Chronograph Calibration ?

[zooma]

We have an F1 Chrony fitted with the Chrony LED lighting umbrella that we occasionally use on the club range to check the fps of our airguns.

Although not very old, and not used that much (and carefully kept in "mint" condition) we have been happy with the constant results it gives and the results we have taken compare favourably with readings that have been obtained from alternative sources, but it has recently been suggested that perhaps chronographs need to be re-calibrated now and again to ensure that they maintain their accuracy.

I know that there is a requirement for speed cameras to be recalibrated now and again, so I suppose that the same could be said for airgun chronographs too, but does it really make any practical "real life" difference if a well cared for and seldom used good quality system such as ours is not re-calibrated from time to time?

We use our Chrony to check for any changes in fps over a range of consecutive shots so we can see how consistently any airgun we choose to test are shooting. These figures can then be converted to find out the ft/lbs when used with the weight of the pellet (in grains) if we wanted to check the power too.

A well established mathematical formulae exists for this conversion to ft/lbs and is also available online (and with some phone apps) for a quick reference. This quick reference is only a guide but we can double check the results "long-hand" if we wanted to be certain or if any result comes close to giving a higher power rating than we would be "uncomfortable" to use on our club range as we will not allow any air rifle over 12 ft/lbs and no air pistol over 6 ft/lbs to be used.

Most target grade match pistols and match rifles fall well below the legal maximum permitted legal limits of 6 and 12 ft/lbs respectively, but some of the sporting rifles come a lot closer and so we would always want to check them to make sure that they were "legal" before allowing them to be used on our club range.

I believe that the results that we obtain from our Chrony system are accurate enough to use as a reliable guide to help us check how consistently a regulator is working on a PCP or CO2 air pistol or air rifle and can also be used to check how consistently any other type of airgun ( spring powered or SSP) is shooting too.

To ensure good results, we never look at the result of just one shot, but aways take a string of at least 10 shots with one known pellet type and then repeat the test with further 10 shot strings with at least two other different known brands of pellets to get a fair spread of readings that the airgun is capable of achieving. We also take a few shots before measuring the fps (especially with springers) to allow the rifle to "settle" as we have noticed that sometimes the first few shots can be more variable.

Until now we have always been happy to consider the results that we take to be more than good enough to identify any airgun that is over the legal limit accurately enough to prevent it being used in our club if we can see it has more power than we are legally entitled to use under the current UK law.

If our results identify an airgun as being too powerful (ref above) I would also advise the owner/user not to shoot it without suitable firearms certification outside of the club range either as it could be considered to be an "illegal" airgun and there is a possibility that the airgun could be seized and the holder/owner/user could face prosecution under the current firearms laws if it were to be tested by an enforcement agency before it had been adjusted to shoot with less power and within the legal limits.

I understand that if a prosecution should be made that the enforcement agency engaged by the police to support the charge would need to be able to prove that their equipment had been calibrated recently enough to pas as "fit for purpose", but I believe that to be unnecessary when we only use our Chrony to assist with accuracy and pellet speed testing or to identify and advise if an airgun produced more power than our current law permits us to use without appropriate certification.

[Jesim1]

Just check it against other Chrono's - they are pretty much all rated to within around 1% of true, so on an 800fps .177 pellet you have about 8fps either way, and if your guns is running at 99% on the power limit more fool you

Just make sure they are all set up fairly and accurately with the correct distance, light and angle - a pellet 1m from the muzzle is already slowing down

James

[zooma]

..... and if your guns is running at 99% on the power limit more fool you


James

I agree James - its close to stupidity and I really don't see the point in "sailing so close to the wind".

Most of the airguns that I have checked to date and rejected as being too powerful are springers!

For some reason many amateur spring powered air rifle "tuners" seem to think that making the rifle "kick like mule" and having the power to knock lumps out of a stone wall is a good thing, but then they forget that the gun has probably lost most of its accuracy - so even if they were successful at hunting down that dangerous lump of concrete that they need to destroy - they probably couldn't hit it if they were stood on it

[AC all day]

When I had a Skan Mk7 it had a selfcheck button on it but I did send it back for recalibration once or twice, but it was being used in a professional capacity, airguns and full bore loads development.

