JackKit

Jack Ingle is the BSACs Technical Diving Adviser. He is a BSAC National Instructor, an IANTD Technical Instructor, a TDI Tri-mix instructor and co-author of NSAC nitrox courses.

IN SEARCH OF THE ULTIMATE BC

Which in your view is the best BC system on the market I know Halcyon has a stand-up reputation, but do you have any insight into the Abyss Explorer wings
Adrian Bradley

Before the BC, divers had to weight themselves correctly to dive safely, especially if they had to do decompression stops. In the late 70s the adjustable buoyancy life-jacket (ABLJ) or horse-collar system arrived. As with all new ideas, some people said it was dangerous and would kill divers. In fact it made a great contribution to diving safety.
In the 80s, the new jacket-type BCs were far more comfortable, as the gas was in pockets surrounding the diver rather than under his chin, cluttering up the chest area.
During the late 80s and 90s, as technical diving took off, the benefits of putting the buoyancy pocket on a divers back became obvious to many of us.
By using what became the wing, divers could mount important equipment such as regulators and inflators where it was easy to get at quickly.
What we were doing was going back to the pre-ABLJ harness, but adding the benefits of a buoyancy system. The harness allowed us to add tekkie D-rings to give anchoring points for the extra equipment we needed for deeper and longer dives. Thats why I prefer wing to jacket BCs.
To narrow down the choice of manufacturer and model, you must identify what you want the equipment to do for you, and examine the quality of the materials used.
You are right to say that Halcyon has a very good reputation, and many technical divers are big fans. Its product is well made, durable and robust.
Abyss Explorer wings look interesting. I have seen an early prototype and it is made to sit snugly around the cylinder and seems to offer a very compact system on the divers back. I havent had a chance to try one yet but Abyss is another company which prides itself on quality of materials used in its equipment.
My preference is for the OMS wing, which has rubber bungees to keep the material packed compactly. I try to ensure that there is no extra drag from the wing while I am swimming, and as a drysuit diver I normally use the suit rather than my wing for buoyancy during the dive.
The OMS wing has 100lb of lift and I use the single-bag inner, but a twin-bag system is available for wetsuit divers.
I also use this wing with a stainless-steel backplate, as this offers a solid mounting point for all my equipment, and it is very adaptable when mounting to other cylinder configurations. My harness is a home-made system with D-rings and straps adapted to my own equipment and needs.

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How do I stay at deco-stop depth
On a couple of dives recently when I have been with a group, we have had to abort, once due to a diver panicking and bolting to the surface, once due to losing a diver in appalling visibility. On both occasions there was no visual reference and I had trouble maintaining the 5 metre stop. Do you have any tips to assist us in this, or does it just come in time

Jonathan Hall-Smith

I know it happens, but I do feel that divers carrying out decompression-stop diving should not be panicking and bolting to the surface. Build your diving experience slowly and dont carry out compulsory stops until you are completely relaxed in the underwater environment.
In British waters we do get bad visibility and you must be able to cope with difficult conditions through practice. For a technical diver planning to carry out deco-stops, it is critical to be able to control your buoyancy at all stages of the dive, especially at the shallower deco-stop levels.
As for loss of visual reference, a shotline gives you a point of reference during an ascent and can be very comforting in poor visibility. It can be used for decompression stops even in moving water, by using a Jon line attached to both the shotline and the diver. The Jon line is there only to maintain contact with the main shotline and should not be used to maintain the depth of decompression stop, which should be achieved by using correct buoyancy control.
Another system used by many divers is the delayed surface marker buoy, which is deployed at the end of the dive so that the diver can ascend while reeling in the line. A DSMB again gives you a point of reference but requires practice in deployment and, once again, correct buoyancy during the ascent and stops.
Many of these skills are taught on advanced nitrox or decompression-stop diving courses. If you are moving into more technical dives, it would be worth signing up for this sort of training.
Why doesnt a breath last longer
I have a question which I have put to lots of people but have failed to get a straight answer to. If I breathe air at 10m, then the pressure will be twice normal atmospheric pressure, so for every lungful I take, I actually inhale twice the amount of air (ie twice the number of oxygen and nitrogen molecules). If my body converts O2 to carbon dioxide at the same rate, why doesnt one breath last twice as long I know the breathing rate is regulated by the partial pressure of CO2, not the O2, but if my body is using O2 at the same rate, it should take twice as long for the ppCO2 to reach the threshold.

Nick Bendelow

You are not the only diver to wonder about this, and its a question that regularly comes up during courses. I like your idea that the deeper we go, the fewer breaths we should need to take. I wish Mother Nature had taken that on board when we were put together!
src="http://diverfiles.net-genie.co.uk/data/images/0302qanda.gif"This table shows examples of partial pressures (pp) for inspired and expired oxygen, nitrogen and carbon dioxide at various pressures
 
The answer is in production of CO2. It doesnt matter how many molecules of oxygen you inhale, though you are right that they increase with depth as pressure increases. Your body will still metabolise the same amount of inspired oxygen for each breath, and produce the equivalent amount of CO2. The partial pressure of expired CO2 remains 0.04 at any depth.
Aluminium or steel for stage cylinders
I have been using a 7 litre steel stage cylinder and need another one. I am wondering whether to go for aluminium. What are the advantages

Jon Ashfold

Many UK technical divers go for steel stage cylinders. I assume that they got used to using steel during training.
Some divers, especially drysuit-wearers, want the heaviness of steel. The more weight we get from the equipment we must carry, the less lead we need to put onto a weightbelt. When I dive with my smallest twin-set - 12 litre steel - while wearing a neoprene suit, I can do without a weightbelt altogether.
However, add a steel stage cylinder to your rig and you will be quite heavy, and need extra buoyancy to cope. Steel cylinders also tend to hang below you if worn side-slung and mounted incorrectly, causing you to be pulled to one side. To prevent this, the top of the cylinder should be mounted on a high D-ring on the harness, with the lower mounting well behind you - on a rear belt-mounted D-ring or the side of the twin-set - so that the base sits high.
Aluminium cylinders are used a great deal abroad, and I prefer them as stage cylinders. They are bulkier than steel, but as the gas they contain depletes they become buoyant, causing the base to rise behind the diver and ensuring that there is no downward drag.
Your weighting has to allow for the additional buoyancy, of course.
I find aluminium cylinders very comfortable. Little protrudes in front of you, so the chest area is left clear for easy access to important equipment, and your legs dont bang and crash against the cylinders while finning.
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