tmassey
Contributor
I'll say what my thoughts are on this in Imperial/US tanks; you can translate into Metric as you wish.
First: *Which* 10L and 12L?!?
There are narrow 10L and wide 10L; there are narrow 12L and wide 12L. The volume does not sufficiently define the geometry of the tank.A perfect example of this in US tanks is the HP119 and HP120 tanks. They have virtually the same total capacity *and* virtually the same water volume (and even identical service pressures); but they are shaped *very* differently. One is 24" long by 8" wide (we would call that short and wide) and one is 28" long and 7.25" wide (we would call that long and skinny.)
Again: these are virtually identical tanks spec-wise. They are both '15L' tanks. They're both hold the same amount of breathing gas. But on your back, they couldn't be more different.
In my opinion, it's the length that makes the biggest and most important difference. And as a rule, you want the narrowest tanks you can get for your purpose. The use of wide tanks should only be because you *need* the extra gas capacity *and* can't handle a longer tank. Otherwise, go for the narrower (and potentially longer) tanks.
For us, we have 3 primary lengths of tanks: 24", 26" and 28". Which is better depends largely on the length of your back. Once the tank gets to even the top of your pelvis, you lose the ability to arch your back. So you need a tank that ends at least a couple of inches before your pelvis. Of course, this is determined by how long your back is, how high you mount the tanks, how flexible you are, etc. (It also assumes you're in proper horizontal trim, with your hands stretched out in front of you and your knees, pelvis and shoulders all in line, and all parallel to the bottom.) Height itself is not the determining factor, torso length is; but height is usually used as a proxy. (If you know you have a long or short torso for your height, adjust accordingly.)
For a person 6 feet tall or shorter, the 28" tanks are just too long. The tank will extend well past their spine, and they won't be able to arch their back -- so they won't be able to look forward. They would be much better off with shorter tanks.
What about a person 6 feet tall or taller using 24" tanks? For them, the tanks end somewhat near the mid-back. The problem there is that all of the weight is concentrated far forward (if you want to reach your valves), which tends to make you head-heavy and hard to create a horizontal, stable profile. It also concentrates the weight into a small area, and with the long legs and arms of the tall diver makes it harder to remain in a stable position: too much leverage.
So, as a rule, someone under 5'6" or so has a harder time with tanks longer than 24 or 25 inches, which means they benefit from HP80's or AL63's, or maybe AL80's. LP85/HP100's tend to be about the maximum for such a person, and pretty far from ideal.
From 5'6" to 6'1", they are squarely in that middle length. AL80's, LP85's and HP100's are perfect. LP95/HP119 are the choice if they need more capacity, but HP120's and HP130's tend to be too much -- likely way too much in the case of the 120's.
Above 6'1", the small tanks become unreasonable. Trying to deal with AL63's or HP80's is just a waste of time. LP85/HP100's become the "small" tanks for such a person. Again, don't bother with the LP95/HP119's: jump to the HP120's and the HP130's only if you really need that extra capacity.
And again, only use the "wide" (8") tanks when you truly need the extra gas capacity frequently (and you've been diving long enough to have a control on your breathing and buoyancy). The difference in effort to push 8" tanks through the water vs 7.25" tanks is noticeable. So wherever possible, get the longest tank that can be comfortably and effectively used, and only then get a wider long tank because you *have* to.
I know that doesn't directly answer your question, in large part because I have no *idea* which of the myriad of non-USA tanks are actually available, and which are available to you. But hopefully this will give you a little bit better understanding of the principles I would use to select the proper tank, and with a general idea of the lengths involved (both in tanks and in people!) it will help you to answer the question for yourself.
And I'm far from an expert: I'd love to hear other thoughts on this as well!
