Beginner's guide to nitrox

Back in 1989 I was introduced to nitrox. Being naturally cautious and not prone to taking unnecessary risks I wondered what possible benefit could be derived from trusting another diver to manufacture that most precious of commodities – breathing gas. But even in 1989 the writing was on the wall. I was five-feet four-inches, losing my hair, putting on weight, and had no control over the urges to don a rubber suit and jump into freezing cold water. I was desperate.

“So, will it cure me of these ailments?” I demanded to know of my instructor, Richard Bull.
“Probably....”, he hesitated, “definitely not”. I was crushed. Shortly after this I did my first nitrox dive; almost 200 years after an English physician named Reddoes had used it in medical procedures and 120 years after Henry Fleuss of Siebe Gorman had conducted a dive using it. Talk about new kid on the block. A few nitrox dives later and I had the hang of it too.

As every diver knows, air is made up largely of nitrogen – about 78 per cent. We have evolved on land to cope with a pressure of oxygen that is about 0.21 bars and that's all we need to survive (not including wine, beer, chips and chocolate). If our atmosphere was 100 per cent oxygen we probably wouldn't be here today. Man’s first experiments with lighting fires would have been terminal for the species and our own cells would have oxidised pretty rapidly denying us the opportunity to become pyromaniacs.

You could regard nitrogen as being a diluent that prevents us having too much of a good thing but this changes when you go scuba diving. We have to make controlled ascents to the surface to prevent nitrogen causing decompression illness. Also, decompression stops become unfeasably long unless we accept depth and time limits for each dive.

Sometimes those limits are a restriction on our aspirations. For example, the photographer who wants to film for over an hour at 18m and then do it all over again four hours later without incurring decompression stops. Nitrox can radically change those limits, the whole principle being to reduce the amount of nitrogen in your breathing gas. In practice this is done by increasing the amount of oxygen in the mix, hence the term ‘Enriched Air Nitrox’. A dive to 18m using EAN 36 would allow a bottom time approximately twice the amount of time you can have using air. My Galileo computer will allow me to spend 95 minutes at 18m instead of 48 minutes using air.

There are other advantages too. Nitrox may not cure old age but the one thing I have noticed is that I often end a day’s diving feeling a lot less tired than I normally do diving on air. This holds true for me if:
a) I use nitrox on dives where I assume I am diving on air, therefore I have absorbed less nitrogen than my dive computer thinks I have, or:

b) I do a deeper dive using air, then during the ascent I switch to using a nitrox mixture for my decompression stops.

Nitrox can therefore give you a substantial safety margin when you use it whilst adopting the normal time limits of diving on air and leave you feeling better as well.

However, as with most things there is a downside. Oxygen starts to become significantly toxic when it reaches a pressure of 1.4 bars. If you are breathing air, this occurs at a depth of 60m but if you are breathing EAN 36 this happens at only 30m. The myth surrounding the use of nitrox is that it is a deep diving gas - it isn't! Like decompression illness, oxygen toxicity is avoided by combining dive planning with rigorous dive discipline particularly in respect of depth limits.

Another disadvantage of nitrox is fire hazard. Cylinder filling equipment, your cylinder and regulator must be cleaned (usually annually) in order to be fit for use with higher oxygen content mixtures. Mercifully the risk of fire and explosion is very small and happens rarely but complacency must not creep in because the consequences could be disastrous. The cost of nitrox fills is therefore greater than air, the reasons being the capital and maintenance costs of the equipment involved but also the cost of consumables such as detectors for oxygen analysers. Although nitrox is widely available at most dive shops it's always best to phone ahead to check that they will be able to provide fills when you require it.

To get started with nitrox you don’t need to invest huge sums in equipment. Depending on the gear you already use you may need nothing other than your cylinder serviced for nitrox use. Your local dive shop will be able to advise you. You do need to be trained to use nitrox. A PADI Nitrox Diver course will qualify you to use mixes up to EAN40 and takes only one day. I have now completed several hundred dives using nitrox. As an instructor I use it primarily to give me a decompression safety margin but it does come in very handy for long exploratory dives – especially on liveaboard dive boats to far flung locations where you want to make the absolute most of your time underwater.