Scuba diving: A gas guzzler's guide

Filling your body with performance-enhancing substances is something most sporty types keep to themselves. But scuba divers are different. Andrew Spooner samples the benefits of 'enriched air'

I've decided to give up breathing air. It gives me a headache, induces an itchy, dry throat and leaves me exhausted. From now on I will only breathe enriched air, commonly known as nitrox. This gas has a higher oxygen content, boosts energy and, much more importantly, extends my bottom time and shortens my surface intervals.

At this point it is probably a good idea to point out that "bottom time" and "surface intervals" are important terms to scuba divers. Bottom time is, literally, the time you can spend on the seabed. Its length is determined by two important factors: the depth you reach and the gas you breathe.

As you go deeper, partial pressure increases the density of the gas you suck in through your regulator. With normal diving air, about 21 per cent oxygen, 79 per cent nitrogen, the longer you spend at depth the more likely you are to suffer from nitrogen narcosis. This will leave your head spinning, thoughts confused and, if unchecked, could lead to blackout and death. Because of this narcotic effect, scuba divers stick to strict depth and time limits.

Nitrogen is an inert gas; unlike oxygen, the body can't absorb it, instead storing it in cells and tissues. This can cause that well-known diving condition, the "bends"; if a diver ascends too quickly, the nitrogen expands, often leaving the diver bent over in agony. Even if they ascend at a safe rate, divers must wait at the surface for a specified period - the surface interval - dependent on the time and depth of the dive so the body can expel most of the nitrogen before they dive again.

The first divers to use a form of nitrox frequently were Royal Navy Commandos during the Second World War, but it was not until the 1980s that leisure divers began to use it regularly. In the Nineties the main dive-training bodies followed suit, introducing nitrox qualifications.

I have come to the Egyptian Red Sea resort of Sharm el Sheikh to take a Professional Association of Divers (Padi) nitrox course with Emperor Divers. "In the course we'll be completing, Padi refer to nitrox as as enriched air," says my Italian instructor, Danny Zanoni. "The usual gas mix for [nitrox] divers is around 32 per cent oxygen, 68 per cent nitrogen.A few years ago you'd only find the occasional diver using it. Now, almost every single dive boat in the Red Sea will have nitrox divers on board." It's the future of diving then? "I would say so," says Danny.

I spend a lot of the course in the classroom, getting my head around a series of complicated mathematical formulae. There are some important differences compared with diving on "normal" air, which is what I used when getting my entry-level Padi qualification. I have to learn a whole new set of dive planners, charts that provide fixed limits for dive time and depth, depending on the surface interval. With an adjustment in the gas mix, all these change: I can't dive as deep, but can stay down longer and dive more frequently. I also need to be alert to the fact that the risk of blackout increases when using enriched air.

The day's studies done, my head filled with gas percentages and my exam pending, I hunker down in a quiet corner for a revision session. "Don't worry," says Danny, "the questions are multiple choice. You should breeze it."

Thankfully, I pass with flying colours. The only thing that remains is to test nitrox at the sharp end, and the next day I enter the waters of the Red Sea to visit one of the world's most famous dive sites: the wreck of the SS Thistlegorm.

Over the years the story of the Thistlegorm has become a mixture of fact and legend. What is without doubt is that it is a big money-spinner - some claim it earns more money for Egypt than the Pyramids.

What is beyond doubt is that this 126-metre long British supply ship was sunk in October 1941 by a German Heinkel bomber, who chanced upon it after failing to find the Queen Mary.

"The wreck is a spectacular site," says Danny as we load up the boat at 5.30am. "The great thing about diving it on nitrox is that we will be able to stay down for longer - our only real limits will be how quickly we consume our gas."

We set sail just as the skies begin to mutate into a range of sunrise hues. Two hours later, we arrive at the dive site. All that is visible from the surface is a couple of buoys and several other dive boats. Danny disappears into the water to check the dive conditions, surfacing a few minutes later.

"We should penetrate the wreck first, as there is quite a strong current," he says, "It normally eases during the day, so we should explore the exterior later."

We don our dive gear, check and log the nitrox mix (31.9 per cent oxygen in this case) and jump in. Within seconds the massive super-structure of the Thistlegorm looms up out of the murk. Giant, twisted sheets of metal, smothered in weeds and muck and surrounded by thousands of gloriously bright fish, lie on the seabed.

Danny leads a group of us into the bowels of the shattered boat. We swim through tiny gaps and into vast chambers filled with encrusted trucks and motor- bikes. There are rifles, toilets and thousands of assorted storage cases.

What is also noticeable is my physical condition. The extra dose of oxygen stimulates mild exhilaration as opposed to the usual headache. There is no scratchy throat, and by the time we hit the surface, after 40 minutes down to depths of almost 30 metres, I feel fantastic.

"How did you find it?" asks Danny. I let him know how much better the dive felt. "We'll be able to go back in again soon," he says. "It's a major benefit of nitrox."

We spend about 90 minutes on the boat, drinking tea and chatting. Normally after such a long, deep dive on air, most divers are dulled and tired, but the nitrox divers on board are all upbeat, as opposed to the fatigued, wrung-out air divers.

"You ready to go again?" says Danny. I need little encouragement; just don't offer me ordinary old air.

THE COMPACT GUIDE

HOW TO GET THERE

British Airways (0870 850 9850; ba.com) has five flights a week to Sharm el Sheikh from London Gatwick, from £218 return.

FURTHER INFORMATION

Andrew Spooner dived with Emperor Divers (020 69 360 1734; emperordivers.com) who offer nitrox courses from £50 (not including equipment) and day trips to the Thistlegorm from £65, including nitrox.

He stayed at the three-star Rosetta Hotel (020 69 360 1888; tropicanahotels.com). Singles from £21.30, room only. For more information about holidays in Egypt: egypt.travel; gotoegypt.org