denton,
I have owned two Shooting Chronys which declined in accuracy over time--and also have a local friend who had the same thing happen. They did not just quit working. Instead two started showing velocities that were obviously much higher than they should have, and changing batteries didn't help. The third started showing variation between sunny and cloudy readings of 100-150 fps. Sent that one back to get fixed, and it worked OK for a while, but I was starting to mistrust them.
Back then I was doing a LOT of shooting with a wide variety of cartridges, and eventually concluded the muzzle blast of some of the bigger rounds was shaking something loose. That's when I acquired my first ProChrono, and those problems ceased. (Purchased another PC maybe two years later, in large part to see if it worked as well, and also tested it against my Oehler 35P while shooting a wide variety of cartridges from .17 to .416 caliber. The second one worked just as well, despite me using the plain old overhead filters, instead of the furnace filters that Al Nyhus has used to good effect.
But the first also just kept trucking, so I gave the second one to a young friend with a growing family. This was several years ago, and he's still using it, with cartridges as large as the 33 Nosler.
John
Interesting and useful information.
Yes, muzzle blast can have an effect on readings. Besides the optical possibilities, disc ceramic capacitors (among other components) tend to be microphonic. You can strap one across the input of a high gain audio amplifier, tap on it, and hear the tapping at the output. So muzzle blast can possibly mimic/interfere with the effect of the passing bullet.
I used a Shooting Chrony for years, and did a formal analysis of the Probable Error. IIRC, the real precision (ability to get the same answer, given the same stimulus) of the system was between 1 and 2 FPS.
That said, in addition to the passing cloud issue, if you use only one section of rod to support the screen, you will get a different reading vs. if you use both sections of rod. So I had a selected set of conditions for chronographing, and carefully followed those. I got very consistent, credible results, but could only do measurements when conditions were just right. So now I have a LabRadar.
I'm a little nonplussed by your experience with declining accuracy. That should be un-possible. But anything that happens must be possible. There was a design change part way through the production run, with early units running a 4 MHz clock instead of 12 MHz. My unit was a later unit. Wonder what else they changed, and if that had any effect?
