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Chronograph accuracy?

1305 Views 15 Replies 11 Participants Last post by  imashooter2
How accurate are chronographs? I often see the same loads posted with far diferent velocity given. I know there are varibles. but sometimes the fps are far apart. powder charges could be off but A scale is easy to check. can you tell if your chrono is right?
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I think a lot depends on the chronograph you're using. I have a Chrony Beta, and noticed one day that the numbers seemed a little high compared to previous sessions. The unit unfolds, and the hinge hadn't opened all the way, meaning the skyscreens were closer together than they should have been. Little things like that can skew your readings.

As far as the electronics go, I think most chronies are extremely accurate. It's actually a pretty simple sees a shadow, starts the timer, and shuts it off when it sees a second shadow. Then it calculates how fast the bullet was going. I think if they're used correctly, the results are probably within a few decimal points of perfect.
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Like any other measurement instrument the technique with which it is used will influence the results, and there are many factors that play into that. One of those is the distance that the machine is from the muzzle.
PJ, did you make that skyscreen yourself? My unit is like yours except it came with a single strip about two inches wide for both ends. BTW, I discovered those foot long shish kabob sticks sold at WalMart are the same diameter and length as the rods for the chrony. Works great and a lot easier on the chrony should you accidentally hit one.
Right now I think mine is running slow. I'm sending it in for an upgrade and I'm going to have it calibrated.
I have heard that a simple & inexpensive test is to run some .22 RF across your chrony. It should check out pretty close to the advertised velocity. I am going to have to try that sometime.
Gohon said:
PJ, did you make that skyscreen yourself? My unit is like yours except it came with a single strip about two inches wide for both ends.
+ 1
To be honest, I don't remember where I got the idea for that setup, but the skyscreens that came with my Chrony were a pain in the butt, so I made that rig. It's nothing more than a sheet of clear plexiglas that I spray-painted white, then glued some 40S&W brass to it in the right places. When it gets windy I put rocks on top of it to hold it in place. Pretty high-tech stuff!

When the wind is really howling it stays in the trunk of my car. ::)
I think I may try to make something like that. Looks like it would work a lot better than the standard setup.
Gohon said:
I think I may try to make something like that. Looks like it would work a lot better than the standard setup.
+1 - next time I go to Home Depot I'll look around and see what they have - I assume PJ used 40 caliber to accomodate the angle of the Chrony metal legs with its wider case.

Neat setup, started me thinking. My problem is my Chrony is an older one, that didn't have the metal supports for the light blocker sheets. Mine just used small blockers attached to the cardboard things that acted as sighting aids. I will have to get around this, but it is a great idea. With my setup, I don't use it in the sunshine, just wait for cloudy days. I will try to adapt your idea to my older Chrony.
My chronograph is a Pact 1, and on bright, sunny days, I can get erroneous readings using the sky-screens as they were supplied with the chrono.

I like the idea of someway to increase the shading affect of the translucent plastic, or at least subduing the ambient light. I might have to get creative, rather than relying on a certain amount of cloud cover to acheive the same effect. Gets a guy to thinking... ;)

Are you still using them cardboard skyscreens? How many holes have you shot thru them? Mine were a little ventilated when I traded it in. Sounds like you need an upgrade.

I took the cardboard from paper tablets and traced the cardboard skyscreens. Cut them out and used them instead. The original ones are prestine, the ones I made have been replaced many times.
Even the best chronographs suffer some degree of inaccuracy and they don't get better as the price decreases. But most are plenty accurate for what we do with them. It's unlikely a chrongraph will be off more than a few dozen fps, but it's possible of course.

There are several electronic factors to consider about chronograph accuracy. One is the basic frequency of the "clock." As a general rule, the higher the frequency, the better the potential for accuracy is, but accurate velocity timing also requires very precise "start" and "stop" triggers. It's more costly to make the clocks run at the precise design frequency and that is what will be checked for ''calibration" in a repair lab. It's also a bit costly to get the photocells and trigger circuity highly precise. No chrongraph maker I know of tells us what their clock freqency or what the clock frequency stability is. Nor do they tell us how well the phototcells are aimed or how wide and deep their "look" angle is.
Set up a friend's F1, another friend's ProChrono and my F1 master back to back to back. Only inches between them. The F1 Master and the ProChrono were within a couple fps. The F1 was consistently 5% faster (all tests were pistol cartridges, though some of them were out of 1894s).

I guess that's one of the reasons you put a little buffer on your "power factor" loads.
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