In the early 1990s, the use of nitrox was seen as something of a 'black art' and was limited to the technical-diving market. Move on 15 years or so and it's all change - nitrox has become an integral part of entry-level recreational dive training across all the major agencies throughout the world, and has well and truly shrugged off its hard-core technical image.

The benefits of using nitrox are wide-ranging, from providing a greater safety margin on standard recreational dives to offering accelerated decompression for deeper dives. BSAC announced last year that nitrox training was to be included in its ocean diver course. The aim to empower divers at an earlier stage of training, in order to make their diving safer.

As an introduction to the basic uses of nitrox, ocean-diver students complete a theory-only exam about the gas. Once qualified, their certification allows them to use nitrox on air tables or air computers to encourage an ultra-safe diving profile up to their 20m depth limit - nitrox offers a greater safety margin than air, as less nitrogen is absorbed by the body. And when training to become BSAC sports divers, students learn to use the gas as a vehicle for extending bottom times, where their maximum depth limit is increased to 35m.

The numbers of divers - most notably new divers - becoming certified to use nitrox is increasing each year. Dive centres overseas are seeing a particularly dramatic rise in nitrox users; however, it seems that the uptake of the gas in the UK is yet to fully compete with that of air.

Mike Calder, managing director of Mike's dive stores, says that only a small percentage of his customers request nitrox fills. Although more divers are becoming nitrox-qualified, he believes that the overall cost in the UK often puts people off.

'The problem is nitrox cylinders have to be oxygen-cleaned once a year, which is yet another cost,' explains Calder. 'It also puts it out of sync with standard cylinder inspection time of every two and a half years. There is a lot of talk of going back to yearly visual tests for cylinders, which would put it back in sync. However, British diving conditions can also be a problem where you have a specific mix for a dive, which you may be blown out from because of the weather.'

Calder also sees the new EU standard - which is set to become law in August 2008 and requires a specific pillar valve to be fitted to cylinders containing more than 22 per cent oxygen - as a turn-off for typical UK dive use. The new M26 valve thread is similar to the well-known DIN fitting; however, it is a different size and not compatible with most regulator set-ups.

This is echoed in popular dive centres throughout the UK, including Cornwall's Looe Divers, which sees a considerable number of divers coming through its doors for air fills. Nitrox fills only cover about ten per cent of this business.

In contrast, nitrox is rapidly becoming the gas of choice among British divers when overseas and on liveaboards. In the Red Sea, where a 30m depth limit is usually set for visiting recreational divers, nitrox is seeing a huge rise in popularity. Not only are depths up to 30m an ideal environment for nitrox use, many of the centres offer it free to guests as part of a standard dive package for suitably qualified divers.

'About 30 per cent of our divers are using nitrox,' says Miki Clark, dive centre manager of Camel Dive Club, which is based in the Egyptian Red Sea resort of Sharm El Sheikh. 'This is split about 50-50 between people who are already nitrox users, and those who complete the nitrox qualification when they are here. We encourage guests to take the course with a 'free nitrox for life' offer - if they do their course with us, they get free nitrox on all subsequent visits. Otherwise, it normally costs €6 per tank.'

Safety is the main benefit for divers using nitrox for dayboat diving in places such as Sharm. Divers generally do not follow a square profile on the sites, so decompression time extension is not really applicable. However, on wrecks such as the Thistlegorm, where divers are following a square profile, extended bottom times are certainly beneficial.

What is nitrox?

Nitrox is defined as a nitrogen-oxygen mixed gas that contains an oxygen percentage other than 21 per cent. Oxygen Enriched Air or Enriched Air Nitrox are nitrox mixtures with oxygen levels greater than 21 per cent. The most common nitrox mixtures are 32 per cent oxygen and 36 per cent oxygen, which offer maximum operating depths of around 34m for and 29m respectively when working to a maximum partial pressure of oxygen of 1.4 bar.

Nitrox was introduced into recreational diving in the 1970s, when Dr Morgan Wells of the US' National Oceanic and Atmospheric Administration (NOAA) began experimenting with the gas. Dr Wells was quick to recognise the advantages for the type of diving performed by NOAA divers.

The International Association of Nitrox Divers (IAND) formed in 1985 to teach nitrox use to sport divers. In 1992, it became the International Association of Nitrox and Technical Divers (IANTD).

