I've looked at the steel tanks versus aluminum. The difference in
initial cost doesn't seem to make sense for a little more capacity. If a large
reserve of air is needed, it seems more economical to use a welding gas tank.
Most any weld supply can fill them with CDA ( clean dry air ) as this is used
in industrial applications.
A new aluminum scuba tank is $140 for 80 cu. ft.
A new Faber steel scuba tank is $350 plus for 100 cu. ft.
A new steel industrial tank is $160 for 140 cu. ft. If you get the big
one, at 240 cu. ft. it's only $189.
Now, loading one of those into a sub will be a serious pain, but if it
was mounted outside, it would be fairly easy.
I'm not sure how "breathable " that air is.
For some serious capacity, you could use liquid CO2, but can't use it to
breathe.
Anyway, I'll be using the aluminum 80's because it's just simpler. Easy
to fill at any scuba shop, small enough to handle easily, rust proof, and
replacement or extras are fairly cheap. Keeping all the tanks the same makes
them interchangeable.
Frank D.