Chrono anomalies


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Walkalong
August 1, 2010, 12:27 PM
Shot some Hornady 200 Gr XTP's today with a charge of N110 well below their max in the PDF.

6 shots - 84 Degrees and seriously humid

Hi=1846
Lo=1844
Avg=1840
ES=2
SD=0

Only 6 shots, and probably not repeatable, but wow. Had three in a wallowed out hole at 7 yards, but had three a little outside the group as well.

A charge below their minimum gave velocities over their max.......

I will be pulling the one I saved down, and then repeating the test if the charge is what I logged it as. Could be a fluke. The sun was at a sharp angle on the chrono. I had one set of six .357's that was whacked out, but it was not consistent like these. High of 13?? and low of 4??. Don't remember exactly. Shot it again and it was normal for that load.

The same load of N110 with a 180 Gr XTP gave 1310 FPS last time out and 1816 today. Something was wrong.

Moral of the story? Don't always believe your chrono. (Humidity mess with it?)

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snuffy
August 1, 2010, 02:08 PM
What caliber? I see a reference to .357, but 1840 for a 200 XTP sounds like rifle velocities.

I would suspect bullet glint to be your problem. Bullet glint is the sun reflecting off the ogive of a shiny bullet, then that reflection triggering the screens prematurely. Low angles of the sun is usually the culprit. I carry a can of sight black with me to the range. I spray that on the bullets IF I get a suspiciously high reading. You can use a magic marker also.

Did the shells extract harder? Or any other signs of high pressure? I would re-test under different light conditions.

rcmodel
August 1, 2010, 02:11 PM
I think you need to use a Lee FCD.
On your chronograph.

rc

Walkalong
August 1, 2010, 02:43 PM
That's too funny rcmodel. :D


Sorry. .44 Mag from a 5.5" Redhawk. No way it's 1800 plus for 21.7 Grs of N110 & a 200 Gr XTP. Just no way. I believe the 1310 the first outing, but not 1800. :eek:

Low angles of the sun is usually the culprit.The sun was at a sharp angle on the chrono.was what I posted, but yes, a very low angle.

TimRB
August 1, 2010, 03:57 PM
"Moral of the story? Don't always believe your chrono."

I'll say. It gave you an average that is less than your lowest value.

Tim

bluewater
August 2, 2010, 11:50 AM
If your Hi=1846 and your Lo=1844, how can you possibly get an Avg=1840. That's impossble. I Think you need to recalculate and pay more attention to detail!

ReloaderFred
August 2, 2010, 12:30 PM
It's called the "Mason/Dixon Correlation Effect", Walkalong. It's caused by shooting too far south, with too high humidity, at the wrong time of day.............

Or, just an anomoly.........

Hope this helps (but I doubt it).

Fred

Steve C
August 2, 2010, 01:42 PM
I always bring some factory ammo, usually .22 lr and a gun that I've chrono'd it from before and know the factory specs for a reality check of the chrono's readings. If these are on the money I have to believe any other readings even though they may seem unusual.

SlamFire1
August 2, 2010, 02:09 PM
I plumb my Chrony with a Harbor Freight Tools laser level. It is important to have the bullet path parallel with the sensors.

I have also found that you have to shoot down the middle. Off axis shots will create obviously erroneous velocities. Such as your warp speed 45 ACP loads.

I start out any recording session with a well defined load. I typically shoot a 158 L 3.5 grains Bullseye 38 Special load over the sensors when I am shooting handguns. If the numbers are not right, then I have instrumentation error.

Put the chrony out far enough that powder particles donít become part of the problem.

http://img.photobucket.com/albums/v479/SlamFire/Reloading/ChronographwithlaserplumbontopDSCN1.jpg

Walkalong
August 2, 2010, 02:21 PM
If your Hi=1846 and your Lo=1844, how can you possibly get an Avg=1840. That's impossble. I Think you need to recalculate and pay more attention to detail!
Not impossible, just improbable.

