Chrono Calibration

[RobertBrindley]

I think what colinb is offering is a device that will confirm the sensor part of your chrony is working. He will trigger it with a known time and as long as you measure you sensor distance correctly it will show the speed the chrony should be indicating. If it's differnet then your chrony is out of calibration.

 

[colinb]

Evening Colin
Thats the big fall down for me. Pellets or should we just say the projectile. as we know they all do different things in different rifles .
take my Ripley set with JSB 10.34's 700fps . 11.2 fpe . it then puts out bizzy mags at 695fps then take my HW100 set exactly the puts the same bizzys out at 634fps at 9.4fpe .
The thing with chrones is there is no master chrono and no master test projectile or rifle . which to set them with .
thats why they all work a plus/minus percentage.

So even with advanced software you still have to pass a projectile between the two points to get a time.
then even the distance between the crown and the first cell will make a difference . spec the chrono at 6 inch from the first cell . then you get someone chrono at 8 inch or 4 inch it then out .
there are so many variables that you would probably never know what you were actually looking at .
Its kind of the same with car speedo's . see the actual clock with a machine will say one thing . then fitted to a car will say something else . and if you change wheel size say 15's to 17's

I wouldn't blame the chrono for variations too much. They pretty well tell the truth within a jitter/resolution of a couple of fps and if projectiles behave in a surprising and different fashion I'd be attributing that to a difference in the gun characteristic, maybe peaking at different pellet weights? My homemade chrono tells the truth within the resolution capabilities of the counter. It triggers when 8% of the light quickly stops. A pellet causes a 25% drop for 4.5mm, so a pellet waist does not cause a false end pulse as no waist is less than 2.5mm or so, with 15% drop input drop remaining. (There are other buried safety factors too).
Also, electronics introduces a characteristic called hysterisis. This acts so when a pellet arrives the decision threshold shifts so a small subsequent signal implying that the pellet has left is ignored, and almost all the signal has to disappear before "pellet gone" is signalled.
If they ever agree on a standard projectile, it really has to be a lightweight slug.
A major reason for the +/- spec is that the manufacturers cannot be certain about the actual distance between sensors. (Not dissimilar to your wheel size analogy) A bigger distance is a lower (better) spec BUT then the pellet slows down more between the sensors. I know that the Nate chrono has sweated blood, tears and sleep in the design stage to address all these issues. A second spec reason in a cheapy is the cost of oscillators and dividers, but that factor is not great nowadays.
So you can't win easily.....but.....
Dead right about distance from the crown. So a cheap chinese chrono on the muzzle should read about the highest. A longer chrono must read rather less, etc. I do have a fix for this. but it needs a little bit of maths. If a chrono had 3 sets of sensors you can get two answers., with the one furthest away being slower. Assuming a linear deceleration over a short distance, and knowing the distance from crown to first sensor it's quite easy to extrapolate (a wonderful word) the pellet speed back to the instant of leaving the crown.
2 cheap chinese chronos might succeed in doing the same job but I suspect they may be a bit "noisy" This is also ignoring the argument about does a pellet speed up slightly on leaving a crown. The three sensor crono, muzzle mounted could settle that old chestnut! I might just make one....

 

[colinb]

On my Chrony I wrote a Sensor Calibration function. You slowly insert the depth gauge of a caliper into the chrony and the display tells you when each sensor triggers. You then take the first reading off the last and it gives your actual sensor distance. This is then input into the menu.

colinb's tester would then give me the final bit of information needed to calibrate my chrony correctly without having to compare to another calibrated device.

I like that feature!

 

[1260engineer]

I like that feature!

As alreadt state . it works from the back of the pellet not the front so pushing something through will trigger of the front .which means you then have to take into acount the length of the pellet .

 

[ed_t]

The biggest problem in all this is the pellet .weight . dimensions. damage . and you can only fire it once .
and thats before you start looking at the barrel . dirty / clean . just cleaned . leaded in .
rifle mechanics. reg /unregged . blar blar blar .

That's a secondary function so irrelevant to the primary function of measuring speed.

It is relevant to energy but that is ke=1/2.m.v^2, and the chrony only measures v.

 

[RobertBrindley]

As alreadt state . it works from the back of the pellet not the front so pushing something through will trigger of the front .which means you then have to take into acount the length of the pellet .

We are only interested in the sensor length so in this mode you could just check for dark (pellet cutting off the sensor). I always measure from the front on my chrono anyway as I have a norrow sense tube. Edit or you measure from the rear of the chrony?

 

[ed_t]

Assuming a linear deceleration over a short distance,

Is that a fair assumption or a bodge? Linear deceleration in air at a 2nd power versus drag in air with a cubic function?

I've always been iffy about people putting chronys in series because the drag is so high.

