Tales from the chrono

[zhunter]

Alan

I was aware that the same manufacturer was being used match/year in and year out, but I think that it would benefit the membership WHEN the EXACT same Chrono's were used, rather than new ones. This would end speculation about inconsistent readings.

Jay

Edited November 18, 2008 by zhunter

[AZ38super]

Z

We use the same chronos for A2 and WSSSC, minus any parts change from accidental damage and such.

Fellas, Please stop trying to determine guilty parties. It makes no difference at this time.

[larry cazes]

ETA - and they were checking mags? Where? I didn't see/hear of anyone getting checked...

Dave, Yup, they pulled mags at the chrono from our squad saturday morning and my wife's 170 didn't make it the first time through. I asked for the RM/MD to come out and it was found to be OK when he rechecked it. I knew somethin' was up since all of ours had been through a gauge in the past. Honest mistake I am sure. I was happy to see them checking. I have found the overall chrono arrangement at area2 to be top notch in the past and this year was no exception.

[wide45]

Alan

This would end speculation about inconsistent readings.

Jay

No, it would not.

[zhunter]

Alan

This would end speculation about inconsistent readings.

Jay

No, it would not.

Why not, same chrono match after match, year after year, you will NOT get more consistency than that!!!!

So go ahead and make your point, how would it not end speculation about inconsistent Chrono's from match to match?

[HoMiE]

Alan

This would end speculation about inconsistent readings.

Jay

No, it would not.

Why not, same chrono match after match, year after year, you will NOT get more consistency than that!!!!

So go ahead and make your point, how would it not end speculation about inconsistent Chrono's from match to match?

Using the same chrono match to match does not gurantee that the chrono stays calibrated.

[zhunter]

I give you that, but the same 2, used all the time, back to back, will be the most consistent you can get.

UNLESS you or anyone else has a better idea. I have not heard of a better idea yet to make it as good as the idea I have put forth.

Without getting in a pissing contest, IF you come up with a better idea, great, but don't knock the best we have come up with to this point!!!

[XRe]

Yup, they pulled mags at the chrono from our squad saturday morning and my wife's 170 didn't make it the first time through. I asked for the RM/MD to come out and it was found to be OK when he rechecked it.

You definitely have to know how to use the gauge when the mags are close... Interesting... AFAIK, they didn't gauge anyone on our squad (and I handed them a 170 to use in the gun, actually...), but I'm glad to know they were gauging some folks. Personally, I'd like to see more gauges in use "in the field"... randomly grabbing a mag out of the dirt and gauging it on the stage...

Alan

This would end speculation about inconsistent readings.

Jay

No, it would not.

Why not, same chrono match after match, year after year, you will NOT get more consistency than that!!!!

So go ahead and make your point, how would it not end speculation about inconsistent Chrono's from match to match?

Using the same chrono match to match does not gurantee that the chrono stays calibrated.

Nor does it guarantee that something doesn't get broken, etc, etc... And... if it happens to be a battery powered chrono, as the batteries weaken, strange things start to happen... And it'll never stop people from whining if they don't make the desired power factor

Having one set of chronos to send around to Nationals and Area matches (or, say, two sets) is an interesting idea, though, and one that couldn't hurt anything, in the end. It does have some potential for unforseen issues (like, the thing shows up unusable for some reason, so some other chrono has to be used...).

[shred]

The problem with shlepping the exact same chronos doesn't make it much better. Hunt down somebody with an Oehler 35P with the 3-screen setup and shoot some rounds across it. Notice how the very same round shows two different velocities 95% of the time, on the very same chrono.

I suspect some variance comes from the temperature of the chrono electronics. Yeah, they could do some temperature compensation, but do they? And do they reliably shift as the temperature and battery voltage changes? IIRC the infrared lights also run off a battery-- as that discharges the light curves will change too.

Chronographs are really far too vague for what we use them for-- it's akin to sticking your hand out the car window and trying to decide if the outside temperature is above or below 72.0'F.

[ChuckS]

I give you that, but the same 2, used all the time, back to back, will be the most consistent you can get.

UNLESS you or anyone else has a better idea. I have not heard of a better idea yet to make it as good as the idea I have put forth.

Without getting in a pissing contest, IF you come up with a better idea, great, but don't knock the best we have come up with to this point!!!

