With its manifolds, shut-downs and isolator valves, twin-set diving is whole new world for many divers. DIVE’s experts, Richard Bull and Richard Trevithick, offer their advice on making the step up
set up on BC
of a twin system
There are essentially two types of twin-sets to choose from: the manifold and the two-separate-cylinders system. Separates provide the ultimate in redundancy. When using two separate cylinders, the diver will breathe down each cylinder together, by switching regulators, say, every 50 bar used. A contents gauge will be required for each cylinder and both should be monitored. The advantage of having two separate cylinders is that if one regulator malfunctions, it does not affect the other.
Manifolded twin-sets are joined together by a bar, essentially making one large cylinder. Only one contents gauge is required. Most, but not all, manifold bars have an isolator valve in the middle. This can be closed off to isolate each cylinder. This enables the diver to preserve the remaining contents of one cylinder should there be an uncontrolled loss of gas in the other during the course of a dive.
What the experts say…
At what stage might divers think about making the switch to twin-sets?
Bull: The reasons vary. A diver may simply want to make the switch in order to have more air on standard dives. Generally, however, a diver should switch when he feels that his diving has progressed beyond what is safe with the available air and the limited redundancy of a single cylinder.
Trevithick: Before paying out for that new twin-set, it’s important to consider the alternatives and make an informed decision first. Would a move up from a single 12-litre to a 15-litre cylinder with a three or five-litre pony cylinder as a bail-out be a better alternative?
What are the main benefits of twin-sets?
Bull: The two main benefits are the possibility of a great deal more gas and, with task-specific configuration, some meaningful redundancy. Twin 12-litre cylinders might be a bit heavy and awkward, but a single 24-litre cylinder would be absurd. And, chances are that the sort of diving which would require that amount of gas would also need adequate redundancy and that means a twin-set.
Trevithick: The weight distribution on the back is more balanced than, say, a 12-litre cylinder with a three-litre pony cylinder, although it may take some getting used to. Also, as hoses are spread between two first stages, it becomes easier to achieve clean hose routeings and reduce any risk of entrapment. Then there is the sheer laziness vote, because a single twin-set will often suffice for two recreational dives.
How costly is it for divers to upgrade from a single cylinder to a twin-Set system?
Trevithick: the cost of upgrade is dependent on how much equipment can be used from your single cylinder set-up and whether you buy new or secondhand kit. However, assuming items are bought new and no kit is used from your existing collection, the cost is likely to be between £1,000 and £1,500. This includes cylinder, manifold, regulators, stainless-steel bands and backplate, harness and wing.
Bull: Cost is certainly one thing against twin-sets. It’s not just buying them in the first place, either. Every time cylinder testing comes around, it’s at least twice the cost. Good-quality steel cylinder bands are the heart of the system – these are more expensive cam bands (used to hold cylinders together). You will probably have to replace your existing BC with an industrial model wing system which has more lift. Yes, it’s a lot more expensive, but I think that the benefits are worth the cost.
How experienced should divers be in order to use a twin-set?
Bull: Experienced enough to be able to make a reasoned and rational decision about using twin-sets, and experienced enough to know your own abilities and limitations. For instance, if you are capable of diving to 45m with a single cylinder, then using a twin-set has the potential to make this safer. However, if you do not have the skills and experience to dive to 45m with a single cylinder, then a twin-set will not suddenly endow you with those skills.
How does using a twin-set affect your buoyancy and weight distribution?
Trevithick: The key difference is the reduction in weight carried around the waist or pouches. However, it is also important to take account of buoyancy change during a series of dives, as a greater volume of gas will be consumed. A wing will lift weight off the back and therefore you may find a wing preferable to a traditional BC. Whichever buoyancy device is used, it must have sufficient lift to bring you to the surface in an emergency. Remember that you may decide to carry one or more stage cylinders before too long, adding to the overall weight and lift required.
Bull: With little exception, a twin-set will be more negatively buoyant than a single cylinder holding the same amount of gas. I have a twin-set that I use specifically for ice diving. It is a twin six-litre set up, with a 300-bar working pressure. It is monstrously heavy for its size, which is why I use it for ice diving as it counters the amount of thermals I wear under my drysuit.
What are the main advantages and disadvantages of using A manifold or two-separate-cylinders system?