Six shots with a hi of 1846 and a low of 1844 leaves four shots. Lets say they are 1844, 1844, 1844, & 1845.

1844
1844
1844
1844
1845
1846
divided by 6=1844.5

Who knows how the chrono rounded it. Technically that should round up. I just wrote what the chrono said. ;)

I am pretty detail oriented, but appreciate the advise, and will strive to continue to do so.

Walkalong. It's caused by shooting too far south, with too high humidity, at the wrong time of day.............
It was very humid Fred. I logged it as "84 Degrees and seriously humid". It took 15 minutes for the moisture on the guns to evaporate. They were still cool from home.

I think snuffy has it, the angle was so low it played havoc with the chrono.
After the first 6 shot string read in the 1800+ range, erased it from the chrono and tried again, but it gave 1800+ readings again.

The .357 string I alluded to gave a 1300ish hi reading (expected, but then the low was 4 hundred and something. No way. Shot that string again, and all the readings were in the low 1300's.

I chronoed some .223 later and it looked normal.

I made a note on all entries that the chrono was acting weird, and that I needed to re-shoot the numbers.

I always bring some factory ammo, usually .22 lr and a gun that I've chrono'd it from before and know the factory specs for a reality check of the chrono's readings. If these are on the money I have to believe any other readings even though they may seem unusual.
That's a good idea Steve C

It is important to have the bullet path parallel with the sensors.

I have also found that you have to shoot down the middle. Off axis shots will create obviously erroneous velocities. Such as your warp speed 45 ACP loads.
I am always careful to get it level both ways, and shoot in the same spot through the sensors as best I can. Good advise. A small error going over the sensors would make a big difference.

TimRB
August 2, 2010, 07:16 PM
"Not impossible, just improbable."

Yes, it's impossible. You can't get an average that is lower than your lowest value.

Tim

1858
August 2, 2010, 07:52 PM
Walkalong, I don't want to hijack your thread but have you ever seen any published data on the accuracy of commonly available chronographs i.e. +/- fps? I have a CED M2 and a Shooting Chrony Beta Master (which I no longer use) but haven't been able to find any information on their accuracy.

:)

Walkalong
August 2, 2010, 07:55 PM
No I haven't. I would assume the big dogs chrono's are better than the cheap ones, but I really don't know.

1858
August 2, 2010, 07:59 PM
Hmmm ... thanks ... but I hope someone knows. :confused:

Walkalong
August 2, 2010, 08:01 PM
Yes, it's impossible. You can't get an average that is lower than your lowest value.
Dang, I just realized I posted it wrong. The average was 1844.

And me going on about being attentive to detail. :o


It was
Hi=1846
Lo=1844
Avg=1844
ES=2
SD=0

So sorry. You are quite right. Impossible the way I originally posted it..... :banghead:

1SOW
August 3, 2010, 10:44 PM
I don't have a lot of experience with my pro Chrony Digital, but I did have similar problems late in the day on a sunny day.

The sun diffusers can't function well when the sun is hitting at an angle below the diffusers.

If you check a 'chrono box' on line, it compensates for this problem.

A cloudy day is actually best and no diffusers are needed.

Otherwise, having the high noon sun with diffusers is also good.

The instructions (which I normally read after-the-fact) mention this.

snuffy
August 4, 2010, 01:26 PM
The sun screens can't function well when the sun is hitting at an angle below the screens.

If you check a 'chrono box', it compensates for this problem.

A cloudy day is actually best and no screens are needed.

Otherwise, having the high noon sun with screens is also good.


1SOW, what you're referring to as "screens are actually diffusers. The screens are down inside the box, and on other chronos are in the bottom of the plastic holders. There's a lense on the top. A bullet causes a shadow to fall on the electric eye sensor, the drop in light is what triggers the timer/oscillator.

The reason they're called "screens" is the original chronographs actually had wires that had to be clipped or broken by passage of the bullet. The loss of continuity or connection is what started and stopped the timer.

