Handy tables and charts.

[Rich]

The 232 bar point would indeed be near enough, an error of about 3 percent I think.

The error at 300 bar is still only around 8 percent. As I said, it hasn't led to complaints from users - only yours, Dave.

Compressibility is actually also temperature dependent so unless yet another input parameter is called for, it's going to have to be an approximation.

[Rich]

And it's Rich, by the way, not Rob.

[Harry's Lad]

The 232 bar point would indeed be near enough, an error of about 3 percent I think.
The error at 300 bar is still only around 8 percent.

Sorry Rich, but I don’t know where these percentages came from (or to what they refer). As I mentioned before, thermodynamics isn’t really my thing but, ‘Standard’ air behaves as the Ideal Gas that your model assumes at around 232 Bar. Above or below that pivotal pressure a mass-to-pressure error exists - below 232 Bar, the vessel’s pressure drop is slightly less than the Ideal Gas assumption anticipates; at higher pressures then the pressure drop is progressively much greater than your assumption would anticipate. Unfortunately, in the real world, the greater-than-anticipated pressure-drop means that at every iteration (fill) the pressure discrepancy increases and so, by cascading that error, there are considerably fewer fills to be had.

As I said, it hasn't led to complaints from users - only yours, Dave.

I haven’t complained. Far be it for me to complain. I’m just flagging an error.

Compressibility is actually also temperature dependent so unless yet another input parameter is called for, it's going to have to be an approximation.

Indeed it does but the temperature range we’re considering is comparatively small. Any temperature variation would apply equally to both models anyway.

And it's Rich, by the way, not Rob.

Noted and corrected. Sorry about that.

ATB
Dave

[Rich]

Perry's Chemical Engineers' Handbook. I can't give you a page reference, I no longer have a copy. It was a useful document when I was in industry.

Compressibility varies by around 5 percent between 250K and 300K, say one percent per 10K variation. 232 bar at factor 1.0 is true at 250K, maybe 3 percent adrift at "room" temperature. That is why I estimated a 3 percent error at the "base" point.

[RobF]

It's academic when you don't know the length of line used to fill the cylinder. We also don't shoot at a fixed temp... i think last year I covered 35 degrees.

I don't think anyone is going to get upset about the fill count not meeting x person's predictions... the benefits of 300 are that if your shooting at 200 bar you can hold a lot more air in the same volume for a longer amount of fills.

Reminds me... I need to get my 3L filled... i've forgotten how many fills it's done... but it's down to 185 and i'm scrounging air. Even though I shoot almost every weekend, and regularly each week, I can't for the life of me remember how many fills I get out of my 4 pcps... all with different sized cylinders.

Rob, not Rich

[tomosmond]

will they open with open office. Im downlading it at the moment and just wanted to know.

[Rich]

Tom, yes, I just tried it and it seems to work OK

Number of fills does not contain any macros.