Z,

Don't take it personally but there is no "fix" for the current equipment. 2 chrono's may match or they may break. They might both read low or high. They may read higher, the same or lower than the chrono you have at home. but the real problem is that none of them are of known calibration. The manuals I have looked in have no specification for accuracy! I am sure that there is some checking in the factory but I suspect they will not quote an accuracy since there is so much variability on how they are used and the lighting issues. (and they don't want angry phone calls from guys who made 164.95 at the nats!) Personally, if there was reliable, repeatable and accurate way of recording the results of a ballistic pendulum, I think that would be much better than the optical sensor trick. Perhaps a radar solution would work too but there may be some cost issues. I am not sure what the answer is but any juju we do on our current gear is going to be nothing more than ceremonial.

<soapbox>

If you load for 165.1, you might not make it!

</soapbox>

Later,

Chuck

[RePete]

I hate to say people, but I was the ammo nazi at the IDPA Nationals and we used 2 CED M2 chrono's and the check chrono crapped out but the primary chrono was giving false readings too. They were in a chrono coffin.

I wouldn't give you a piece of dog crap for one.

I have a PACT Professional.

Don't get me wrong, I'm not saying that the PACT is the be all and end all of chrono's, I just saying that thew M2 is shitte.

[HoMiE]

I give you that, but the same 2, used all the time, back to back, will be the most consistent you can get.

UNLESS you or anyone else has a better idea. I have not heard of a better idea yet to make it as good as the idea I have put forth.

Without getting in a pissing contest, IF you come up with a better idea, great, but don't knock the best we have come up with to this point!!!

I do what you have proposed in earlier post. I try my ammo over as many chronos I have access to. I record the load data, time, temp., location, chrono used, etc. I come up with some numbers. I then allow for a 3~5% variance and if I want to make 165 PF everytime, I better make sure I load to 172+PF give the conditions I think I am going to encounter.

Then if that doesn't work, you could always spray some silicon down the barrel and get more velocity (never done it, but I've heard it works?)

[double_pedro]

The problem with shlepping the exact same chronos doesn't make it much better. Hunt down somebody with an Oehler 35P with the 3-screen setup and shoot some rounds across it. Notice how the very same round shows two different velocities 95% of the time, on the very same chrono.

I suspect some variance comes from the temperature of the chrono electronics. Yeah, they could do some temperature compensation, but do they? And do they reliably shift as the temperature and battery voltage changes? IIRC the infrared lights also run off a battery-- as that discharges the light curves will change too.

Chronographs are really far too vague for what we use them for-- it's akin to sticking your hand out the car window and trying to decide if the outside temperature is above or below 72.0'F.

I have an Oehler chrono with 3 screens. I purchased this model in order to have some measure of redundancy/confidence. It's not ideal but better than a single measurement of each shot. FWIW, in my experience, during a given chrono session the two measurements are generally within 1 fps of each other, however, it is not uncommon on some days for the difference to run 4 - 6 fps. BUT, the difference in the measurements isn't random, it always goes one way. When I chrono, I generally pick a spot within the triangles and shoot all of the shots there - usually this is just a few inches above the detectors but sometimes I may shoot the groups higher within the triangles.

As for why there is a difference on occasion (and speaking to the broader question of why different chronos may not agree), I suspect the reason is probably not in the electronics, but rather in the mechanics of the detectors. The chrono is measuring a time-of-flight and converting this to a velocity based on an assumed knowledge of the distance between the two detectors. Typical distances between detectors is about 2 feet, so an error of 1" is about a 4% error. The distance between detection points should be known better than this, but I suspect when one considers the (angular) orientation and acceptance cone of the detectors, the distance between detection points in the bullet's path may (systematically) vary by 5% or more. Further, an angular orientation error would likely lead to an error that depends upon how close to the detector the shots pass (due to a different lever arm).

I haven't tried running this down (yet) but if my hypothesis is correct for my chrono, I will be able to demonstrate it by looking at the difference between the two measurements as a function of where I shoot within the screens (i.e. high or low within the triangles). I will do this the next time I chrono. If it pans out, this sort of effect could be eliminated by collimating the acceptance cone of each detector to better define the distance over which the time-of-flight is measured. A downside may be a decrease in the chrono's efficiency to detect the bullet's passage.

Anyone know what the top chrono manufacturers use to develop/calibrate/QC their instruments? Perhaps one of them could provide a little technical field support (at least at major matches) in exchange for visibilty.