[zooma]

To be fair, I have also checked some HW97K in .177 that have turned-out to be bog standard inside, but they have been over the legal limit when tested with a range of different pellets as the power had increased from when they were bought new!

Some spring powered air rifles just gain power after they "bed-in" with use, often leaving the unsuspecting owner quite surprised to find that when their air rifle was tested at a later date it had gained enough extra power to become "illegal" (ie - over the 12 ft/lb maximum limit).

I suppose the answer is to check your airgun over a chronograph from time to time and be prepared to take some advice from an experienced air gun gunsmith or tuner who can suggest how to lower the power enough to make it legal again.

I have yet to come across an over-powered spring powered air pistol that had been brought into the club - lets hope it stays that way!

[PhatMan]

Bob,

With a rifle new to me, I always test on two (2) different 'reference' type chrono's - usually a SKAN and LMBR R2A.

I too have found that all my chorno's agree within reasonable limits.

To get a reasonable estimate, you will need to chrono' at least thirty (30) shots, and have a good idea as to the average pellet weight (weigh 100 pellets, and divide weight by 100).

Calculate the hypothetical maximum velocity : mean and standard deviation (SD) of the thirty (30) shots, then the mean + (3.3 X SD).

Then work out the hypothetical maximum energy from hypothetical maximum velocity (HMV) and weight of pellet :

( 'pellet weight in gr.' X HMV X HMV / 450660)

The value 450660 is g X 7000 X 2, where g is acceleration of free fall, and is ca. 32.16 f/s² where I live.

This hypothetical maximum energy is unlikely to be exceeded in 99.9 % of times.

For me, this hypothetical maximum should be 11.5 ft.lb or less.

Have fun & a good weekend

Best regards

Russ

[zooma]

Bob,

With a rifle new to me, I always test on two (2) different 'reference' type chrono's - usually a SKAN and LMBR R2A.

I too have found that all my chorno's agree within reasonable limits.

To get a reasonable estimate, you will need to chrono' at least thirty (30) shots, and have a good idea as to the average pellet weight (weigh 100 pellets, and divide weight by 100).

Calculate the hypothetical maximum velocity : mean and standard deviation (SD) of the thirty (30) shots, then the mean + (3.3 X SD).

Then work out the hypothetical maximum energy from hypothetical maximum velocity (HMV) and weight of pellet :

( 'pellet weight in gr.' X HMV X HMV / 450660)

The value 450660 is g X 7000 X 2, where g is acceleration of free fall, and is ca. 32.16 f/s² where I live.

This hypothetical maximum energy is unlikely to be exceeded in 99.9 % of times.

For me, this hypothetical maximum should be 11.5 ft.lb or less.

Have fun & a good weekend

Best regards

Russ

Hi Russ,

I understood a couple of things in your method and guess that it can calculate the power of an airgun very precisely- but I am not sure that I understand all of it.

We thought that we tested quite well, but your testing methods are “laboratory inspired” compared to ours - and could become a complete self contained hobby within itself - and probably needs more time and kit than most clubs have to check a visiting airgun or a new addition to a club members armoury.

I do have a new SKAN to supplement the Chrony but we need to be able to check the power a little more quickly if we are to get any shooting done on the same night, so I guess that our method of checking strings of 10 shots (often repeated with 3-4 different types of pellet) can only be regarded as a “good guide” - but good enough to identify any airgun that is shooting with more power than we find acceptable for use on our club range.

Meanwhile, I will read your method a few more times whilst I am away in my caravan this weekend and maybe send you an email to ask about the things I find puzzling (or just don’t understand fully!).

Enjoy your weekend!

Bob.

[tunnelrat]

Bob,

With a rifle new to me, I always test on two (2) different 'reference' type chrono's - usually a SKAN and LMBR R2A.