 

[smartshooting]

From my own experience with with test equipment, if its quality to start with then it is cost effective to get it calibrated but at the cheaper end of the market its simple economics just to replace with a new one. I have a 40r old Fluke 77 DMM thats still in calibration tolerances now but have binned and replaced many other peoples cheap DMMs in that same time period.

 

[colinb]

Is that a fair assumption or a bodge? Linear deceleration in air at a 2nd power versus drag in air with a cubic function?

I've always been iffy about people putting chronys in series because the drag is so high.

We are only talking about a few inches. So over such a small distance it's essentially linear. But if one had the processing power the velocity drop would let one estimate a 2nd power curve. However, close to the muzzle no-one really knows what other effects are in play.

 

[colinb]

From my own experience with with test equipment, if its quality to start with then it is cost effective to get it calibrated but at the cheaper end of the market its simple economics just to replace with a new one. I have a 40r old Fluke 77 DMM thats still in calibration tolerances now but have binned and replaced many other peoples cheap DMMs in that same time period.

I still possess a Datron 1281 and a Wavetek 9100 calibrator. Not quite a good match but one checks the other and the 1281 has self cal.

 

[smartshooting]

I still possess a Datron 1281 and a Wavetek 9100 calibrator. Not quite a good match but one checks the other and the 1281 has self cal.

Nice. The issue with calibrating anything is always having the confirmed reference to start with. With DMMs we used to get one calibrated to NAMAS standards every year and then check every other against. Not sure what the standard would be for a chrono or where to get one certified that could be used as reference?

 

[ed_t]

We are only talking about a few inches. So over such a small distance it's essentially linear. But if one had the processing power the velocity drop would let one estimate a 2nd power curve. However, close to the muzzle no-one really knows what other effects are in play.

Fag packet calculation and you're looking at clocking 200 microseconds over 50mm. 50m acceleration of the order -300 m.s^-2. Those numbers add a big error if peĺlet speed and air pressure flow are critically choked.

Give me some helium and i'm sure we could up the numbers.

 

[NateChrony]

Is that a fair assumption or a bodge? Linear deceleration in air at a 2nd power versus drag in air with a cubic function?

I've always been iffy about people putting chronys in series because the drag is so high.

You're correct to be iffy, but most chronographs dont have the resolution to be able to detect the difference.
With my setup & my units I can detect a ~0.15fps difference at about 100mm. i.e. if I swap the order of calibrating units on the muzzle, I can notice a drop of 0.15fps at around 800fps using wad cutters.
There is a HUGE amount of work in hardware and software to get down to this level of resolution.

 

[RobertBrindley]

I use my Chrono for the following.

Priority 1 The Law. Does my airgun stay below the power limit at the muzzle with all weights of pellet?
We can theorise all we like about how a pellet behaves on leaving the muzzle. The law gives us sticked rules which we have to follow.

There is no chronograph available that will tell you the actual speed at the muzzle we rely on a average over a set length.

The Durhan Constabulary have released their Air Gun testing procedure to the public. They use a SKAN Mk10 which I think has an 8" sensor distance. They minimum distance they are allowed to shoot from the scan is 12". In their procedure no where does it mention doing calculations to correct for the loss of power over this quite considerable distance. Other forces may well differ.



Priority 2 Accuracy. PCP what pressure range is my airgun firing at its flattest power range? What pellet works best for my gun?

Again I am quite happy to use an average reading over the 120mm sensor distance of my chrony I don't care if it drops .5fps over 120mm it's consistency I am looking for.

Priority 3 Fault finding. Has the power dropped or become inconsistent?
Same again here my chrony must be consistent or how can I tell if it's my airgun or the Chrony.




What colinb device will give us is a way of proving one part of our Chrony is performing as it should. It will tell you if the sensor readings are accurate and it will tell you if they are consistent.

In real life you will have to be careful about weighing your pellets correctly (a big problem) and also is your chrony hardware/software sensing pellets correctly.

Chronographs readings are a collection of cumulative errors and colinb kind offer to design this device will help knock one of those on the head.

 

[colinb]

Nice. The issue with calibrating anything is always having the confirmed reference to start with. With DMMs we used to get one calibrated to NAMAS standards every year and then check every other against. Not sure what the standard would be for a chrono or where to get one certified that could be used as reference?

There is not really a single standard for a chrono, because speed is not an easily repeatable fundamental quantity, unless you could use the speed of light. It is derived from distance and time. (Like OHMS is derived from VOLTS and CURRENT) It's possible, though difficult, to make a standard of sorts, but it would have very limited use. The early days of ballistics pre electronics make a fascinating history. I've always wanted to try some of these, brought up to date.

 

[NateChrony]

unless you could use the speed of light

In a vacuum right?
whats the vacuum?

How about a light source, split with a prism which partially reflects into both sensors?
Same beam, split in 2 - extra distance, known time... easy!

