IT'S STRANGE TO THINK THAT, JUST OVER A DECADE AGO, technical diving was the new big thing. Not the deep wreck exploration that gets the press these days, just simple things like using nitrox on an average 24m dive to double no-stop times.
Nitrox was the gas of the future, the popular topic at technical diving conferences, the advance guard of technical diving of which we could all be part, a tool we could employ in our everyday dives.
Unless, of course you were a member of BSAC or PADI, in which case it was the devils work, didnt actually exist, and, if it did exist, would be guaranteed to kill you as soon as you sniffed it.
But we could all rebel, sneak off on a course run by IANTD or TDI, poke the establishment in the eye and do our own nitrox thing.
Eventually the establishment realised that it was losing out, invented nitrox for the unwashed masses and introduced courses that made it all legitimate.
Hardly anyone thinks of nitrox as technical any more. With no basic nitrox to talk about, technical diving discussion has shrunk into ever diminishing spirals of extreme technicalities. Nitrox has become just part of the regular diving scene.
So with more than a decade of nitrox diving behind us, what is the state of play for nitrox diving How did we get here, and what does the future hold Dive shops, skippers and divers give their views. But first, lets review the basics.
Nitrox is, like air, made up of nitrogen and oxygen. In air the proportions are roughly 79% to 21%. In nitrox the proportion of oxygen is greater and the proportion of nitrogen consequently lower. The mixture is described by the oxygen percentage, so a nitrox mix with 60% nitrogen and 40% oxygen is described as nitrox 40, and air can just as easily be described as nitrox 21.
I am sitting here doing a dry dive on nitrox 21 while writing this article. The chances are that you are also breathing nitrox 21 while reading it.

One way to make nitrox is to begin with air and enrich it with oxygen. This gives us oxygen enriched air or enriched air nitrox, which leads to the term EAN40 as an alternative to nitrox 40.
Starting with air and adding oxygen is not the easiest of methods. Its a lot easier to get air to high pressures than it is oxygen, so we end up either mixing the nitrox before we compress it into a diving cylinder (continuous blending), or starting with a cylinder part full of oxygen and diluting it with air (partial-pressure blending).
The second of these methods is the more common. The process involves decanting oxygen into an empty cylinder to a calculated pressure, then either decanting or compressing air on top of the oxygen to bring the cylinder up to full.
Because the oxygen proportion comes from the pressure to which the oxygen is decanted, added to the partial pressure of the oxygen in the air that is then added, this is called partial-pressure blending (see panel overleaf for more detail).

Once you have the maths sorted out its all very simple... except that it isnt. Stuff burns in oxygen. If it burns violently enough it can explode, and the greater the proportion of oxygen in a mix, the more likely it is to burn or explode.
The magic number is 40%. Somewhere above 40% oxygen content, some familiar materials can ignitespontaneously. Below 40% you need a spark (lots of things burn in straight 21% air if you set fire to them).
So what stuff is dangerous We can break it down into two groups: stuff thats supposed to be there, and of which diving equipment is made; and stuff thats not supposed to be there, such as dirt, grease and other contamination.
Diving equipment made of materials that wont burn easily in oxygen is referred to as oxygen-compatible, and when equipment is clear of contamination it is referred to as oxygen-clean. Put the two together and the equipment is in oxygen service. Any diving equipment that comes into contact with oxygen above 40% has to be in oxygen service.
The main reason why partial-pressure blending is not as straightforward as it may at first seem is that, even for a final mix of less than 40%, we have to add 100% oxygen first. So it is safe to fill nitrox by partial-pressure blending only if the cylinder and filling system is fully oxygen serviced.
Which brings us back to the continuous blending method of filling nitrox. Because you have mixed your nitrox of less than 40% before it goes into the diving cylinder, the cylinder doesnt need to be in oxygen service. Nitrox is mixed at low pressure, then compressed into the diving cylinder.
A third system which is now widely used throughout the world, but which has so far largely passed us by in the UK, is called de-nitrogenised air (DNAx). You create low-pressure nitrox using a membrane filter system that lets more oxygen through than it does nitrogen. The gas remaining is oxygen-enriched and ready to be compressed.
So why doesnt everybody do it Because the membrane filter systems are expensive, and you still need supplies of pure oxygen to make richer mixes by partial-pressure blending.

