Mortimer Schnerd, RN wrote:
George E. Cawthon wrote:

mawtg wrote:

does anybody know how a gas cylinder measuring 6" dia and 32" tall can
have 80 cu ft of gas? how is this figured?


The answer is that it can't. It can only hold
about 1/2 cubic foot (if I did the math
correctly). Doesn't matter what the gas is,
doesn't matter what the pressure is, it is always
the same volume because gases expand to fill the
volume of the container. The question is
illogical probably based on a statement that was
incomplete, or part of which was ignored in
stating the question.



Not quite. Your statement is incomplete as well. That .5 cubic foot that you
suggest only holds true at atmospheric pressure. If you compress the gas
within, you can hold virtually any amount up to the bursting pressure of the
tank. On 80 cf scuba cylinders which are only a little bigger than his example,
you'd charge them to about 200 atmospheres (3000 psi) to get the 80 cubic feet
packed in there. It is understood that the 80 cf measurement is what the
cylinder holds under pressure, not empty.

No it isn't. The volume of the cylinder is about
0.5 cubic foot, so the amount of gas that it
contains is ALWAYS 0.5 cubic foot; it doesn't make
any difference what the pressure is. Your 4th
sentence is also incorrect. At some pressure and
temperature you get liquid air which fills the
cylinder and at that point you can't put any more
air into cylinder since a liquid is only slightly
compressible. Your last point is also incorrect;
there was is no assumption about the pressure, and
it still matters not a whit since the cylinder
volume is 0.5 cubic feet so that is all the air it
can hold no matter what the pressure is as long as
it is gas.


AGA Divator used to make twin 40 cf systems that required them to be pumped up
to 4400 psi. Now that is one hell of a lot of pressure. They were little
things that fit closely to your back but they were ungodly expensive and most
dive shops couldn't fill them. But they packed the same 80cf as the larger
cylinders.



Your assumption is that we are talking about
cylinders of air for scuba diving at specific
pressure. The OP did not say anything that would
indicate that. He did provide enough information
that you could figure out the pressure needed in
the cylinder to have 80 cf of gas at one
atmosphere. Since it is 80 cf and cylinder is 0.5
cf it needs to be compressed about 160 times. One
atmosphere is about 15 psi, so the psi needed is
15 x 160 about 2700 psi. All of which has nothing
to do with what I said and the general lack of
understanding of states of matter.