Handy tables and charts.

**Rich** · 06-04-2009, 10:12 PM

Being the original author, let me say that I am and was at the time aware that the maths were simplified and ignored the Van Der Waal issue, rather like the earlier versions of Chairgun and other ballistic software had simplified approaches to residual energy and trajectory.

Number of Fills was released at a time when money was being raised by members of this forum to support breast cancer charities, and the spreadsheet was given away on the basis that recipients who found it useful would make an appropriate donation. I have the papers on file somewhere, as I dare say the posts and threads from that era have long gone. We made about £170 for breast cancer charity on the first evening; I recall sending about 75 emails with a copy of the spreadsheet attached, before I went to bed at 2am.

There seems to be little point in re-releasing number-of-fills as pretty well everyone these days will choose a 300 bar bottle over a 232 so long as they can get it refilled relatively easily. The cost of getting to the filling shop is for many the largest single factor, so it makes sense to buy the biggest and highest pressure bottle you can manhandle.

If I were to update it, I would not bother with a true Van Der Waal correction, but simply use the present elementary maths substituting an arbitrary value of something like 285 bar whenever the user thought they were selecting 300 bar. That would give a closer approximation. In any event the accuracy is affected by whatever allowance is made for dead volume in the hose and gauge, and experience has shown me that some decant sets are far more wasteful than others.

The other factor to consider is that number-of-fills has a look-up table with the reservoir capacities for several popular rifles (and that's out of date now too.) Readers may think that getting the reservoir capacity from the manufacturer only requires a phone call. In practice one firm steadfastly refused to let me have the information declaring it was a trade secret. I ask you, how can it be a trade secret of any worth when firms such as Theoben specify the size of the buddy bottle?

The example of the S410 with 200cc is actually incorrect as later information has shown this reservoir to be 216cc. The carbine and export models have also been amended.

If I could move to the am-I-legal spreadsheet, I do commend this to all members especially in the light of current issues regarding rifles approaching the legal limit. This time the maths part does not rely on any science or engineering, it's purely a matter of statistics, more specially probability.

An over-the-counter chronograph will have an accuracy of something like plus or minus 1 percent of reading. The owner will not know whether his chrono reads high, low, or spot on. The chrono used by the Home Office testing station will be a precision piece of kit and will have a calibration certificate, with a tolerance of something like 0.4% of reading.

It would be possible for your domestic chrono to be at one end of its limit and the testing station chrono to be at the other end of its limit, a variation of 1.4%. Remember that the chrono measures velocity, but it is muzzle energy that is specified by law. As there is a square function from velocity to energy, 1.4% variation in velocity equals 1.96% variation in power.

The maths part looks not only at the rifle's velocity but the consistency of that velocity. It is the case that a rifle that is inconsistent is more likely to throw a rogue high value at some time. That is why the spreadsheet can give you the surprising and maybe unwelcome answer that ten or even twenty shots close to the limit (but all under it) bring a risk of failing a Home Office test if the rifle is inconsistent. When it says, there is a 3 percent risk of failing a test, think of it this way. You and 32 friends line up to have your rifles tested, all supremely confident that your trials at home have kept you under the limit, albeit close to it. One of you will be disappointed to learn that you are in possession of a section 1 firearm, without a licence.

Rich

**Harry's Lad** · 06-04-2009, 11:17 PM

Originally Posted by Rich

Being the original author, let me say that I am and was at the time aware that the maths were simplified and ignored the Van Der Waal issue, rather like the earlier versions of Chairgun and other ballistic software had simplified approaches to residual energy and trajectory.

The first ChairGun version, as it evolved through several iterations, was an adequate model of airgun ballistics as used in the context of airgun usage at that particular time. ChairGun2 evolved from a different ballistic model that more accurately met different (and more challenging) criteria – mostly driven by the requirement of long-range shooting in Oz and the USA. In both cases, the software operated accurately within its stated restraints.
That’s the difference; your spreadsheet is capable of giving results that, because of the known and ignored oversimplification, are erroneous. I was simply pointing out that, given the deficiencies of the simple model used, the 232 Bar + results may be seriously in error.

Originally Posted by Rich

There seems to be little point in re-releasing number-of-fills as pretty well everyone these days will choose a 300 bar bottle over a 232 so long as they can get it refilled relatively easily. The cost of getting to the filling shop is for many the largest single factor, so it makes sense to buy the biggest and highest pressure bottle you can manhandle.

Perhaps that’s a good reason for re-doing/releasing the spreadsheet since it is the > 232 Bar predictions that are in error. The conjecture was, on acquiring a 300 Bar bottle on the basis or your spreadsheet, that the punter could be very disappointed in the number of fills realised.

Originally Posted by Rich

If I were to update it, I would not bother with a true Van Der Waal correction, but simply use the present elementary maths substituting an arbitrary value of something like 285 bar whenever the user thought they were selecting 300 bar. That would give a closer approximation.

You could do that . . . or you could do it properly

although writing an accurate working algorithm in VBA could be quite challenging.

Originally Posted by Rich

In any event the accuracy is affected by whatever allowance is made for dead volume in the hose and gauge, and experience has shown me that some decant sets are far more wasteful than others. The other factor to consider is that number-of-fills has a look-up table with the reservoir capacities for several popular rifles (and that's out of date now too.) Readers may think that getting the reservoir capacity from the manufacturer only requires a phone call. In practice one firm steadfastly refused to let me have the information declaring it was a trade secret. I ask you, how can it be a trade secret of any worth when firms such as Theoben specify the size of the buddy bottle? The example of the S410 with 200cc is actually incorrect as later information has shown this reservoir to be 216cc. The carbine and export models have also been amended.

True. But then, we can only use the data that we’re sure of. Otherwise it’s just GIGO again.

ATB
Dave

**Rich** · 06-04-2009, 11:30 PM

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

**Harry's Lad** · 07-04-2009, 09:16 AM

Originally Posted by Rich

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** · 07-04-2009, 06:46 PM

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** · 07-04-2009, 08:33 PM