common use of nitrox mixtures is when it contains oxygen in higher proportions than atmospheric air because the reduced partial pressure of nitrogen is advantageous in reducing nitrogen uptake in the body's tissues, thereby extending the practicable underwater dive time by reducing the risk of decompression sickness.

This definition seems fairly straight forward, however there are MASSIVE missunderstandings related to the use of Nitrox. Primarily because of the flippant use of terminology that confuses people. Some of them are:

Nitrox allows you to dive deeper.INCORRECT - It's actually MORE dangerous to dive "deeper" on Nitrox than Air. (Oxygen Toxicity)
Nitrox allows you to dive longer.INCORRECT - The amount of time you have before you run out of gas is NOT determined by the blend of the gas you use. 80cft of Nitrox is still 80cft of breathable gas. Volume doesn't change. What is actually meant by this statement is that you will have more time before you reach your Nitrox calculated No Decompression Limit (NDL) than if you are using "Air" calculated NDL. However, most recreational divers will run out of gas before they hit their NDL on "air" anyway. won't get a longer dive (generally).
Above [certain depth] there is no value in diving Nitrox. INCORRECT - "Value" is often assumed to be only a cost analysis instead of looking at the reduced risk. Above the related MOD (Maximum Operating Depth) Nitrox ALWAYS lowers the risk of decompression sickness. Only you can determine if an extra $4 is worth the reduced risk...which may be approaching 0...but is never actually 0.
Nitrox reduces post dive fatigueINCORRECT - There are no scientific studies that show anything other anecdotal evidence of this phenomenon. You may feel better sure...that's known as the placebo effect. But don't discount that feeling...maybe that is a valid benefit to you!
Common "air" is not Nitrox. INCORRECT - A mix of 78% nitrogen and 21% oxygen IS Nitrox by definition. It is just not commonly used in that terminology. You can in fact set your computer to Nitrox mode with an EAN21 blend and it work exactly as expected, cause science.
Nitrox is always EAN32 or EAN36INCORRECT - 32% and/or 36% Oxygen is simply a standard mixture that is commonly "banked" by air fill stations. You can use any particular blend and call it Nitrox. However, you may have to make a special request to have that blend created, which may or may not be offered by that shop.
All Nitrox tanks must be marked with a Nitrox stickerINCORRECT - There are no scuba police. The shop that fills your tank can require whatever they want. It's their shop. However, it can be generally said to be a wise decision to mark tanks that contain a greater than EAN21 ("Air") with a "Nitrox" sticker.
You must measure the mix yourself and mark it on the tankINCORRECT - There are no scuba police. There are certainly "best practices" such as measuring or watching the testing of the mix in the tank you will be diving and then physically writing it on the tank, but not everyone follows those "best practices". It is not uncommon on a boat, for example, for a rented Nitrox tank be tested while you watch and then you mark/sign on a log paper instead of on a piece of tape physically on the tank.

Well if none of the above is true, what is the point of Nitrox?

For ONE simple reason....and only one.

Using a gas blend of greater than 21% oxygen reduces the risk of decompression sickness compared to the same dive on "air" (21% oxygen) because it reduces the amount of Nitrogen uptake.

That's it...there is nothing more. There are a bunch of if/then and technicalities and specific scientific details about why this is true. But at the core...that's the only is a risk reduction technique.

Well heck...dive Nitrox all the time!!

Unfortunately, even though Nitrox reduces the risk of decompression sickness, it has one fatal flaw. Oxygen is toxic to our bodies at higher levels. How much of a higher level is based on the depth in the water at which we breath it. You will learn about partial pressure and Maximum Operating Depth in your Nitrox class. In short though, when diving the "standard" EAN32 Nitrox long as you stay above the MOD of 110ft you will receive more benefit (reduced risk of decompression sickness) than the detriment (increased risk of oxygen toxicity) when compared to "air" (EAN21). However, how much more is up for debate and how much that amount is worth to you in dollars is up to you.

Okay cool....but what do you do?

*Note: I currently have (4) AL80 tanks. 100% of my dives locally have been no greater than 50ft where Nitrox has marginal risk reduction (though not zero) for the additional cost ($4 a tank). As I venture into Lake Michigan with repeated dives below 50ft I am likely to acquire additional tanks of a larger size and of Steel instead of Aluminum. So until I decide what size those will be, I do not want to mix/match Nitrox and Air in my current AL80 tanks for various reasons.