[shred]

I have an Oehler chrono with 3 screens. I purchased this model in order to have some measure of redundancy/confidence. It's not ideal but better than a single measurement of each shot. FWIW, in my experience, during a given chrono session the two measurements are generally within 1 fps of each other, however, it is not uncommon on some days for the difference to run 4 - 6 fps. BUT, the difference in the measurements isn't random, it always goes one way. When I chrono, I generally pick a spot within the triangles and shoot all of the shots there - usually this is just a few inches above the detectors but sometimes I may shoot the groups higher within the triangles.

As for why there is a difference on occasion (and speaking to the broader question of why different chronos may not agree), I suspect the reason is probably not in the electronics, but rather in the mechanics of the detectors. The chrono is measuring a time-of-flight and converting this to a velocity based on an assumed knowledge of the distance between the two detectors. Typical distances between detectors is about 2 feet, so an error of 1" is about a 4% error. The distance between detection points should be known better than this, but I suspect when one considers the (angular) orientation and acceptance cone of the detectors, the distance between detection points in the bullet's path may (systematically) vary by 5% or more. Further, an angular orientation error would likely lead to an error that depends upon how close to the detector the shots pass (due to a different lever arm).

Anyone know what the top chrono manufacturers use to develop/calibrate/QC their instruments? Perhaps one of them could provide a little technical field support (at least at major matches) in exchange for visibilty.

Your Oehler is more consistent than mine then. It often shows 3+ fps difference between the shots and while it is usually predictable, it isn't always. Interestingly, turning the screens around does not always change the error in the opposite direction. Long ago mine was calibrated by Oehler themselves and found to be within spec.

The real trouble is the shadow of a bullet is a very tiny, very dim thing and not nice square pulse like electronics like to see, so the detectors pretty much have to figure out 'where the mountain starts', as it were.

Also think about this; A bullet at 1000 fps travels across 2 feet (24") of skyscreens in 2/1000 of a second (.002). A bullet at 999.9 fps takes about 0.0020002 seconds, for a .0000002 second difference. That's about 0.2 microseconds. How far does that 1000 fps bullet go in 0.2 microseconds? 1000*.0000002 * 12 inches = 0.0024", or roughly the thickness of a piece of paper. Put the screens the thickness of a piece of paper too close togther and the bullet now reads 1000.1 fps. One sheet of paper too far apart and it's 999.9 fps...

So if the skyscreens spacing is off by even the thickness of a piece of paper in their spacing, the chrono is not capable of reading tenths of a fps correctly, even if everything else were perfect, which it never is.

Now let's consider temperature. A 20'F temperature swing will change the length of a 24" piece of steel by 0.003" or so. Some plastics move that much with a 5 degree temperature change. Who knows what temperature their chrono was calibrated for?

So who believes chronos are accurate to .1 fps now?

[double_pedro]

So who believes chronos are accurate to .1 fps now?

Hopefully no one....but for verifying PF at a match, I'd be happy with a set-up that is reproducible and calibrated to within 1%.

[shred]

So who believes chronos are accurate to .1 fps now?

Hopefully no one....but for verifying PF at a match, I'd be happy with a set-up that is reproducible and calibrated to within 1%.

Yeah, but even then with a 1% accuracy and repeatability, Major is +/- 1.65 PF and Minor is +/- 1.25. So a round that actually shot out the barrel at 165 PF could show up as 163.5 if the chrono was measuring less than 1% low.

I've yet to meet anybody knowledgeable and serious that believes chronos are overall better than 1% accurate and repeatable unit to unit, place to place, time to time and temperature to temperature.

We are effectively measuring powder with a postal scale and expecting our reloads to be correct. But it's OK, at Nationals, we use two postal scales!

[double_pedro]

I could live with 1%. The thing I hate is having to build in worst case combinations of several percent for temp, humidity, etc., and then another several percent for possible chrono miscalibration (maybe twice that since mine could be low and the match chrono high). Before you know it, I'm having to load what might be 135 PF (only 8% over 125) to make sure I make minor under worst case conditions. (end of gripe)

FWIW, I ran a couple of quick tests this morning with my chrono. I shot alternating shots low, medium and high in the screens (27 shots in all). The shots at the top of the chrono ran about 12 fps (out of about 870 fps avg) slower than those at the bottom - 1.3% difference based on where in the screen I shot them. The ones in the middle fell in between (about 4-5 fps slower than the shots at the bottom). Goes to show that shot placement can also have an effect...this would also have to be controlled and accounted for in the calibration.