Having set a goal of diving the wrecks of the First World War German High Seas Fleet, in this blog, part 2 of 2, Martin Maple discuss the team and personal equipment needed for our adventure.
Twin-sets (called doubles in the USA) are two breathing gas cylinders mounted on the diver’s back. They have two obvious advantages over a single cylinder configuration: increased volume of breathing gas for the dive and redundancy should an equipment failure occur. This is especially true of twin-sets that are manifolded together, as this allows all the breathing gas to be accessed from either regulator. However they are heavier than single cylinder configurations when out of the water and are also significantly more expensive to buy and maintain.
The use of independent cylinder configurations, including pony bottles and sidemount/backmount systems are not well suited to sea diving below 30 metres and so are not considered further here. Similarly the use of closed circuit rebreathers (CCR) are for another time – we are only concerned with open circuit (traditional scuba) configurations for now.
A typical configuration for UK wreck diving is as follows:
- Two manifolded euro-12 litre, 232 bar steel cylinders with wide cylinder bands
- Primary regulator with a 2.1 metre (7’) rubber hose, and a wing inflator
- Necklaced backup regulator on a short hose, dry suit inflator (if not using trimix), and a submersible pressure gauge (SPG)
- Steel backplate with continuous webbing harness supporting five D-rings, waist-mounted cutting device, and crotch strap
- Doughnut-shaped single bladder wing with 18 kg (40 lb) lift capacity, with a kidney dump on the left, and free of any bungee on the buoyancy cell.
A separate 1 litre steel suit inflation cylinder attached to the backplate is needed if using trimix because helium provide insufficient thermal insulation to use in a drysuit. If using rich nitrox to accelerate decompression a suitably sized and rigged aluminium decompression cylinder equipped with a regulator on a 100 cm (40”) hose and an SPG on a 15 cm (6”) hose is needed.
A redundant air supply alone is insufficient to dive deeper safely. Divers also need to be competent in the use of all their equipment, which requires knowledge, training and experience. Moreover a team approach, using agreed ‘standard operating procedures’, becomes increasingly important below 30 metres. Both you and your buddies must have the capacity to plan and execute the dive, and critically, to respond appropriately if failures develop. A mandatory decompression obligation is a virtual overhead environment. Home becomes a long way away with increasing deco stop time!
What is hypercapnia?
Carbon dioxide accumulates in the body if it is produced faster than the lungs can eliminate it. In open circuit diving, this can occur as a result of heavy exercise in the water, such as finning against a current, and is made worse by the increased density of gas breathed at depth.
It is for this reason that BSAC recommends that breathing gas density is maintained at 5.2 grams per litre or lower, with an absolute ceiling of 6.4 grams per litre. The practical result of this is that divers need to consider adding helium to their breathing gases if diving deeper than 30 metres, and it is absolutely required to safely dive below 40 metres.
Divers starting out with a twin-set need to be aware of the decompression limits as their dives become controlled by this more than the breathing gas availability. No one likes surprises and so a more disciplined approach to diving and planning becomes necessary. But the personal satisfaction and enjoyment to be gained from adventurous diving are not to be under-estimated!
Back to part 1.