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As a “person of maturity,” when lugging my tanks or, especially, when scrambling over rocks on a shore entry, I often have wished for lighter tanks. This got me thinking about an “ideal” tank. At first, I thought such an ideal tank would have no weight at all (purely hypothetical, of course). Oh, the joy of diving with such a tank!—or so I thought. But then it dawned on me that were such a tank to exist (with conventional size and working pressure) it would be positively buoyant (because, weighing nothing, it would displace a greater weight of water than its own weight). For example, my (steel) HP80s hold 80 cubic feet of gas at a working pressure of 3442 psi, which works out to an internal volume of 0.34 cubic feet (9.7 liters). In water, an empty weightless tank with that capacity would displace 0.34 cubic feet of water and would thus be 21.9 pounds positively buoyant (in salt water). This led me to revise my definition of “ideal” to be a tank that was neutrally buoyant when empty. Well, my HP80s have an empty weight of 27.7 pounds and are 3 pounds negatively buoyant when empty. Since the negative buoyancy allows me to use three pounds less lead, the effective weight of my tank is 24.7 pounds—only 2.8 pounds more than my so-called “ideal” tank! All this has led me to be very happy with my HP80s (but somehow I still wish they were lighter!).
(Put another way, "good" tank weight reduces positive buoyancy; "bad" tank weight does not. My HP80s have 24.9 pounds of "good" weight and (only) 2.8 pounds of "bad" weight.)
While I had to re-read your math a few times, I completely agree that my HP80s are my ideal tank I wonder why wrapped fiberglass cylinders haven't been applied in diving.
I agree on the HP 80. I don't know of a more efficient package. More here.
Since the cylinder will have volume it might as well have the weight to match the displaced water. A light weight cylinder would be nice for lugging to the dive shop but would need to be counteracted with lead to sink when diving.
A higher pressure cylinder would further reduce displacement and the associated needed ballast. Fills would be a challenge and regulator wear & selection would be impacted.
On top of the impracticality, composite cylinders are expensive to buy and have a severely limited service life
As long as we're talking ideals and not constrained by the limits of modern technology, my ideal tank is as follows:
The weight/size would be constantly variable, since the tank would be made of the same "liquid metal" the T1000 was made from in Terminator 2. This would allow the tank to show no buoyancy-swing from gas, since as you breathed the gas down, the tank would change size to alter it's buoyancy characteristics on the fly.
You could pre-program it to start the dive neutrally-buoyant and not affecting your weighting scheme, and end the dive the same way. Drysuit divers could pre-program it to be constantly heavier, (obviating the need for a belt), and end the dive at the same weight/buoyancy characteristics. Vacation divers in nothing but a skin could program AL80-characteristics if they so choose.
You could also use it to dial in trim if you are head or feet-heavy, the tank walls would get thicker either up top or down low to help dial in that perfect trim.
It could even have an "emergency mode" where if you suffer wing failure at depth at the start of a dive, (at your heaviest) the tanks elongate or otherwise get bigger to assist with getting neutral and ascending.
The optimum scuba tank would be made out of titanium, hold 100cf at 5000psi and be about the size of a lunchbox thermos bottle. ...
It would take a lot more than 5000 PSI to get 100 Ft³ down to a thermos bottle. I dive an old set of AGA Divator 324’s — 300 Bar/4,351 PSI, 2 Bottles, 4 (3.8 really) Liters/bottle that holds about 79.5 Ft³ configured as doubles.
Each bottle is 110mm/4.33" OD x about 500mm/19¾" high, hemispherical end to end. That being said, this form factor is superb and that would hold 100 Ft³ at 5500 PSI. The rating for what we call 300 Bar DIN here in the US (DIN 477 part 5) will actually go to 450 Bar/6,526.7 PSI test pressure. The DIN connection rating is probably fine but, to the best of my knowledge, no first stage Scuba regulators are available that are rated by the manufacturer to operate above 300 Bar/4,351 PSI.
Titanium would probably get this configuration down from around 12 Lbs negative to around neutral. They are worn valve-down and have rubber boots. Double hemispherical ends would provide the best strength to weigh ratio.