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Nitrox : What, Why, How, Is It Worth It?

(By Keith Lawrence : Originally written for the club magazine)

What is it?

Put simply, Nitrox is a ‘modified’ form of the normal compressed air that has been used for diving for many years. Normal air is roughly 20% Oxygen, 80% Nitrogen – a Nitrox mixture (normally called a mix) alters this percentage. For ordinary diving mixes of 32% O2, 68% N2 through to 36% O2, 64% N2 are common. When talking about Nitrox mixes the normal abbreviation used is something like ‘EAN32’, EAN stands for Enriched Air Nitrox, the number ‘32’ stands for the O2% in the mix.

Why Use It?

Going back to our basic training, Henry’s Law of gas absorption in liquids is the one that causes us problems, as we dive the increased N2 pressure gradient between our blood and the air in our lungs causes N2 to be absorbed. It is this N2 absorption and its release that determine the dive times calculated by our tables and dive computers.

So, if we were to decrease the amount of N2 (by using Nitrox), less N2 would be absorbed, we could dive for longer, decompression would be shorter… Well, yes… BUT what have we replaced that N2 with? – O2! This in itself causes all sorts of other problems – remember that ppO2 limit of 1.4 and how normal air becomes toxic at around 60m?, well EAN36 becomes toxic (according to BSAC) at around 29m!! Even if you don’t exceed a ppO2 of 1.4 the O2 itself builds up in your body and can lead to an Oxygen Toxicity ‘hit’.

So why bother? The main reason is safety, there are two ways of using Nitrox –

Use Nitrox but dive ‘assuming’ that it is air and calculate your no-stop and decompression requirements as though you were using air (but remember the ppO₂ limits!). The use of Nitrox adds quite a safety factor when using normal air tables.
Use Nitrox with Nitrox tables and computers. Assuming that a no-stop dive is safer than a decompression dive the use of Nitrox will give far shorter (if any) decompression requirements than exactly the same dive done using normal air.

There is a third reason often given – you feel a damn sight better and a lot less tired after a Nitrox dive than an air dive. I didn’t really believe this until I experienced it, I’m still not convinced that it is all down to Nitrox, there could be psychological or other reasons – but it’s true for me at least! I can drive home after a good weekends diving without dozing off at Fleet services…

How Do I Go About Using Nitrox?

You need to do a course, BSAC do a couple but many go for one of the independent agencies like IANTD (International Association of Nitrox and technical Divers). There are some differences between the courses, do an advanced course rather than the basic ones on offer - the advanced courses give you more flexibility with mix choices and include a practical element.

But then there is the equipment –

Your cylinders must be O₂ cleaned (about Ł15 a time) every year, once cleaned they can only be filled from a double filtered Nitrox compatible compressor.
Your air computer needs updating to a Nitrox model if you don’t want to use the tables.
If using mixes greater than 40% O₂ (normally only used as a decompression gas) your regulators need to be O₂ cleaned.

If buying new equipment and you think that you may want to use Nitrox in the future, then think ahead. Make sure your new cylinder has a Nitrox compatible valve, spend an extra Ł30 or so on your new computer and buy the Nitrox model. The cost of new Nitrox cylinders and a new computer is what puts most people off.

Is It Worth It?

This is purely a personal choice, there are some other hassles to think about –

Your fills are more expensive, around Ł5 at the coast, up to Ł7.50 inland.
Not everywhere sells it (yet…), and you don’t want to contaminate your nice O₂ clean cylinders by using a normal compressor.
Having a choice of mixes could mean that you have the wrong mix for the dive being planned.

For me I personally think it is worth it, as I planned my equipment purchases with Nitrox in mind the ‘extra’ cost of using Nitrox was about Ł200, most of that was the course fee. When using air I run out of no-stop time long before I reach my air reserve, using Nitrox has allowed me to enjoy some nice long dives that would have involved long decompression stops had I been using air. I have completely off-gassed from the Saturday dives when it comes to the Sunday dives, I feel a lot better after the dives than I did when using air. An appropriate Nitrox mix is now my normal diving gas.

As a club we are quite unusual in that not many of us actively use Nitrox, but the number of club members qualified to use it is increasing all the time and several people are talking about using it next season.

Finally I’ll give some figures from my dive computers on actual dives done with Nitrox and air. During the Weymouth weekend I was diving with Steve Shepard (thanks Steve!), as he was using air I had an air computer and a Nitrox computer on the same dives, the following are the actual figures from the two computers –

Dive	Max Depth	Time to 6m	Minimum No-Stop
Dive	Max Depth	Time to 6m	Air	O₂% Nitrox
Aelonian Sky	31m	32	3	32% 19
Prevrel Ledges	20m	41	8	40% 53
Landral	35m	25	1@3m	30% 6
Portland Bill	20m	32	32	40% >99

When compared to normal air the use of Nitrox on a dive can make a considerable difference to the no-stop time, talk to one of the Nitrox divers in the club if you want to know more.