Little confused here. One very common dive tank size is listed as 80 liters at 3000psi. The tank itself is obviously much smaller than 80L - it’s a cylinder about 8 inches in diameter and 24 inches tall (thus around 1230 cubic inches or 20 liters).
Where does the 80 liter measurement come from? It doesn’t appear to be the volume of air at normal atmospheric pressure (14.7psi) that you’d get if you filled the tank to 3000psi (wouldn’t 20L at 3000psi give you about 4000L at 14.7psi, just p1v1=p2v2?).
Yes, 80 cubic feet, not 80L. Oddly enough my little spreadsheet that I’ve been using for calculations has 80*1768 (to get it into cubic inches) all over the place.
Anyhow the question still remains, where does 80 cubic feet come from?
1230 cubic inches * 3000psi/14.7psi gives around 140 cubic feet which is certainly closer to 80 cubic feet.
If the above calc is right “in spirit” if not in numbers, output pressure of about 26psi (2 atmospheres) would make the numbers work out about right. This would correspond to pressure about 30-some feet underwater and makes a certain amount of sense.
And it’s also 1728 cubic inches per 1 cubic foot, not 1768. That might be a typo, though, since your computation looks ok.
Also, they might be sold as “80 aluminum”, but their actual capacity at 3000 psi is somewhere around 75-77 cubic feet. I suspect there’s an engineering standard that goes back through the ages that explains that. It might involve hogsheads. I don’t want to go there, for at the end of that route lies madness. Madness!
Plus, having looked inside scuba tanks, the outside shape is not a very good indicator of the inside capacity - those things have seriously thick walls. Especially the aluminum ones. I wouldn’t be suprised if your back-of-the-envelope size is about 2x too big, which would give you your 140 cubic foot capacity.
The physical size of the tank isn’t a good indicator. SCBA tanks, very similar to those used in scuba diving have come a long way in the last 30 years. The steel 30 minute tanks pressurized to 2015 or 2216 psi have been replaced with aluminum/fiberglass and aluminum/kevlar or aluminum/carbon fiber composites pressurized to 4500 psi, allowing the wearer double the breathing capacity at roughly equal weight.
SCUBA cylinders sold in North America are typically rated according to their surface equivalent air volume. To use a common example, the Luxfer aluminum 80 contains approximately 80 cubic feet of gas at ambient pressure, compressed down to the volume of the cylinder, which is only a couple of cubic feet. Conversely, cylinders sold in Europe are most commonly rated according to their actual volume (water capacity) - so the European equivalent of the 80 cu.ft. cylinder here would be a 12 L cylinder.