Accuracy of Chronoghraphs

[Donkey]

Hello guys.

I'm looking at the Oehler 35P and the Kurzzeit PVM 21 Chronographs. The 35P quote 4fps accuracy @ 3000 fps with the 4ft rail and a 4Mhz clock which is around 0.1% http://www.oehler-research.com/specs.html The PVM21 however quote < 1% with a 14" rail and a 16Mhz clock http://www.kurzzeit.com/eng/bmc18.htm this would be roughly 20-30 fps at 3000 fps. Considering the light screen spacing and clock speed of the PVM 21 it should, in theory, have 2.3 times the resolution, and you would hope accuracy. Has any one had any experience with these and can comment on their real world accuracy?

We are getting loads down to about 4ft/s SD measured on the 35P so we really need to have something that will be accurate to at least .5 fps and I'm thinking about the PVM 21 but am worried about the factory accuracy figures.

Has any one lengthened the rails on the PMV21? I see they are connected by RJ45 ethernet cables so it should be pretty easy. Is there enough calibration available to be able to say extend it to 4ft and still get actual speeds?

Also has any one used one of Kurzzeit's higher end Light Screens for instance the LS600 or even better the dual LS600. http://www.kurzzeit.com/eng/dual_lichtschranken.htm with the PVM system?

Is the software that comes with the Kurzzeit QuickLoad or is that only included if you get it through Neco? With a Multi Light Screen unit like the 35P or the Dual LS600 can factory software or QuickLoad calculate BC? That is something I would really like to be able to do for custom and modified bullets.


Lots of questions there. Thank you for any answers in advance.


Cheers all
Jason

 

[normmatzen]

I don't think a rated accuracy of 1 % will preclude a single digit SD measurement. The 1 % is probably absolute velocity accuracy, not repeatability. You may find that even with the absolute accuracy spec at 1 %, 1. the real short term accuracy may be better than that. 2. the reading to reading may in fact have at least an order of magnitude better accuracy on repeatability so the 1 % absulute lets you measure MV of, say, 3000 fps to within 30 fps, but the measurement to measurement may be as good as one digit or +/- 1 fps. Remember, the +/- 1 digit is an artificial number as it indicates the indicated value where the statistical calculations are made in memory at the best accuracy of the A/D and D/A converters in the system and the mechanical rigidity of the unit.
If your unit says 3000 fps with an SD of 3 for 10 shots, I would expect the MV to be accurate to better than +/- 30 fps but the SD to be accurate to what is possible statistically with only 10 data points.

My Shooting Chroney does not make me worry about its accuracy, now that I know how to set up the bullet path!

 

[RonAKA]

The Kurzzeit PVM 21 appears to be a two plane measurement system. If so, it has only one timing for bullet speed Plane 1 to Plane 2. The Oehler uses three planes, and in theory could get three timings 1-2, 2-3, and 1-3. Some believe they use three, while I think their literature say two. In any case I would suggest due to the multiple planes and ability to average and reject bad readings the Oehler should be more accurate.