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When choosing a scuba cylinder, we are presented with a lot of choices between steel and aluminium cylinders. So which ones do we choose for which purpose and why?
Let’s start by identifying what the correct names for cylinders are; how they are used and what the best choice is.
Single cylinder on the back – steel is the best choice here because, as the cylinder empties, it still remains negative in buoyancy.
Twin cylinders on the back – again, steel is the best choice due to its negative buoyancy characteristics.
Deco cylinders/Bailout – used for decompression gasses and can be nitrox or trimix. Aluminium is best as they start negative, but become neutrally buoyant at around 140 bar.
Stage cylinders – used for additional back gas and staged (removed and left) on the way into a cave. Aluminium is best as they start negative, but become neutrally buoyant at around 140 bar.
Pony bottles/Bottom mix off boards – used to extend the time spent on the bottom or as redundancy - Aluminium is best as they start negative, but become neutrally buoyant at around 140 bar.
So, it is fairly simple, anything on the back, should be steel and anything that is carried off board, should be aluminium. Luxfer aluminium cylinders are preferred in 80cuft as they become neutral earlier than most other 80cuft aluminium cylinders by 20-30 bar and have the characteristic of being lighter in the bottom. This allows them to sit up nicely under a divers arm, thereby creating less drag as they tuck into the divers sidestream. Being neutral also makes them easier to manipulate on the dive and on the surface when handing them up to the boat.
Luxfer AL40 – used for 100% oxygen, deco, bailout
Luxfer AL80 – used for deco mixes, bailout, stage and pony/bottom mix
Back gas single – 10.5 litre (85cuft), 12.2 litre (100cuft), 15 litre (125cuft) – which is best?
The 12.2 litre Faber is the best choice for back gas, singles or twinsets for reasons of length, capacity and versatility.
10.5 litre – length is 560mm, which makes most divers very head heavy, forcing them to drop out of trim to correct the high centre of balance. This in turn makes the diver inefficient in moving through the water column, resulting in higher gas usage and higher levels of exertion over the dive.
12.2 litre – length is 625mm, with an ideal balance point close to the centre of the cylinder.
12.2 litre compact - length is 515mm with a lot of weight towards the neck of the cylinder, making divers very head heavy. These are the least desirable of all steel cylinders.
15 litre – length is 610mm with a lot of weight towards the neck of the cylinder, making divers very head heavy again. If you feel you need this much gas, a better choice would be twin 7 litre cylinders due to much better weight distribution of the cylinders on your back.
Back gas twins - 10.5 litre (85cuft), 12.2 litre (100cuft), 15 litre (125cuft) – which is best?
The 12.2 litre Faber standard is the best choice for a twinset too; as all of the length and centre of balance issues with the other sizes presented above are now doubled in a twinset.
2x 10.5 litre = 4872 litres of gas at 232 bar = weight on back 22.6kg = 215 litres of gas per kilo carried.
2x 12.2 litre = 5660 litres of gas at 232 bar = weight on back 25.8kg = 219 litres of gas per kilo carried.
2x 15 litre = 6960 litres of gas at 232 bar = weight on back 33kg = 210 litres of gas per kilo carried.
So as you can see, the 12.2 litre is most ideal, as it has the best balance point and is the lightest with having the most efficient gas to weight ratio. When you add to this, the increased drag from the larger diameter 15 litre cylinder at 204mm, against the 12.2 litre 178mm and you can see why we choose the 12.2 (100cuft) cylinders.