Hi,
Frankly if you have limited or no previous experience diving a twin set, I wouldn’t start on a trip unless you want to focus on learning to dive a twin set as part of some kind of “intro to tec” or similar course as taught by a good “tec” instructor.
If that is the case, I wouldn’t take the twin set to 40m either until you have 20 dives or so under your belt in this configuration.
The reasons for this are as follows:
A twin set will only really give you redundancy if you can quickly and efficiently operate the valves for a shut down. Most people require several practice dives to master this skill.
A twin set diving rig in a traditional OC DIR configuration (& yes – I know that BSAC divers can dive their twin sets with a Buddy Commander) will also require specific adjustments to the diver in terms of position of the weights, position of the d-rings, tank bands on the twin set and slack in the harness. Configured incorrectly the twin set will be uncomfortable and difficult to dive in trim.
Now if you had a lot of experience diving a twin set them you would be able to tell within one dive exactly what fine tuning was required – without this experience however it will be hard to pin-point where exactly adjustments need to be made.
Further skills you should aim to master in this configuration in the interest of yourself and your buddy include primary donate (& possibly – full deployment of the long hose) proper venting with the rear dump (or potentially going way out of trim when using the corrugated hose which is often short on a DIR rig to prevent trapping by a stage tank)
The whole situation will be further complicated by learning to dive a twin set at the same time as diving with a dry suit.
PADI (for example…) teaches that when diving a dry suit all buoyancy should be controlled using only the suit with the wing reserved for emergencies i.e. loss of buoyancy due to flooding of the suit. This isn’t practical with a 2x12l twin set since you will be ca 6.2 kg negative at the beginning of the dive which may exceed the lift capacity of the dry suit.
In view of the above you will need to be able to efficiently add and dump gas from both the wing and the dry suit throughout the dive while maintain correct buoyancy – again a skill that requires practice.
For similar reasons the PADI (for example…) method of determining correct weighting with a full tank of adding enough weight to sink on exhale and then adding the weight of the air in the full tank won’t work with a twin set either given that you are likely to be negative without weights with a large twin set at the beginning of the dive. If you can’t breathe the twin set down to empty without losing buoyancy you don’t have much redundancy either.
If you are diving in the Mediterranean a free flow short of a burst hose or torn 2nd stage diaphragm is unlikely. If you still feel you need a redundant air supply, I’d ask for a 7l stage tank or so rigged as a bottom or deco stage where you can operate the valve and see the stage SPG (i.e. not strapped to the back-gas tank(s).
regards,
Rory
