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> 
> Due to molecular interactions, in real world physics at very high pressures
> we get less volume than predicted via the ideal gas law.
> Using the Van der Waals equation of state we get the following quick
> example (at 70 degrees F):
> 
> Pressure           Volume                   Volume
> (PSIG)             Ideal Gas Law            Van der Waals
> 
>  1000               68.05                     71.13
>  2000              136.09                    143.55
>  3000              204.14                    207.14
>  4000              272.18                    257.10
>  5000              340.23                    295.33
>  6000              408.28                    325.14
>  7000              476.32                    349.06
>  8000              544.37                    368.77
>  9000              612.42                    385.39
> 
> These calculations were done on a 1 liter tank, with resultant volume
> expressed as delivered gas liters...

tank capacity rule(approximation for N2):
300bar tank: `ideal` capacity - 10%
400bar tank: `ideal` capacity - 25%
At a higher pressure (less than 1000bar) there is no increase of the `real`
capacity if you increase the tank pressure... 





John,


I have also done some calculations with Inertgases in MATLAB, it is interesting
to look at the big deviations from the ideal gas law at high pressures.
For example He and H2 are much more `non-ideal` than Nitrogen.
If anybody is interested, I can mail those Matlab files to this group. 
I also used the Van der Waals equation for calculation. But there is my problem:
I am not very fit in physical chemistry and so I do not know the limits of this
equation.

I have two questions concerning your calculations:
a) Is the van der Waals equation the proper tool for calculating the p-V
Relation
   in our pressure range (100bar-600bar)?
   How accurate is it? Do we need (much more uncomfortable) equations like
   those of Beatty-Bridgeman?
b) How do you calculate the p-V Relationship for mixes (for example: air, 
   79% N2 and 21% O2)? Is a linear superposition of the two gases in the 
   van der Waals equation possible if you want to calculate (for example) air?
   (I don`t think so personally, but I can`t give a detailed explanation)



-- 
Juergen Poniatowski, dg78@hr*.th*.de*
Schlossgartenstr. 1, 64289 Darmstadt, Germany
Tel: [49](0)6151/781555; 
            621/301602

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