Chrono Calibration

[colinb]

This post makes me think about my amateur repair to my very cheap, £21 Ebay chrono, after accidentally shooting out the second LED, through careless alignment.

I quickly sourced a supply of new leds and bought ten for a few pennies, and I desoldered the broken LED.

When I soldered in the replacement, I just tagged it onto the stumps of the old LED leads which were sitting a touch proud of the board... herein lies a significant potential error. The new LED is not only perhaps 2mm higher than the original, but it is also possibly not exactly the same distance from the other LED as was the original. This if true, will have an impact on the timings, and the calculated velocity.

To be fair though, a short base-line chrono like mine, with maybe a 60 mm baseline between the light sources, would have been inherently less accurate from new than the other type. Also, perhaps it is true that the more significant part for spacing is the sensor rather than the light source LED. I need to think more about that - it may be a mistake.

Also, I am certain that neither of my unlicensed air rifles is anywhere close to the maximum power allowed on an uncertifcated rifle. The WH99s is reading 748 fps or thereabouts which is under 11 ft pounds in .177, and the little Baikal IZH38 is sitting at around 4.5 ft pounds, so I need fear no handcuffs.

My Webley Axsor which is on my firearms certificate comes out at somewhere about 25 ft pounds. All I need a chronograph for is to check the rifles are behaving normally and don't need new seals or lubrication.

I might try to calibrate mine by firing a .22LR over the thing. CCI claim the velocity of the sub sonic ammo as 1050fps. Id the chono comes out near that, I might just accept it and go with that.

I wouldn't worry. Your replacement will be near enough based on your figures. The led position is relatively unimportant- only a small effect - it's the sensor position that really counts.

 

[RobertBrindley]

ON teh problem of accurate clock timing:

I am into amateur radio building and accurate clock timing is a long standing issue there, since oscillators are the core feature of RF generation. I go back far enough in that game to the days of oscillators governed by inductance and capacitance networks (extremely vague and inaccurate and wandering with temperatures) to more modern crystal control, but they are far from great too. Then we had crystal ovens to hold the quartz oscillators at a fixed temperature, thus getting rid of some variability, and now even more controlled systems with TCXO systems. In the end, the best frequency control we can get is often derived from GPS receivers which can provide incredibly precise, timed pulses to allow frequency calculation and timing.

In order to check my clock frequency of my Chrono I used a bolt on GPS receiver about £3. Over short time periods jitter can be a problem with GPS but over longer periods they are fantastic. My Chrono basically runs overnight comparing it's clock to the GPS clock. I was quite amazed how good the ESP32 clocks were.

 

[colinb]

Hi colinb this all sounds very interesting!
I have my DIY chrono and one of my cheap HT_X3006 Chronos along with a couple of Nate devices to test with.
Let me know if I can make up any circuits and test/draw out any diagrams for you. I don't make PCBs but can do complex Veroboards. Knowing your designs they will be bare essentials anyway.

I have a great interest in calibration having gone through the pains of designing my own chrono.

There are several ways of doing this and I'm happy to draw up and post a circuit using old fashioned parts. My present device is optimised for my home made chrono with a sensor spacing of 4.8 inches (don't ask!) . I didn't bother to minimise the parts count - what was in the spares box was more important- all expense spared. I have the facility to lay out and generate Gerbers for PCB manufacture, but unless you are making several dozen boards for this small device, the setup cost is prohibitive, hence this thread.

 

[Flinty]

That just leaves some way to ensure the calibrator is calibrated.

The device that Colin is proposing is for chronographs like the LMBR.

That's correct, the calibration equipment must itself be calibrated to a higher standard than the equipment being calibrated.
It's been 30-odd years since I was the ISO Calibration system manager in a very large food factory, but a simple example would be that if I required a temperature gauge on the production line to read within +/- 1 deg, my calibration equipment must be calibrated by an external source to within +/- 1/2 deg, their equipment must then be to +/- 1/4 deg on a cascade system all the way back to the National Physical Lab.

 

[colinb]

In order to check my clock frequency of my Chrono I used a bolt on GPS receiver about £3. Over short time periods jitter can be a problem with GPS but over longer periods they are fantastic. My Chrono basically runs overnight comparing it's clock to the GPS clock. I was quite amazed how good the ESP32 clocks were.

Whatever clock is used- around 4MHz to get our sort of resolution - the error will be tiny for our purposes providing it's not faulty. I used the radio 4 off air standard to double check my timer counter, which revealed itself to be "off" by 0.3 part per million at an age of 40 years. It won't move much now, so I tend to trust it. Radio 4 has gone now- it was killed of about a month ago.

 

[tony1951]

It is surprising how cheap GPS has become. You can buy boards with precise pps pulses for about £4. For any not in the know on clock timings, who wonder why we are mentioning GPS in this context, the chronograph you use, must be able to calculate the time difference as the pellet or bullet passes the sensors, very precisely, if the velocity figure given on the screen is to be accurate.

