Volume and pressure

[East West]

I had a question which has been difficult to fully trust the answer . We were taught that volume and pressure remain constant so the exact cylinder of gas with 500 psi and 20 cubic feet will have exactly 40 cubic feet with 1000 psi . I believe there must be a variation with that formula . As the pressure increases there must be resistance which will affect the volume . I was even going to make a telephone call to a industrial gas supplier to ask this question . thanks Charles

 

[cloudflint]

The pressure will be inversely proportionate to the volume.

If you have 40 cubic feet of air at 500 psi and you halved the size of your container then the pressure would double, likewise if you doubled the size of the contained the pressure would half. if you put in more gas either the volume would have to increase keeping the pressure the same of the pressure would have to increase keeping the volume the same.

Not sure exactly what your asking so i just gave a generic example of how pressure vs volume works with Boyle's law?

 

[DevonDiver]

Charles.... not sure what you mean? resistance from what?

Volume and pressure in a flexible container.... therefore the elasticity of the flexible container needs to be calculated and included?

 

[rongoodman]

The ideal gas laws are only "ideal" over a relatively narrow range of temperatures and pressures. Beyond that, more sophisticated models have to be used. I've seen references to this begining to happen when doing fills on 300 bar(4500 psi) cylinders.

Here is a little more about it.

 

[gander]

The ideal gas law is simply PV=nRT; Therefore, if comparing two volumes you have

(P1V1)/T1 = P2V2/T2

If the temperature is constant, then you can see the relationship is inversely proportional. If you assume that you are outside of ideal gas laws things change a bit, but it is generally insignificant for our purposes.

 

[fnfalman]

PV = nRT.

If you want to get fancier then you need to expand to include Equations of State (Redlich-Kwong, et al).

 

[PfcAJ]

PV = nRT.

If you want to get fancier then you need to expand to include Equations of State (Redlich-Kwong, et al).

That equation works pretty well in scuba diving pressures. Past 4500ish psi, it doesn't really work at all.

 

[rjack321]

I had a question which has been difficult to fully trust the answer . We were taught that volume and pressure remain constant so the exact cylinder of gas with 500 psi and 20 cubic feet will have exactly 40 cubic feet with 1000 psi . I believe there must be a variation with that formula . As the pressure increases there must be resistance which will affect the volume . I was even going to make a telephone call to a industrial gas supplier to ask this question . thanks Charles

Its close enough for diving calculations, but not perfect. The math goes increasingly arwy with helium since its compressibility is quite different than air.