The popularity of the BSAC's CCR (closed-circuit rebreather) course, which was launched a couple of years ago, has prompted the BSAC to introduce a new technical-diving course, namely the mixed-gas diving course. But what is 'mixed gas' and why should we consider using it?

Essentially, mixed-gas diving gas can be described as any breathing gas that is made up from a mixture of oxygen, nitrogen and helium. Other gases, such as neon and hydrogen, are also considered to be in this category, but these are very rarely used outside commercial diving operations.

The reason why helium is used to substitute some of the nitrogen and oxygen found in normal breathing air is that nitrogen and oxygen have a narcotic or toxic effect on the body at deep diving depths. The depth at which these effects can be felt when breathing air is generally around 30m in the case of nitrogen narcosis and 70m for oxygen poisoning. Helium, on the other hand, is an inert gas which is also less dense. Its advantage is that it demonstrates virtually no ill effects on the body and is absorbed and released more readily than nitrogen. With this in mind, the next question is how much helium should we use? To find out the answer and to learn more about the practical side of diving with mixed gas or trimix, as it is often known, I attended the first BSAC advanced mixed-gas course, conducted in November last year off the Egyptian coast at Sharm El Sheikh. A prerequisite to signing up is to have BSAC extended range diver or similar qualification. The course is divided into a series of lectures, dry practical kit configuration lessons, open-water skills sessions, at least five open-water dives totalling a minimum of 360 minutes underwater and a theory exam. Having already gained entry-level status as a trimix diver from another agency, I found that most of the first day was a refresher exercise. However, for the mixed-gas diver rookie, the instructors take you through the theory behind trimix as a breathing gas, decompression models, safety considerations, planning and execution.

Next up was how to assemble all the equipment so that it is neat and tidy while at the same time being readily available. Correct kit configuration, as it is called, is paramount for a safe and enjoyable dive. When carrying a minimum of four cylinders and four regulators, it is essential that you are able to assess any one of them instinctively and at the same time be absolutely sure about which gas you are breathing. We learned that there is no single, definitive way to configure your equipment, but that the majority of technical divers adopt a similar pattern.

With the theory understood and gear all correctly set up, the next step was to head to confined waters to learn how to use it all. As with most courses, the first step was to achieve neutral buoyancy - without this, progression on the course would be extremely difficult. Following this, various skill drills were shown to us using waterproof prompt cards. These ranged from relatively simple tasks such as mask clearing, regulator recovery and correct gas selection, to more complex procedures such as removing and replacing side-mounted stage cylinders and carrying out gas shutdowns. The latter often proves to be the most challenging procedure as it involves turning off one of your back-mounted cylinders. As the valves are behind you, you have to master a technique that involves lifting the twin-set slightly and reaching behind and feeling your way around until you can locate the valve you wish to turn off. The reason for this is that, should you have a catastrophic failure in one of your regulators or valves, you can quickly isolate it and save as much gas escaping in the shortest possible time. After a couple of hours in shallow water all these procedures started to become second nature, which allowed us to progress to the next stage.

Day Two saw us carry out a couple of dives to a maximum depth of 35m. Although some of the previous day's skills were repeated to ensure we had thoroughly mastered them, the main objective of these two dives was to use and become familiar with run times. As deep diving usually involves long and complex decompression schedules, these have to be worked out prior to the commencement of the dive and meticulously adhered to. Not only is the time at each stop important, but so is the total ascent time. In addition, various contingency plans have to be prepared just in case the dive doesn't go according to plan. Decompression and ascent times must also be calculated to take into account the possibility of a dive being deeper or longer than planned. The alternative run times are not for optional dive plan changes, but are a back-up to the planned schedule. Most of our decompression was carried out on a specially prepared underwater trapeze, which consisted of three metal bars roped together at a depth of 3m, 6m and 9m. This allowed several divers to decompress together in relative comfort. Once we had learned how to dive using these run times, we could progress to a deeper depth.

The next three days saw us carry out three dives to a maximum depth of 55m, 65m and 75m in succession. Under the guidance of the instructor, bottom times were left to us to determine. This was achieved by using the recommended, not-so-easy-to use (it runs on DOS!) software program. By trial and error we would eventually obtain the maximum allowable bottom time for an acceptable run time. It's a pity that the BSAC stipulates that all dives must be conducted using tables (this is an advanced course) instead of computers as the vast majority of divers use electronics to calculate their decompression times, only using tables for back-up purposes.

No skills exercises were undertaken on these deeper dives, but we did pick up some useful tips and techniques for making long decompression stops more enjoyable. I found two of these that were particularly useful included using a Jon line and being taught how to re-hydrate underwater. A Jon line consists of a short piece of rope usually between 1-2m in length with a clip at each end. It allows you to attach yourself to the shot-line or decompression trapeze without having to use your hands, as long as there is a current running. Long decompression stops often lead to an extremely dry mouth, so a good technique to master is being able to drink while underwater. We used a squashable foil-packaged soft drink. The method involved undoing the top and immediately sealing with your thumb. The next stage was to remove the regulator and slide the top of the carton into your mouth while releasing your thumb. You then sucked the juice out of the bag then reversed the whole procedure.

BSAC's entry into technical diving has been overdue, but this comprehensive course certainly instils the importance of the skills and planning that trimix diving requires.

DIAGRAM

1. isolator manifold
2. trimix gas cylinder
3. primary regulator
4. wing BC
5. back-up regulator
6. D-rings
7. & 8. decompression regulators
9. first deco gas cylinder
10. second deco gas cylinder


Thanks to…

All the staff at Tekstreme Diving based at Emperor Divers in Sharm El Sheikh, Excel Airways and course organisers BSAC instructors Ray McKee and John Dunwell.