 

[tony1951]

Radio 4 has gone now- it was killed of about a month ago.

Tragic that was. 198KHZ was an institution, chopped down to save a few quid in powering its 500KW output valve. Some big tube that must have been.

I use a Russian time signal and WWV on 10 and 15MHZ to calibrate the frequency resolution on my KIWI SDR receiver.

http://tony1951.proxy.kiwisdr.com:8073/

EDIT:

If you look at this example you can see how my own web based sdr receiver exactly fits with the WWV frequency standard broadcast from teh USA on 15 MHZ. The signal line on the waterfall is exactly lined up with the frequency displayed by my receiver. It didn't used to be when it was set by the internal crystal oscillator in the Kiwi hardware.

 

[RobertBrindley]

There are several ways of doing this and I'm happy to draw up and post a circuit using old fashioned parts. My present device is optimised for my home made chrono with a sensor spacing of 4.8 inches (don't ask!) . I didn't bother to minimise the parts count - what was in the spares box was more important- all expense spared. I have the facility to lay out and generate Gerbers for PCB manufacture, but unless you are making several dozen boards for this small device, the setup cost is

Look forward to seing your design. Maybe I could use it to iron out the problems with my HT_X3006? It's not miles out just inconsistant fast on one test then slow on the next. I can't really add a correction factor to it. Using your tester I can play with the hardware and maybe iron out the issues.

I have a 3D printer as well so maybe that would come in handy for members of the forum to print out any standard design hardware.

 

[tony1951]

In fact, if you have a precisely calibrated modern radio receiver, or SDR, you can use it to check the actual frequency of any electronic clock circuit, because by definition, such clocks emit radio frequency energy at their clock frequency. All you need do is move the circuit near to the receiver antenna input that you wish to test and you can see it on the radio watefall or hear it if it is an audio only receiver.

 

[Steve636]

First off it's very reliable in that it always registers and it was about 3fpe more than my friend's Chrony chrony but we didn't go to great lengths as that's more than good enough for me.

Oops, meant to be 3fps not 3fpe

 

[1260engineer]

From my point of view there are so many problems in calibration. starting with the manufacturer of the chrono . body its self has to be perfect . never mind the placement of the cells which also has to be perfect . every manufacturing drawing has a tolerance +/- x amount .
which will affect the distance between the cells. then you you have to figure when the first cell actually sees the pellet . start or middle . which will start the clock then when the second cell sees the pellet. which will stop the clock . this gives you the time between the two points . throws up other stuff does it change with pellet head types. and so on .

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 .



Anyway i did a few experiments with my Chrony and LMBR . as i othen run the 2 in line .

simply what i do is work on the average. though the Chrony has a trick . because it has a hinge in the middle . if you pack under the far end the cells move closer together. this means the distance between them is shorter so the pellet appears to go faster or you can slow it down .
see 1.2fps between them .


( This in mind chaps with a Chrony make sure its perfectly flat open . )
between my to chronos i normally see 3 to 4 fps between them which its always been .
its also the way i can keep an eye on them calibration wise .

 

[Andy from Sandy]

If you place chronographs in a line there will be a finite difference in speed anyway.

If you check the LMBR instructions there is a feature to enter the distance between the rows of cells if it's quite a bit different to the 50mm they believe them to be.

 

[Steve636]

From my point of view there are so many problems in calibration. starting with the manufacturer of the chrono . body its self has to be perfect . never mind the placement of the cells which also has to be perfect . every manufacturing drawing has a tolerance +/- x amount .
which will affect the distance between the cells. then you you have to figure when the first cell actually sees the pellet . start or middle . which will start the clock then when the second cell sees the pellet. which will stop the clock . this gives you the time between the two points . throws up other stuff does it change with pellet head types. and so on .

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 .



Anyway i did a few experiments with my Chrony and LMBR . as i othen run the 2 in line .
View attachment 1022893
simply what i do is work on the average. though the Chrony has a trick . because it has a hinge in the middle . if you pack under the far end the cells move closer together. this means the distance between them is shorter so the pellet appears to go faster or you can slow it down .
see 1.2fps between them .
View attachment 1022910

( This in mind chaps with a Chrony make sure its perfectly flat open . )
between my to chronos i normally see 3 to 4 fps between them which its always been .
its also the way i can keep an eye on them calibration wise .

Very close, impressive really.

