Getting to grips with nitrox
Two new BSAC courses aim to equip sport divers for nitrox diving, and simple tables save you the trouble of doing complex calculations when you use mixed gases.
Getting to grips with nitrox
Why use Nitrox?
HENRY Royce of Rolls-Royce cars always insisted that he was not an inventor, but that his skill was to improve on the innovations of others. And so it is with Jack Ingle, BSAC Nitrox Chief Examiner, who devised the BSAC Nitrox Diver and Advanced Nitrox Diver courses. By waiting and examining the efforts of other training agencies, he has been able to come up with two courses of Rolls-Royce quality.
The BSAC Nitrox Diver qualification is for those who wish to use nitrox 32 and nitrox 36 for no-stop diving to safe depth limits of 35m and 30m respectively.
Advanced Nitrox Diver is for those who wish to do more adventurous diving, using a wider range of mixes and involving dives which might need decompression stops. This includes using nitrox as both a diving gas and, up to nitrox 50, as a decompression gas.
Jack has followed the spirit of the BSAC '88 tables by relieving divers using nitrox from complex calculations. He appreciates that most divers cannot manage such complexities as the Buehlmann tables or complicated oxygen-exposure tracking charts, so everything is determined from simple look-up tables.
Of prime importance in all BSAC nitrox training are the Club's new Nitrox Decompression Tables. Dr Tom Hennessy, the BSAC's decompression-table guru, has reworked a set of tables suitable for use with nitrox 21 (air), 27, 32 and 36.
These tables are similar to - and as simple to use as - his original BSAC '88 tables. They can be used for repeat diving with different gas mixes each time, and are designed to work in the direction of increasingly rich mixes in any sequence of dives, in the same way that it is best to do deepest dives first. It is not only their simplicity in use that distinguishes the BSAC Nitrox Decompression Tables from those used by other training agencies; the information they hold is obtained from base data rather than by using an "equivalent air depth".
The tables also include a simple look-up Oxygen Toxicity Table for both Unit Pulmonary Toxicity Dose (UPTD) and Central Nervous System percentage (CNS%).
Another table allows the user to look up the partial pressure of oxygen (ppO2) for any combination of depth and nitrox mix.
The Student's Course Pack also includes a Nitrox Dive Conduct Slate. As with all the new BSAC nitrox materials, including cylinder stickers, the tables are finished in a characteristic "nitrox yellow and green".
All decisions on use of mixes at varying maximum depths, whether as straight breathing gases or as decompression mixes, have been determined around a set point of a maximum 1.4 bar ppO2.
If all this sounds complicated, I should point out that for a competently trained and open-minded air diver, the additional information needed to use nitrox is extremely straightforward.
Both of these BSAC courses are to the point, devoid of mumbo-jumbo, and readily applicable to diving in the real world.
You need a BSAC Sports Diver qualification to embark on the Nitrox Diver course, which at 70 includes a day of theory, an optional "taste nitrox" dive, registration, certification, student notebook and the tables.
Dive Leaders and above who hold the BSAC Nitrox Diver certification qualify for the Advanced Nitrox Diver course. This takes two days: one of theory and one of diving. It costs £100 and includes student notebook and certification.
The practical session includes two dives concentrating on gas-switching techniques, buoyancy and depth disciplines, and decompression techniques. The second dive is something of an assessment.
Those with a Dive Leader qualification and above but no nitrox certification can combine both courses over two days for £130.
Courses in development include BSAC Extended Range Diver, using gases up to nitrox 80 as a decompression gas in conjunction with air (nitrox 21) to increase safety in the deeper depth ranges now being explored by some divers.
For further details check with your local BSAC Diving School or Regional Coach.
Why use nitrox?
WHEN we learn to dive we are given compressed air to breathe. Air is a mixture of gases, mainly oxygen and nitrogen (approximately four parts nitrogen to one part oxygen).
However, our bodies are designed to breathe air at one bar of pressure, so breathing it under increased pressure when diving can cause a serious side-effect.
All trained divers know about the hazards of decompression illness (DCI). Depending on how deep and for how long we dive, this is caused when we absorb some of the nitrogen in the air we breathe. We need to use decompression tables or a computer to ensure that we either do not run out of no-stop time or make the appropriate stage decompression stops during ascent to avoid injury.
We can reduce the hazards of decompression sickness by reducing the amount of nitrogen in the mix. This is done by increasing the amount of oxygen, and the resultant gas is called nitrox.
A diver can use a nitrox mix to build in an extra safety margin by applying decompression calculations for air. Alternatively, by applying decompression calculations for the relevant nitrox mix, he can enjoy extended bottom time.
Another way to use nitrox is to breathe a mix rich in oxygen during a decompression stop to increase the rate at which nitrogen is eliminated from the body.
Surprisingly, oxygen is hazardous when breathed under pressure at depth. With normal air, divers rarely reach the depths (60m+) when this might occur, but nitrox is enriched with varying amounts of oxygen, so you have to be careful.
Oxygen poisoning under water can be terminal.
The equipment you use with nitrox may differ from that used for normal compressed-air diving, so special nitrox training is essential.
Appeared in DIVER - April 1997