Handy tables and charts.

[Harry's Lad]

I really can't get into a protracted dialogue over this I'm afraid.

I've never had anyone complain that they have been hoodwinked or duped or misled into buying the wrong bottle. Quite the reverse; many have written to say how close the predictions are to their own experiences. I know that is only their perception and that it almost certainly differs from actuality, but for many folk their perception is 90% of reality.

Writing code to fit the Van Der Waal correction when only two points on the curve have any relevance, i.e., 232 and 300 bar (as bottles aren't rated as far as I know to any intermediate values) would be a waste of time and I don't rise to such challenges any more.

ATB

Absolutely fair enough.

I made the comment in the first place because I could only imagine that you were unaware of the considerable error - at 300 Bar - caused by your oversimplification. It now seems that you were well aware of the errors but did it anyway.
Personally, I wouldn’t do that but each to his own.

Only your 300 Bar point is affected – the 232 Bar point would be near enough – as explained above.

ATB
Dave

[snock]

Is it possible that by attempting to belittle someone else's work, Dave, that you feel you're in some way trying to bolster Chairgun as the prime source of ballistic data for airguns? I'm afraid that's how it's looking, to me, anyway.

Chairgun isn't particularly accurate either. Sorry to drop that on you, but Chairgun 1.8 said that I will need 68 clicks on my big nikko if I shoot AA fields at 55 yards, zeroed as I am at 23 yards wilth all other parameters as built.

Chairgun 2.? says - for EXACTLY the same physical parameters - I need 58 clicks. Practice tells me I need 56 clicks.

At the end of the day, does it really matter about a few clicks or a few fills? No, it doesn't.

The two can live along side each other in perfect harmony without picking holes in someone else's work.

This thread's sole aim was to provide FREE tables and charts to guide those folk who like that sort of thing.


Atb,


Chairgun can be downloaded here.

[Harry's Lad]

Is it possible that by attempting to belittle someone else's work, Dave, that you feel you're in some way trying to bolster Chairgun as the prime source of ballistic data for airguns? I'm afraid that's how it's looking, to me, anyway.

I’m not trying (or have tried) to belittle anyone or anything – simply pointing out that an error exists. I sited FillCalc (not ChairGun) as a source of comparison (although the same FillCalc algorithm does exist ChairGun2/3). You were the first to mention ChairGun in your post #24. I’m left wondering why you would accuse me of ‘in some way trying to bolster Chairgun as the prime source of ballistic data for airguns’ when it was you who introduced the topic in the first place . . .

Chairgun isn't particularly accurate either. Sorry to drop that on you, but Chairgun 1.8 said that I will need 68 clicks on my big nikko if I shoot AA fields at 55 yards, zeroed as I am at 23 yards wilth all other parameters as built.
Chairgun 2.? says - for EXACTLY the same physical parameters - I need 58 clicks. Practice tells me I need 56 clicks.

As mentioned above (my answer in #25 to Rich’s specific reference in #24) ChairGun has evolved, and continues to evolve, subject to specific and changing criteria. ChairGun1 had problems (and errors) in certain areas (although these were resolved by the last – V1.99 – iteration). Changing requirements lead to the adoption of a better mathematical model that had benefits in all areas and ChairGun2 was born.
Two clicks at 55 Yards is something around ¼ MoA; pretty good when ChairGun2’s specified accuracy is +/- ½ MoA. Did you actually measure the BC or assume a nominal value?

At the end of the day, does it really matter about a few clicks or a few fills? No, it doesn't.

Quite true. However, does 1/8” Inch of error in 55 Yards really relate to 33 to 24 fills difference (37.5% error)? The former is almost negligible, the latter monumental.

The two can live along side each other in perfect harmony without picking holes in someone else's work.

Again, I’m not ‘picking holes’ or ‘belittling’ anything or anyone – simply pointing out that an error (a correctable error at that) exists. The more serious point is that Rich (not Rob, sorry), having admitted prior knowledge of the essential Van der Waal correction, failed (for whatever reason) to make said correction and released the flawed spreadsheet anyway. If you’re not bothered by that then perhaps you don’t understand the problem or my concerns.
It may have been done quickly … it may have been for a good and worthy cause … but, when all is said and done, it’s just plain wrong.

This thread's sole aim was to provide FREE tables and charts to guide those folk who like that sort of thing.

FillCalc is free too.

ATB
Dave

[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