cu ft in a gas cylinder

[Guess who]

When you're right, you're right. Trying to make the point, I said something dumb. It happens.

[jo4hn]

A watched Boyle never goes to pot. [old Swedish saying] j4

[Steve Peterson]

There is a phase diagram of CO2 at

with explanation.

Steve

[WillR]

Yep -- got a few things -- it was small as usual, but I arranged for=20 some purchases for the next few months so...

--=20 Will R. Jewel Boxes and Wood Art

power of accurate observation is commonly called cynicism by those=20 who have not got it.=94 George Bernard Shaw

[Robatoy]

PUN FIGHT!!! (Starts off with a martial arts move from India, aka the Punjab...)

[George E. Cawthon]

Actually that isn't true. The properties of water are quite different from those of carbon dioxide. Sublimation of water also occurs at normal atmospheric pressures.

[George E. Cawthon]

All true. But none of that changes the fact that the question is erroneous.

[George E. Cawthon]

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.

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.

[fredfighter]

Sure it is. Water does just that.

I didn't say that water could did sublimate at normal atmospheric pressure. The vapor pressure of water over ice is non zero, same as for a lot stuff.

I said water goes gas to solid and solid to gas (sublimation) with no liquid phase at very low presssure, which is true. Carbon dioxide does the same, at atmospheric pressure, gas to solid, solid to gas, with no liquid phase.

[Mortimer Schnerd, RN]

Within the cylinder, yes. But cylinders aren't sold according to their empty volume; they're sold and rated according to the volume of gas they can hold at their rated pressure.

OK, I'll give you that point. My bad. But I'll bet the tank will blow long before the air liquifies. 3000 psi scuba cylinders will blow the safety disk at around 4000 psi; get hydrotested at 5000 psi, and will catastrophically fail at around 7500 psi. Will air liquify at 500 ATM? What gases would? I pulled out my trusty old CRC Handbook of Chemistry and Physics but couldn't find the information.

But it can certainly be pressurized to hold much more gas than .5 cubic feet. Otherwise there's no way in hell it can hold more than it holds at one atmosphere.

[George E. Cawthon]

No it can't. How can you get more than .5 cubic feet of air inside a cylinder that only has .5 cubic feet of volume? Please explain how you can get more volume inside a cylinder than the volume of the cylinder. Inquiring minds want to know?

And why are you fixated on assumptions about the cylinder? It is just a cylinder. You don't know the purpose or anything else about the cylinder.

[George E. Cawthon]

I know you didn't. You should have.

Not true, doesn't have to be very low pressure. It happens all the time at atmospheric pressure.

[Doug Miller]

Certainly it *is* true. Or do you claim that water does *not* pass directly between the solid and gaseous phases at very low pressures?

He stated that this occurs at very low pressures. That is true.

He did *not* state that it does *not* occur at normal pressures. You seem to be under the impression that he did.

Yes, thank you, we know that already. Who claimed that it didn't?

[Doug Miller]

By compressing it, of course, so that the same *mass* of air that occupies 80 cu ft at standard pressure now occupies only 0.5 cu ft and easily fits inside the cylinder.

Do you really have a hard time understanding this concept? Or are you just being pedantic? Inquiring minds want to know....

[Todd Fatheree]

There's no need to be intentionally obtuse just to try to prove a point. The subject contains the phrase "gas cylinder". Now, maybe in your world, that's any old cylindrical object that contains matter in a gaseous form, but most humans interpret that to mean a (mostly) cylindrical object designed to contain a gas at pressure. It's obvious from the freakin' question that the gas is pressurized...otherwise the question would have never come up.

todd

[Todd Fatheree]

The phrase I used was "intentionally obtuse", but "pedantic" fits even better.

todd

[fredfighter]

Did you ever see this show called Dr Who?

A gas cylinder is constructed like a tardis you see so that

[fredfighter]

You can if the cubic feet are _standard cubic feet_ because a standard cubic foot of gas is independent of the actual volume of the gas.

...

'tis reasonble assumption, given the question.

[George E. Cawthon]

Like to argue for the sake of it, Doug?

Inclusion of the stipulation of "very low pressure" in his statement implies that very low pressures are needed. Otherwise why throw it in?

[George E. Cawthon]

Possibly being pedantic, but more like trying to indicate that sloppiness in writing results from faulty thought processes or lack of information. The point I was trying to make was a basic understanding of states of matter would negate that kind of question. The point was, and is, that a gas occupies whatever space it is allowed to occupy. And the volume it occupies, of itself, tells nothing about the mass.