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Use the calcualtor at

http://www.processassociates.com/process/property/z_factor.htm

You need to know the critical pressure and temperature ( you can just
look it up, there are different numbers for different gases ).  If you
need the formula, go to

http://members.xoom.com/almuhtaseb/papers/1/

The formula is in the appendix.


Dan MacKay wrote:
> 
> I have a general question for the list dealing with the compressablity
> of the various gases used for mixed gas diving. I have found by
> experience (and Georges video) that ideal mixes of gas do not normally
> come out exact. When using electronic gagues I find that I can be
> quite accurate with the mixes. Unless I allow for the different
> compresibility factors of the component gases in the mix then there
> can be significant variance in the target mixes. This is particulary
> noticable in mixes with a high target fill pressure  (3000PSIG+) with
> a high helium content.
> 
> Another big factor of course is the temperature. I find that if I mix
> up in my garage on one day and take my tanks to the shop to get topped
> up the next then there can really be a significant variance
> particularly if they toss them in the tank to fill them to my
> requested target pressure. For the dive shop I normally frequent I
> have more or less have them trained now to just fill them nice and
> slowly outside of the tank when they have the time. But in situations
> where on remote locations where I am filling here and going there I
> can not always controll the temperature. I can eyeball it (as I do now
> with the O2 and He ratios) and come up fairly close.
> 
> What I was wondering if there were any gas gurus around that have
> access to the formulae that allow one to adjust compressiblitiy
> factors and temperature?
> 
> I know that this is a little anal but I wanted to include this in a
> blending software package I am developing I thought that it would be a
> nice addition.
> 
> Dan.

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