 

[ed_t]

I am OCD so qualify easily! Bear in mind that if a chrono has what it calls a self calibration, it has probably taken a short cut, and self check would be a better word.
We've already said that sensor separation is the biggest fundamental and irreversible error. It can be minimised at manufacture by careful measurement and placement. But after that, you'd need to invent a repeatable, steady pellet or equivalent, and I've posted before on how this could be done. But once that has been done as best, any temperature variations etc have to be lived with as an error. No cheap internal calibration will spot a physical movement of the sensors.
OK- a self calibrating chrono. All this can do is check the timing. The method will probably be incestuous. As I've said, timed pulses can "tickle" the LEDs, fooling the chrono into thinking a pellet has passed at a certain speed. But what is the source of the tickle timing? Probably a master clock internally, that also provides the timing for the time detection from the sensors. It's doing two jobs.
If that master clock is wrong, then the calibration cannot detect it. Imagine that from being correct, that internal clock speeds up. Your chrono will then read slow for a pellet. (More clock pulses) But when the LEDs are "tickled" in self calibration, the time between tickles will reduce. The two effects cancel each other out perfectly. Get it? A "correct" cal answer is unchanged by a faulty clock. Rare, but I have seen this fault happen.
To check a clock requires another, separate clock. And my external tickler provides that.
Anyone claiming an internal cal has to promise to provide 2 completely independent clock sources, preferably of different designs as 2 the same could mirror each others faults or degradations.

That's what we do in safety critical stuff but take it a little further: 2 seperate measuring sensors with different technologies on different processing systems so there are no common modes of failure.

I think airbus were running 5 independently programmed and individually processed control systems for their fly by wire control, but both them and boeing let the safety down with common instruments.

Very ocd for a chrony.

 

[RobertBrindley]

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.

 

[colinb]

From my point of view there are so many problems in calibration. starting with the manufacturer of the chrono . body its self has to be perfect . never mind the placement of the cells which also has to be perfect . every manufacturing drawing has a tolerance +/- x amount .
which will affect the distance between the cells. then you you have to figure when the first cell actually sees the pellet . start or middle . which will start the clock then when the second cell sees the pellet. which will stop the clock . this gives you the time between the two points . throws up other stuff does it change with pellet head types. and so on .

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 .



Anyway i did a few experiments with my Chrony and LMBR . as i othen run the 2 in line .
View attachment 1022893
simply what i do is work on the average. though the Chrony has a trick . because it has a hinge in the middle . if you pack under the far end the cells move closer together. this means the distance between them is shorter so the pellet appears to go faster or you can slow it down .
see 1.2fps between them .
View attachment 1022910

( This in mind chaps with a Chrony make sure its perfectly flat open . )
between my to chronos i normally see 3 to 4 fps between them which its always been .
its also the way i can keep an eye on them calibration wise .

I've agreed with Nate that you look for the end of the skirt. Because that is a sharp edge. You don't get owt for nowt so have to cope with a waisted pellet too, that might cause a double pulse. Different pellet shapes really require a different selection. But as we are testing a gun, a fairly light slug (does it exist?) is the most certain pellet to use. Clever software can also help eliminate false readings but I'm not that clever.

 

[Andy from Sandy]

A very fast camera and some clever stuff to examine the frames. A person could examine the frames and train a program what it needs to look for.

 

[colinb]

ON teh problem of accurate clock timing:

I am into amateur radio building and accurate clock timing is a long standing issue there, since oscillators are the core feature of RF generation. I go back far enough in that game to the days of oscillators governed by inductance and capacitance networks (extremely vague and inaccurate and wandering with temperatures) to more modern crystal control, but they are far from great too. Then we had crystal ovens to hold the quartz oscillators at a fixed temperature, thus getting rid of some variability, and now even more controlled systems with TCXO systems. In the end, the best frequency control we can get is often derived from GPS receivers which can provide incredibly precise, timed pulses to allow frequency calculation and timing.

Fair enough but horses for courses. As we would be generally be very happy with a measurement timing error or uncertainty of 0.2% or 2000ppm, it would take a pretty rubbish clock to make a real contribution to an error. Pellet weight variation is typically around 1%. I have 2 mags that cause up to 2% speed difference and I cannot see why. etc etc.
So in practice, we have to admit that except for a clock fault, external calibration is a comfort factor that many would appreciate. 99.9% of the time, things will work. The 0.1% are broken. I do it because I can. And offer it to others if they also wish to have that extra comfort factor.

 

[Hannah]

The chrony I have is one of these:
View attachment 1022701

It has a built in calibration function which does not involve any shooting.
I've often wondered if it actually does anything at all or just a con?
Maybe it's got a built-in version of what you propose.

I've got this one , it's good

But it's stopped connecting to Bluetooth all of a sudden

 

[1260engineer]

I've agreed with Nate that you look for the end of the skirt. Because that is a sharp edge. You don't get owt for nowt so have to cope with a waisted pellet too, that might cause a double pulse. Different pellet shapes really require a different selection. But as we are testing a gun, a fairly light slug (does it exist?) is the most certain pellet to use. Clever software can also help eliminate false readings but I'm not that clever.

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