Yesterday, I did a formal repeatability study on my Shooting Chrony. This ONLY tests whether two identical Chrony's will get the same result under the same conditions (precision}. It does not test accuracy (calibration), or immunity to changes in test conditions.

Results: The effective resolution of the Chrony is 1.15 fps. That means that the fourth digit is real information, not random noise. The system is good enough to distinguish between bullets only 1-2 fps different in speed. Over the range tested, about 2550 to 2950 fps, 99.98% of the variation observed was from speed variation, and .02% was measurement system variation.

In short, it's an extremely precise (repeatable) measurement system.

Thanks Denton. That's good info. - Dan

Denton: Please describe your methodology, because the indicated results appear to exceed the theoretical resolution of the Chrony system.

The last I knew, the Chrony (as well as Oehler and Pact) all used a 1 million hertz chip. This means that it emits 1,000,000 cycles per second. Therefore, at approximately 3000 fps with a 2 foot screen spacing (which I believe is what the Chrony uses), it would record either 666 or 667 cycles. If it recorded 666 cycles, it would give a velocity reading of 3003 (1,000,000/666 x2), while if it recorded 667 cycles it would give a velocity reading (rounded) of 2999 (1,000,000/667 x2). Obviously, with a 1 million hertz chip, it's resolution at 3000 fps cannot be greater than about 4 fps.

Electronics are advancing rapidly, however, and Chrony may now be using a much faster chip. If it uses a 5 million hertz chip, then it's resolution with a 2 foot screen spacing would be less than 1 fps.

Why don't you check the owner's manual and see what the specs are on the chip? If I'm way behind the times, I'll be glad to know it.

[ 05-26-2002, 21:48: Message edited by: Stonecreek ]

Stonecreek--

Excellent observation! You have detected a subtlety that most would not have thought about. Your mind must be as bizarre as mine.

The specs aren't given in the manual, but before I ever launched into this, I wondered about the same thing, and stuck a little wire loop down by the electronics. I hooked the ends to the antenna jack of a general coverage receiver. The clock now runs at 12 MHz.

Oehler provides spec on their chip.

On the Oehler 35P Proof Channel the crystal oscillation freq. is listed as: 4.0 MHz oscillator for 0.25 microsecond time resolution.

Oehler further notes: Error/accuracy assumes dark bullets, good light conditions, exact screen spacing, and shooting through the center of the screen.

Accuracy of readings correlates to speed of the bullets. Higher velocities result in greater error. Of course, screen spacing determines accuracy too, greater distance between screens affording greater resolution.

denton -

I've been wondering about this since I read your post - how did you set up two Chronys to check the same shot at the same time? I'm guessing you did use two Chronys, right? Am I missing something?

BTW - I see our friend Bill seems to be as close-minded as always.

R-WEST

I had a bit of luck in the Chrony department.

My Chrony is the basic model... no remote control or readout. So I put it in front. My buddy is richer than I am, so he has the remote readout and control. We just put his behind mine. I could read the front panel of mine directly, and the readout for his was right in front of me, on the bench. Bang! Two numbers. (Is it some kind of test of faith--will I let me friend shoot over my Chrony?)

His Chrony had both rods installed on the diffusers, so his diffusers were higher than mine. I think that is probably why his numbers were always about 27 fps slower than mine, instead of the expected 3-8 fps. I have a solution for that problem, but no time to fiddle with it.

I enjoy watching human reactions. It will be interesting to see what he says when my article on that topic comes out. It has already been accepted for publication. Next one I'm working on is the precision of the various methods for measuring chamber pressure. Preliminary estimate--case head expansion is by for the worst, CUP is fair, piezo PSI is probably a little better, and strain gauge PSI is at least as good as piezo, maybe better. For $150, you can make measurements at home that are as repeatable as what the pros do. The problem is getting an accurate absolute calibration... not easy.