My recollection is that propane and butane liquify when they are compressed
to whatever their vapor pressures are at a particular temperature.  There
they sit, with some liquid going to gas as temperature rises, and
recondensing as temperature drops.

Matthew Pollard wrote:

> My thermodynamics says that you can measure the pressure by mass. Weight
> an empty tank and a full tank. The difference is the mass of the propane
> in there. Now comes some math...
> pressureXvolume=molesXgas constantXtemp
> you know v, n, r, and t.
> n=moles= mass of gas/formula weight of gas (propane is 44grams/mole)
> v=volume of the clinder. Lets say 1L
> R=gas constant=0.0821 L*atm/mol*Kelvin
> Temp=298K (or about room temp).
>
> So for 1000grams of gas, that corresponds to.... 556atm which is about
> 8173 psi. Yikes. that is high. Hmmm, somebody check my math here.
>
> This is for an ideal gas though.... Propane is NOT ideal, esp at high
> pressures. We'll need the van der waals equation for a better answer.
>
> But that gives you an idea that we have a lot of pressure here!
>
> ok, back to work
> m

[snip]