While reviewing the basics, it's worth stomping on a myth that seems to come up all too often. Nitrox does not enable divers to go deeper.
While nitrogen narcosis will fuzz the brain, it is oxygen that puts the real limit on how deep a diver can go. High partial pressures of oxygen are toxic. So with enriched air nitrox, oxygen toxicity will actually reduce the maximum depth at which it can be used.
Depending on conditions, the limit set on the partial pressure of oxygen ranges from 1.6 to 1.4 bar, and most rebreathers set a limit of 1.3 bar.
The big advantages of nitrox are all based on lower levels of nitrogen. Less nitrogen means less narcosis and less decompression.
While decompression tables are generally calibrated in metres depth, what they are really showing is the decompression requirements for a partial pressure of nitrogen.
For nitrox, tables can be recalculated using equivalent air depth, the depth at which air would have the same partial pressure of nitrogen as the nitrox mix, using a simple formula.

So who is using nitrox and what are they doing with it I asked a selection of dive centres and divers.
A good starting point was the Old Harbour Dive Centre in Weymouth, at the heart of south of England wreck-diving, where Janine Gould told me: About 40% of our fills are nitrox. Use has been growing at about 20% per year, though I think it has now levelled out. Five years ago we used an average of 10 cylinders of oxygen per week during the season. In the past two years we have used 20 to 30 cylinders per week. The split between main gas and deco gas is 50/50.
Newly trained divers migrate towards nitrox easier than the old salts. Unfortunately, it seems that the new divers dont stay with UK diving for so long. Its a little bit of been there, got the T-shirt, moved on to new stuff, though not entirely, of course.
Nitrox will continue to grow but I dont see it reaching more than 50%.
Zoe Cuthbertson of Scapa Flow Charters notes a similar growth in nitrox sales, with more than 50% of fills being nitrox, though she did point out: We actually had a bit of a drop in nitrox sales in 2003, with some groups entirely air. But there is also a steady increase in rebreather divers, who use less oxygen.
Most groups use nitrox these days, Mike Rowley, skipper of mv Maureen in Dartmouth, told me. The exceptions are the older divers who prefer not to be bothered. Usage ranges from everyone in a group to just the more experienced group leaders. However, we see a steady increase in people using it each year.
Probably more than 50% of our customers used it last year. The days of it being used as safe air seem to have gone.
Most divers use nitrox to take maximum advantage of bottom time and reduced decompression.
To get the perspective away from the coast, I dropped in at a couple of my local shops in Bristol. At Mikes, manager Jo New told me: Between 5 and 10% of our fills are nitrox. Most are mixes up to 36% for twin-sets. Richer fills tend to be 3 litre ponies and a few 7 litres for decompression.
I then spoke to Kate Painter at Bristol Scuba Centre, which is, as far as I am aware, the only shop in the UK to use DNAx. Weve had it for almost a year now, said Kate. She didnt have figures on proportions of fills, but commented: A lot of divers equate nitrox fills with oxygen-clean cylinders, but that doesnt apply to membrane systems.
We need to get the message across that we can fill any cylinder up to 40% as long as it is properly labelled. It doesnt need to be oxygen-clean. The overhead of doing a nitrox course is much lower for students, as I can run it with all their existing kit. Fills of 32% are also£1 cheaper with the membrane system.
One of the constraints on increased nitrox sales is that customers want fills instantly, according to Kate. This can be inconvenient, as we share a compressor between the membrane system and regular air fills. To do a nitrox fill we have to blow it through to clear out any air, then blow it through again afterwards to go back to air.
Well soon have a dedicated compressor for nitrox fills, and then everything will be quicker.
Membrane systems are much more popular overseas. All our staff have been diving on nitrox for the past three years, said Mathias Bret, manager of Red Sea centre Dive Point Hurghada.
It involved a lot of work filling by partial pressure, though last year we installed a large membrane station that makes it a lot easier. We still use partial pressure for fills over 40% and for trimix.
Its noticeable that a couple of years ago it was mainly experienced divers using nitrox. Today we are introducing nitrox much earlier, even during basic open-water training, and demand is increasing. Nitrox use has increased significantly with lower costs and especially with free nitrox for those certified through Nitrox and Rebreather College (NRC).
Our rental department now has many more nitrox than air computers. Nitrox divers are much more likely to rent a computer than air divers.

