What's your favorite set of doubles?

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What height range is the 10L good for vs. the 12L in your view?

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!
 
It depends on what Im doing

For most dives off a boat or quarry - HP100 will work up here in ontario. Unless Im wearing a Halo then I need HP130s
If Im doing cave LP 104s - with a cave fill - so much gas I can stay forever compared to my friend in a set of 130s.

The 100s were my first set of doubles - I still like them best...
 
If Im doing cave LP 104s - with a cave fill - so much gas I can stay forever compared to my friend in a set of 130s.

At the same pressure, LP104’s and HP130’s hold almost exactly the same gas. Depending on exact model of each tank and the resolution of the spec, their actual capacity at the same pressure (i.e. cave filled) is easily within single-digit percentage of each other! :)
 
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.

As a short little troll I say Amen brother. I can dive about any tank but even an AL80 tends to hit me in the butt. On the other hand my HP100 rides great. (Not all HP100s are the same height and I have found out by luck I bought one that was a little shorter).
 
I’m 5 feet 9/10 inches and twin 12l are perfect for my back. They are the gold standard in terms of twins because they are the most commonly used.
 
At the same pressure, LP104’s and HP130’s hold almost exactly the same gas. Depending on exact model of each tank and the resolution of the spec, their actual capacity at the same pressure (i.e. cave filled) is easily within single-digit percentage of each other! :)

Not quite at 3500 the LP104s will each have 150cu ft and the 130s 130cu ft (depending on the specs etc) and the more the overfill the wider the difference. Pretty easy to get fills to close to 4000 in cave country. I get that is sort of close. But that 20 cuft meant it wasn't me who turned the dive.
 
I’m 5 feet 9/10 inches and twin 12l are perfect for my back. They are the gold standard in terms of twins because they are the most commonly used.

Have you tried the 10L Tanks as doubles (I am talking European sizing/Tanks)?
 
Not quite at 3500 the LP104s will each have 150cu ft and the 130s 130cu ft (depending on the specs etc) and the more the overfill the wider the difference. Pretty easy to get fills to close to 4000 in cave country. I get that is sort of close. But that 20 cuft meant it wasn't me who turned the dive.

what math did you use to get there? Even ignoring non-linear compression, 104*3500/2640 is 138.
Fabers equivalents of LP108's and FX133's are both 17.0L water volume so will have identical gas volume at identical pressures. The LP104's and HP130's from PST are about identical for water volume and are all roughly 17 liters. Certainly close enough for horseshoes and hand grenades.
 
Not quite at 3500 the LP104s will each have 150cu ft and the 130s 130cu ft (depending on the specs etc) and the more the overfill the wider the difference. Pretty easy to get fills to close to 4000 in cave country. I get that is sort of close. But that 20 cuft meant it wasn't me who turned the dive.

Nope. The water volume of a Faber LP108 and HP133 are both listed as 17.0. PST HP133 is listed as 16.6. The biggest deviation is Worthington: LP104 is 16.8 and HP130 is 16.0

So even worst case is 16l to 17l. 4000 psi is 272 times ATA. That means that AT WORST, it’s about 9 Cu Ft and likely less — and in the case of Faber on both side they are identical.

Ref: Updated SCUBA tank specifications list -- in PDF and spreadsheet

It pays to look at the specs, rather than using shorthand...
 
what math did you use to get there? Even ignoring non-linear compression, 104*3500/2640 is 138.
Fabers equivalents of LP108's and FX133's are both 17.0L water volume so will have identical gas volume at identical pressures. The LP104's and HP130's from PST are about identical for water volume and are all roughly 17 liters. Certainly close enough for horseshoes and hand grenades.

Didn’t see your reply. It helps to have confirmation!

I’m really sick and tired of such sloppy shortcut and rule of thumb math and thinking. All of that tribal wisdom really twisted me up regarding tank advantages and disadvantages. I’m in the market for new tanks, and it was paralyzing. So I reworked everything from actual accurate (if the manufacturer is accurate...) figures. I was shocked to find how absolutely consistent everything was, not only across similar sized tanks, but up and down the entire line, no matter the brand.

So now I’ve relaxed. I’ll buy what fits me and not care about the fine details. :)
 
https://www.shearwater.com/products/teric/

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