 

[NateChrony]

The Durhan Constabulary have released their Air Gun testing procedure to the public. They use a SKAN Mk10 which I think has an 8" sensor distance. They minimum distance they are allowed to shoot from the scan is 12". In their procedure no where does it mention doing calculations to correct for the loss of power over this quite considerable distance. Other forces may well differ.

I'll be doing a bit of work soon around "testing procedures"... i'm 90% confident there's large gaps & deficiencies in current procedures that would really open up a can of worms. I've seen the "courses" that are provided in training in the use of chronographs - there's a decent lack of standards & consistency.
This specifically relates to the way optical Chronographs are used - will do a few articles/posts about it.

 

[colinb]

In a vacuum right?
whats the vacuum?

How about a light source, split with a prism which partially reflects into both sensors?
Same beam, split in 2 - extra distance, known time... easy!

We are into fantasy land! But what fun! The first measurements of the speed of light used 2 lanterns from 2 adjacent mountaintops. One guy uncovered his lantern and timed the response of the 2nd guy. Conclusion? It's pretty fast.
A prism is a concept but I can't see how it would be practical. How would you slow down or lengthen one path relative to the other? I have no clue as to the speed of light in glass nor of variation in Refractive index with light frequency. As said above, a calibration chain has to consist of stages where each successive upward link is about 4x better that the previous one. And that chain has to be unbroken up to National Standards.

It has occurred to me that for most chronos, let's assume ( very bad word!) that it is correct at delivery. . There has to be an internal clock- the most critical component. If that clock is made available at a test point it would be easy to check it externally. Then subsequently, if the clock has not drifted and the chrono otherwise works, then it's good. The downside is the need for an external measurement device. But being pretty common, they are not stupidly expensive.
But it remains that a cheap external, independent, simple device, simulating a pellet passing the sensors and utilising the full chrono, is a very satisfying method as it tests and calibrates in one go.

 

[RobertBrindley]

I find all this accuracy stuff facinating. Have just ordered a calibrated 1g weight (class M 0.1mg ) to test my 10g digital scales. The weight cost me 3 times the cost of the scales. I have a few scales will do a mini test with my weight.

I think Nate it's about time you built a replacement for the SKANs the police use and colinb built them a calibrator.

Again from the Durham Police official test persedure.
This is one of the tests they perform on their SKAN before carrying out a forensic test on a suspect rifle.

"Use the chronograph to measure the velocities of four 0.177 (4.5mm) calibre air gun pellets(average medium pellet weights) fired from the Webley Alecto pneumatic .177 (4.5mm) aircal. air pistol, serial no. TRAT0111W 02013. The gun and the pellets are kept in the FCEL. Record the velocities on the Chronograph Test Results Spreadsheet (FRM_024_FCEL) andChronograph Check Form (FRM_008_FCEL). If the velocities are between 360 and 390 feet persecond, the chronograph can be deemed to be working normally. If the velocities are outsideof the range 360 and 390 feet per second, the procedures described in the Equipment,Calibration and Estimation of UoM Procedure (SOP_013_FCEL) must be followed."

There has got to be a better test than using an old air pistol!

 

[colinb]

I find all this accuracy stuff facinating. Have just ordered a calibrated 1g weight (class M 0.1mg ) to test my 10g digital scales. The weight cost me 3 times the cost of the scales. I have a few scales will do a mini test with my weight.

I think Nate it's about time you built a replacement for the SKANs the police use and colinb built them a calibrator.

Again from the Durham Police official test persedure.
This is one of the tests they perform on their SKAN before carrying out a forensic test on a suspect rifle.

"Use the chronograph to measure the velocities of four 0.177 (4.5mm) calibre air gun pellets(average medium pellet weights) fired from the Webley Alecto pneumatic .177 (4.5mm) aircal. air pistol, serial no. TRAT0111W 02013. The gun and the pellets are kept in the FCEL. Record the velocities on the Chronograph Test Results Spreadsheet (FRM_024_FCEL) andChronograph Check Form (FRM_008_FCEL). If the velocities are between 360 and 390 feet persecond, the chronograph can be deemed to be working normally. If the velocities are outsideof the range 360 and 390 feet per second, the procedures described in the Equipment,Calibration and Estimation of UoM Procedure (SOP_013_FCEL) must be followed."

There has got to be a better test than using an old air pistol!

How interesting! And absolutely a total joke of a test. Any decent barrister would drive a coach and horses through that one. The pistol goes faulty so they "calibrate" the chrono.....It looks like there may be a market there.....
My belief (no evidence) is that the SKAN is originally designed for nice smooth wide fast bullets. So speed and size are maybe 4 times more than for airguns.
I would wonder if yet more errors are introduced by the smaller pellets, especially the waist that can give a false result.