For a divers perspective, I asked some typical UK divers rather than dedicated tekkies. Ben Knights did a TDI Advanced Nitrox course last season. I had been using a twin-set to do decompression diving on air, so doing a nitrox course was an obvious progression - just a way to expand my diving skills, said Ben.
I didnt have any particular project in mind, but a few weeks after the course a spare place on a club trip to Scapa Flow came up and the nitrox really came in useful. I got a nitrox computer and made the most of it.
I really noticed how much less tired I was after diving, then I managed the 12-hour drive home in one session without falling asleep. I havent had the opportunity to use it since. I think Ill get some clean cylinders for this season.
Someone who has been using nitrox a bit longer is Phil Hogarth. I didnt use it much last year, but did the year before. It was just a reflection on where I went for UK diving holidays and whether they had nitrox or not.
I like to use 80% to decompress on deeper dives, just for safety rather than to accelerate the decompression, and I feel a lot better for it. The sort of dive nitrox is really good for is a known target between 30 and 35m, like the Kyarra. I can go to Swanage for a day trip, get twice as much time under water, and drive home feeling great.
For Sarah De La Rue, use of nitrox depends on the group with which she is diving. Nitrox hasnt influenced my branch diving at all, but I use it all the time when diving outside of the club, said Sarah
On day trips the increased bottom time more than compensates for the cost of a fill. On a week-long trip where every dive involved a minimum of 20 minutes deco, the safety margin added by decompressing on a richer mix did wonders for my peace of mind. I am gearing myself up for mainly nitrox diving and plan to do more advanced certificates, as well as learning to blend.
I met Simon Powell on a boat in Brighton. I use nitrox primarily for boat diving in the 25 to 40m range where deco obligations can build up, he told me. I started off using it in a twin-set based on the planned depth. Now I use 80% for accelerated deco more often than I use nitrox in my twin-set.
By accelerating the deco I can breathe air on the bottom and still incur the same total deco penalty. I accept the risk involved with taking on more deco and off-gassing faster. Overall the cost of gas is about the same, but I have more flexibility.
Not everyone who is nitrox-qualified uses it that often. I have only used nitrox a couple of times, shortly after I qualified to use it two years ago, said Gareth Seaborne. I havent done enough diving recently to justify the investment in oxygen-clean gear, but as I emerge from the long tunnel of parenthood, particularly with an eldest son who now dives, I can see me using it more.
He regrets a missed opportunity in the Maldives: I forgot to take my qualification card and wished I had, cos the nitrox divers got a lot longer.
Their cylinders were bigger and filled to a higher pressure, too!