Nitrox benefits

• Longer no-decompression limit compared with air
• Increased safety margin for intensive diving trip
• Reduced decompression penalty as less nitrogen is absorbed
• Reduced surface interval times and longer subsequent dives
• Accelerated decompression gas for deep dives
• Many divers claim that using nitrox reduces feeling of tiredness post-dive, although this is not proven. It is attributed to the reduction in the number of asymptomatic bubbles in the system

Nitrox disadvantages

• Risk of oxygen toxicity if the maximum operating depth is exceeded
• Cylinders must be oxygen-cleaned every 12 months and regulators must be nitrox-compatible
• Risks of handling pure oxygen when filling
• Price - in some areas you will be charged more than air
• Carrying multiple cylinders when extended range diving increases complication

Liveaboards

With divers clocking up as many as five dives a day for six days, nitrox is an obvious gas of choice on liveaboards. These days, most offer nitrox to guests usually for an extra one-off charge. DIVE's Charles Hood, a BSAC advanced instructor and award-winning underwater photographer, regularly runs trips on liveaboards. He says that at least half of the guests on each boat he travels on use nitrox.

'Most now have a 30m rule,' he says. 'Nitrox is either offered free or unlimited for use over a week at a cost of around €50. They will usually pump one mix at a time and give you a 32 per cent mix if everyone is on 30m. If you do dive deeper in the morning, you simply use air for your first dive. It's about extra safety.'

Hood believes nitrox really shows its benefits on wreck-diving trips. 'Nitrox is excellent for wreck dives,' he says. 'It's particularly useful in Scapa Flow, where people are diving square profiles. You get up to 50 per cent more no-decompression time over a week if you get the exact mix right.'

THAT LITTLE BIT EXTRA

Based on BSAC nitrox tables, we look at how much extra time divers can benefit from bu using nitrox (assuming diver starts at 'A' before the first dive):

FIRST DIVE AT 36M
Air: 14 min no-stop time
Nitrox 32%: 19 min no-stop time
EXTENDED NO STOP TIME:
5 MINUTES SURF CODE F

SURFACE INTERVAL
120 min SURF CODE C

SECOND DIVE AT 30M
Air: 16 min (3 min stop at 6m)
Nitrox 36%: 26 min (3 min stop at 6m)
DIVE EXTENDED BY 10 MINUTES

TOTAL EXTENDED DIVE TIME:
15 MINUTES

Advanced uses

Diving media consultant Richard Bull offer his advice on advanced uses of nitrox. 'Using nitrox is not considered a technical activity in itself any more, but what about its use in technical and advanced diving? The obvious role of nitrox in this context is as a decompression gas, but my message would be think about it and be creative. Make it work for you and don't automatically go down the two decompression mixes - EANx 36 and 80 - route. The choice of nitrox mixes for decompression often involves compromise and balancing conflicting objectives.

'I recently ran a diving operation in the Strait of Messina with the objective of filming our presenter at 35m. The water rips through this bottleneck between mainland Italy and Sicily, so clearly the work rate of the divers was going to be high and plenty of bottom gas was needed. It was decided to use twin-sets filled with EANx 28. This choice of bottom gas gave us a little leeway with a partial pressure of oxygen (ppO2) of 1.4 bar at 40m - 5m in excess of our target depth.

'The team would need to get out of the water reasonably quickly because of the strong currents and tides. To this end, the local guys decompressed on EANx 80 or 100 per cent oxygen. This is sound thinking, but was it really ideal for us? We wanted, and indeed needed, an independent bail-out system. The obvious option was to carry a cylinder of EANx 28, our bottom gas. I was reasonably happy with this, but I was conscious of the fact that our team was carrying increasing amounts of equipment - this was significant, given the complicated tasks that our team had to carry out. We never intended to do long decompression stops anyway, so what if we carried something like an EANx 36 as a decompression gas, and could this double as a bail-out gas?

'This could just about be breathed at our target depth; besides, if you have a problem, you're out of there. Obviously the decompression stop times would be longer, but therein lies the choice: shorter stops and more equipment, or longer in the water and less equipment. Whatever you choose, at least you have thought it through and made a decision based on logic rather than blind adherence to convention. In truth, there is a whole bunch of other factors that would affect your decision: gas consumption, bottom times, risk of entanglement, visibility, water temperature and lots more.

'In many ways, I think that I am applying the early principles of technical diving to the choice of nitrox mixes. That is, look at the mission in hand and decide what you need rather than bodge it with the first thing that springs to mind.'

Nitrox myth-busters

FALSE - 'You can go deeper using nitrox'
TRUE - When the maximum partial pressure of oxygen exceeds 1.6 bar, divers are exposed to the risk of central nervous system (CNS) oxygen toxicity
FALSE - 'Nitrox requires specialist equipment'
TRUE - In fact, most modern diving equipment is nitrox-compatible for use with mixes of up to 40 per cent
FALSE - 'Nitrox reduces narcosis'
TRUE - Not necessarily: oxygen has equally narcotic properties under pressure