In some cases, the reflection of direct sunlight off the ogive of the bullet causes a spike in the light falling on the sensor, then the rapid decrease is what triggers the sensor. But it reflects forward, triggering it early, giving a false high-er reading. Bullet glint is what I call it. Painting the bullet flat black stops it, you COULD just smoke the bullet with a match or lighter.

Asherdan
August 4, 2010, 01:57 PM
I took a quick look at my Shooting Chrony manual and...

On sunny days, at certain angles, light may reflect from the bullet and cancel the bullet’s shadow. Mark the bullets all the way around with a dark-coloured felt marker if this seems to be happening.

Interesting call out on the bullet glint, Snuffy.

Accuracy: 99.5% or better. Displayed velocity will not differ from actual velocity by more than 1 part in 200, i.e., Ī10 fps on a velocity reading of 2000 fps. Typical performance is generally better, and shot-to-shot repeatability is always more accurate, i.e., Reported Mean Instrumental Velocity may differ by as much as 0.5% from actual mean instrumental velocity, but Standard Deviation calculated from data gathered with a Chrony will always be closer than 0.5% to actual Standard Deviation for a string. This is an important fact because an accurate measurement of a load’s uniformity is of considerably more importance than is an exact measure of its average velocity.

IRT 1858's question, above is what Shooting Chrony claims, CED claims +/-1% of measured velocity, or better (http://www.competitionelectronics.com/pages/Prochrono_Digital.html) for the ProChrono Digital. This agrees with my memory of an article by Joe D'Alessandro over at Real Guns in which he stated that all the manufacturer's he had looked at were in the same accuracy range, regardless of price/features.

I'd be wondering along the same lines as Snuffy, BTW. The last time I ran into what I suspected was bullet glint I strung a translucent white plastic shopping bag down the sun side of the chrono as a diffuser and things worked out.

jmorris
August 4, 2010, 05:48 PM
The discussion about diffusers reminded me of a time I was helping my brother in law with a science project and needed to use the chronograph after dark. The photo below is what I came up with and it worked. I bet the white corrugated plastic roofing stuff from homedepot or lowes would work for many angles during the day as it would seem like just one big cloud.


http://i121.photobucket.com/albums/o213/jmorrismetal/tools/chronylight.jpg

moxie
August 4, 2010, 06:07 PM
"you COULD just smoke the bullet with a match or lighter."

Yes, but, PLEASE don't do it at my range. Thanks! Felt tip marker or Sharpie would be more in my comfort zone, being a holdover from my explosives safety officer days.

JDGray
August 4, 2010, 06:09 PM
I had some pretty wild readings on my F1, and took the advice of someone on one of these gun boards, and taped some clear tape over the eyes. The trick worked great, sunny or cloudy.

Walkalong
August 4, 2010, 07:20 PM
Ran some more over it today. Avg 1384ish IIRC. Much more believable. Overcast and humid. Not a keeper load. Just curious how fast they were going so I would know what charge weight to try next. I knew 1800+ was wrong. :)

1SOW
August 4, 2010, 10:25 PM
Another senior moment on the 'screens' vs 'diffusers'---corrected, thanks.

You could try the sure fix: Shoot just about an inch below the diffusers and you shouldn't get any sun reflection off the bullet.:D:uhoh:

Are diffusers cheaper than the rods???

1858
August 4, 2010, 11:30 PM
IRT 1858's question, above is what Shooting Chrony claims, CED claims +/-1% of measured velocity, or better for the ProChrono Digital. This agrees with my memory of an article by Joe D'Alessandro over at Real Guns in which he stated that all the manufacturer's he had looked at were in the same accuracy range, regardless of price/features.

I checked your link to CE (Competition Electronics) not to be confused with CED (Competitive Edge Dynamics) and read the "+/- 1% of measured velocity, or better". So a bullet with an actual velocity of 2,600 fps could be reported as anything between 2,574 fps to 2,626 fps ... yikes!!

:)

Ridgerunner665
August 4, 2010, 11:36 PM
Low batteries will cause them to do strange things too.

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