I too have found that all my chorno's agree within reasonable limits.

To get a reasonable estimate, you will need to chrono' at least thirty (30) shots, and have a good idea as to the average pellet weight (weigh 100 pellets, and divide weight by 100).

Calculate the hypothetical maximum velocity : mean and standard deviation (SD) of the thirty (30) shots, then the mean + (3.3 X SD).

Then work out the hypothetical maximum energy from hypothetical maximum velocity (HMV) and weight of pellet :

( 'pellet weight in gr.' X HMV X HMV / 450660)

The value 450660 is g X 7000 X 2, where g is acceleration of free fall, and is ca. 32.16 f/s² where I live.

This hypothetical maximum energy is unlikely to be exceeded in 99.9 % of times.

For me, this hypothetical maximum should be 11.5 ft.lb or less.

Have fun & a good weekend

Best regards

Russ

Hello PhatMan

Should you be dividing by 450240, rather than 450660, in your calculation for muzzle energy?

[Turnup]

Hello PhatMan

Should you be dividing by 450240, rather than 450660, in your calculation for muzzle energy?

It depends entirely on the value you choose for g. g is not a constant all over the world so to be pedantic, the chosen value for g should be the prevailing g wherever your scales were calibrated. To be practical, the difference is negligible (less than four parts in a thousand).

[PhatMan]

Tunnelrat,

Turnup has it

( I spent the better part of twenty-five years making precision measurements, so I used the local value of 'g' without thinking )

Have fun & a good weekend

Best regards

Russ

[tunnelrat]

Thank you Russ and Turnup for enlightening me. I've just done some sums and it looks like I should be using 450662.

All the best,

Craig

[RobF]

If you have a really consistent rifle then you'll find the limitations of the Skan. It might read to 2 decimal places in FPS but you'll see if you log the speeds that certain speeds are exactly the same to 2 decimal places yet they don't vary by just 0.01 away from that. The results are being rounded. If you shoot a few shots you'll see there's some 'magic' numbers and nothing in between.

[Turnup]

If you have a really consistent rifle then you'll find the limitations of the Skan. It might read to 2 decimal places in FPS but you'll see if you log the speeds that certain speeds are exactly the same to 2 decimal places yet they don't vary by just 0.01 away from that. The results are being rounded. If you shoot a few shots you'll see there's some 'magic' numbers and nothing in between.

All digital devices suffer from this kind of error. The smallest unit of time that the device can measure digitally will determine the smallest variation in velocity which can be calculated. It's caled quantisation error. The more money you spend the smaller units of time (faster internal clock and more digits in the interval value) can be measured and the smaller the quantisation error will be but it will never be zero. There are other sources of error so a good design will balancce these for a given budget.

There is a similar effect in the cheaper digital scales.

[Turnup]

Thank you Russ and Turnup for enlightening me. I've just done some sums and it looks like I should be using 450662.

All the best,

Craig

If you are measuring the weight of pellets using a balance then for an individual pellet it will read the same weight regardless of the local value of g. It is therefore necessary to use the value of g for which the scale was calibrated when converting weight back to mass. It is possible to measure mass directly but that kit is very expensive.

Anyway for our purposes the difference is much less than other sources of error so don't sweat it.

[RobF]

All digital devices suffer from this kind of error. The smallest unit of time that the device can measure digitally will determine the smallest variation in velocity which can be calculated. It's caled quantisation error. The more money you spend the smaller units of time (faster internal clock and more digits in the interval value) can be measured and the smaller the quantisation error will be but it will never be zero. There are other sources of error so a good design will balancce these for a given budget.

There is a similar effect in the cheaper digital scales.

Yep. Good job we don't use them to weigh pellets...

I think club chrono use is limited to giving an indication of legality if that's what it's being used for. Most clubs I know that do this test new members' gear when they first arrive so there's not something way over when it shouldn't be. I think getting one calibrated is probably going to blind you to all the other possible errors in the testing process. I know when we're in the workshop testing over the chrono it becomes a real pain to do try and remove a lot of them.