The main technical change anticipated is the introduction of the EN 144-3 standard for nitrox cylinder valves. This will be phased in between 1995 and 1998.
EN 144-3 introduces a new valve fitting, a bit like a current DIN fitting but wider and with a metric thread. This means that existing DIN regulators will need an adapter, and A-clamps may not fit at all. The thinking behind this is that it will prevent rich mixtures being filled into cylinders that are not oxygen-clean. It will become mandatory for any mix other than air, but clean 21% air fills may be used with the new standard.
I fail to understand the logic behind this. The easy solution for divers will be to convert everything to EN 144-3, whichever gas they want to put into their cylinders, air or nitrox. There is nothing except human procedures to ensure that a cylinder with the new valve standard remains oxygen-clean - which is exactly how the current system works.
Many divers will no doubt choose to use adapters with existing regulators rather than buy new first stages, so EN 144-3 cylinder valves could in fact introduce an additional failure point in the water.
The only people to benefit will be manufacturers and distributors of the new valves, who are onto a nice little earner. Even then, I suspect that many of them would prefer not to have the hassle.
What it amounts to is that handling oxygen-rich gas will always involve human procedures and that no engineering solution can get round that. In which case, why change
Had I missed something I wondered. I asked the dive shops, divers and dive skippers for their reactions to EN 144-3 and, just to make sure, checked with Mike Harwood at the Health & Safety Executive.
As a member of the BSI sub-committee for underwater breathing apparatus, it seems that he had written a letter back when the new standard was first proposed, stating that the HSE did not support the need for it. The UK didnt want the standard but was out-voted in Europe. After that it was a rearguard action to make sure that at least the engineering of the standard was workable.
Reactions from those I asked ranged from complete ignorance of the measure to utter rejection. I failed to find anyone who thought it was a good idea.
Divers are confused by EN 144-3 and those who know more find it incredible that the bureaucrats can come up with such a ridiculous proposition, said Kate Painter. It wont increase safety; no one wants it.
Or, as Mike Rowley succinctly puts it: As a professional engineer, charter-boat skipper and diver, I think its bollocks!

Nitrox can bring greater peace of mind on decompression stops.
Using nitrox as a decompression gas
UK-style nitrox filling - this compressor delivery panel and partial-pressure blending panel is at Mikes in Bristol
Comparison of air and nitrox dive times for no-stop dives and 10 minutes decompression, nitrox mix selected for 1.4 bar, to a maximum of nitrox 40 (note that in practice, for nitrox 40 at 1.4 bar a 25m depth limit would apply)
Proportion of nitrox use or training. BSAC training is gauged relative to other courses such as Practical Rescue Management and O2 Administration
De-nitrogenised air has revolutionised nitrox supplies almost everywhere in the world apart from Europe, writes John Bantin. Instead of adding air to pure oxygen to make nitrox, with all the fire and explosion hazards that involves, a patented membrane system is used to draw amounts of nitrogen out of the air that is being pumped.
The system is expensive to install and requires double the amount of motive power to compress it, but these economic obstacles have not stopped it being adopted by many diving operators.
Once properly up and running, worked in conjunction with a gas-banking system, nitrox percentages of up to 40% can be drawn off just by setting a dial. Not only that, but the mix is consistent to within a tenth of 1% every time.
We cannot say whether it was the popularity of nitrox with divers or the fact that it has become possible to supply nitrox in this easy way that has given rise to the unstoppable march of DNAx, as it is called.
There is no commercial advantage for any operator in supplying nitrox unless he either has a competitor already doing it, or wants to keep ahead of the game, but that is what is happening. One operator installs a nitrox supply system, so his competitors have to follow suit.
So why is nitrox supplied by DNAx so popular in the rest of the world The great majority of leisure-divers worldwide like to dive shallow and often, unlike their European counterparts. They rarely go deeper than 25m, so the increased safety or increased no-stop times allowed with nitrox become very real advantages, especially when doing up to six dives a day.
The maximum operating depth of commonly supplied mixes such as nitrox 32 or nitrox 36 are not really relevant to these divers, as they rarely approach the limits.
There is no reason not to use nitrox, so they now tend to expect nitrox supplies at dive centres, and especially on liveaboard dive boats.
We can foresee a time when leisure divers in this category will learn to dive with nitrox and never really expect to breathe ordinary air while under water.
DNAx online - such efficient membrane nitrox-filling systems are found all over the world, though barely in Britain