As close to light weight, given current pressure parameters is the old steel 72’s pushed up to 3000 psi.
what you need/want is something around the 72 physical size with 6000psi.
call it a VHP 110
We can easily buy 300-bars tanks here in Europe. High-strength steel, so lighter than aluminium.
But getting them filled at their rated pressure is not that easy...
Most filing stations here can go up to 250 bar, 232 bar being the standard.
But when you ask to fill at 300, many will refuse, because it would require to modify the settings of their safety valves.
And they are quite negatively buoyant at the beginning of the dive.
Conclusion for me is that the best compromise between size, capacity, cost, carrying weight, easy refilling and buoyancy is with steel at 232 bars. Possiblly Faber. My favourite size is 15-liters, which gives me a lot of air, enough for some "light deco" when required.
Luckily, this is the most common type and size here in the Mediterranean.
Some years ago, 300 bars tanks made in carbon fiber were presented, much lighter than the steel ones, but they were very expensive and never caught traction.
You have also to consider the problems you will encounter when it is time for pressure testing. These fiber tanks are very elastic, so they expand much more than the allowed tolerance. So most testing labs would discard them, not knowing the fiber technology and not taking care to read the relevant testing standards for carbon fiber tanks, which of course specify a much larger allowed expansion under pressure than steel tanks.
. I've been diving them for 10 years now, no rust, no other issues, passed all TÜV inspections. I would never dive them as main tanks or stage tanks due to their buoyancy, but they are great as bail-out on the 'breather.