Bull: A manifolded set will give you loads of air without the need for changing regulators underwater. A separate set will give you a redundant gas source, but in order to make the most of the increased amount of gas on your back, you will have to switch regulators underwater. A manifold with a central isolation valve gives you the opportunity to run the set in either single or dual air source mode.
What specific skills do you need for twin-set diving?
Trevithick: Good buoyancy control, which is mastered through having the right equipment, training and weighting, developed through lots of diving. Some years ago a friend of mine strapped his two single 15-litre cylinders together to his sports BC. As he jumped off the boat, he clocked up a 10m dive before resurfacing! Clearly, he had not conducted a weight check, nor considered the lift capacity of his original BC. If diving with a manifolded twin-set configuration, effective gas isolation and shut-down drills are essential for the successful management of a gas-failure situation. For those diving independent cylinder twin-set configurations, gas monitoring and regular regulator switching throughout the dive need to be well practised.
Bull: And you must be able to do this when stress levels are up, so practise these procedures under stressful conditions.
Are there any specific courses or training for twin-set use?
Trevithick: The BSAC sports diver syllabus covers open-circuit equipment configurations, including twin-sets. In addition, the subject is covered in the context of enriched air nitrox in the BSAC and equivalent advanced/combined nitrox and extended range diver course. Do seek advice from other twin-set divers in the branch, and if possible, try the system out in the pool.
What is the best way to build up twin-set dives?
Trevithick: Experiment with the set-up at home. Time in the pool will help with initial buoyancy and leak checks. Confidence with buoyancy and familiarisation with the positions and performance of regulators and buoyancy controls will follow. Basic skills, including rescue skills, can be refreshed or adapted and the new ones of gas shut-downs and regulator switching introduced.
Bull: Get your buoyancy right in shallow water, where you can easily abort the dive and get back to the surface, for example, at 5m off a beach.
What configuration changes do you have to make when upgrading to a twin-set?
Bull: If you are going to get the most out of a twin-set, then you may have to change the way that you do things and the way that you think about things. It does, of course, depend on how you are going to use your twin-set. Let’s say that you have switched to a twin-set because your diving is progressing and becoming deeper and more adventurous. Two regulators will be the order of the day and that means two gauges. You need to have these arranged so that you know which regulator you are breathing from and to which gas source each gauge is connected. The regulators will have to be attached so that they are available for switching as required. Think objectives first, not method. Don’t just copy what you have seen done before without asking yourself what has been achieved.
Trevithick: With two first stages, hose routeing may need to change. Where sources of buoyancy, such as wing and drsyuit, had previously come off the same first stage, they can now be split between two. This provides redundancy in the event of loss of gas in one cylinder. Check whether your existing BC is still up to the job. A wing may also be more comfortable in the water. If you intend to carry side-slung cylinders, consider whether a long hose for the alternative air source recipient will make life easier. Finally, buoyancy should be checked and re-checked and weight reduced.
Choose cylinder sizes that are appropriate to your own diving, not the diving that you might do in the future.
Think about your kit configuration. It’s no good having extra kit if you can’t get to it. Keep working to improve your configuration
You will probably spend more time underwater, so plan the decompression schedule carefully.
Don’t use the extra capacity to take on dives that are beyond your experience and ability.
Conduct a weight check using near-empty cylinders.
Regularly practise gas isolation, shut-down and rescue skills.
Work on the balance of your kit. Try the cylinders higher or lower in the bands and try your weights in different positions.
Make sure that you can operate everything in the kit that you will be using, such as a thick drysuit or thick gloves.
Dive with other experienced twin-set divers.
Increase depth and dive time gradually.
Ascend on walls or shot-lines, conditions allowing, until you are completely comfortable with buoyancy.
Practise any drastic re-configuration in shallow water where you can easily bail out.
Meet the panel
Richard Trevithick has just retired from his role as BSAC southern regional coach, but still remains active on the British diving scene. A highly active diver for the past 18 years, he regularly teaches on BSAC instructor training schemes and carries out extended range and rebreather diving.
One of the UK’s renowned technical-diving pioneers, Richard Bull is a freelance diving consultant for film and television productions. His many diving qualifications include BSAC first class diver, IANTD and TDI instructor and Norwegian commercial diver.