bhpeow,
I just stumbled on this question, if you're still interested:
There are hundreds of versions of the equation that dcasto put up. All of them start with the data from the GPSA Engineering Data Book figure 20-4. The equations behind that graph are lost in the mists of antiquity. The oldest version I've seen is in Katz Handbook of Natural Gas Engineering that was published in 1959 and it was an old relationship then.
The GPSA version has so much data on it that it is difficult to see that the curves are not a log-log straight line. They flatten slightly at higher temperatures (the slight flatening makes a huge difference on the log scale).
I blew a copy of GPSA 20-4 up to "D" size and then used t-squares and triangles to very carefully pick the points. Then in a week of very dense MathCad I came up with an equation that honored the curve (dcosta's doesn't, don't use it above 100F). Problem was that the high pressure curves have a different bend than the low-pressure curves. I chose to force the fit to 100% match under 50 psia and accept a 15% error at 1,000 psia and 150F. This lets me have some confidence in my arithmetic below 0 psig.
I haven't published my equation because I'm still trying to decide if I want to. The purpose of this post is to try to point out the process that someone needs to go through to develop their own empirical equation.
