Chronograph question

[DartonJager]

I had a friend ask me if the abilities of a chronograph to give accurate readings can be affected by things such as wind induced movements, being out of alignment with the rifle or not being level.

I replied I honestly didn't know that all I ever had done was set up my chronograph to be level and aligned it to my rifle by eye and measured so the first screen was 10" from my rifles muzzle.

So if I by chance my chronograph isn't set up to be level, aligned or is moving due to wind will it result in giving inaccurate readings?
Thanks,
Arthur

 

[BallisticsGuy]

Short answer, maybe, but at a level that wouldn't be anything but statistical noise at best.

It really depends on the type of chronograph. All have idiosyncratic tendencies in one way or another. Inexpensive optical types (Shooting Chrony, ProChrono, etc...) like I think you're talking about have pretty narrow windows through which one fires so it'd be pretty hard to fire at an angle that would substantially affect the readings it will give and for it to give any reading at all.

The way they work is by sensing a drop in light reaching a pair of little dohicky's (photodiodes) that convert light to electrical signal. A drop in signal starts a timer and another stops it. Those kinds of chronographs also tend to run on fairly slow 8bit processors and the actual bits their circuits are made out of aren't anything special performance wise, so there's a good bit of available slop there. Up to around 3% or more on some has been documented. You could, I guess, get some additional error thrown in if the chronograph as a whole is oscillating backwards and forwards along the axis of the bullets travel and provided that that happens while a bullet is passing through the skyscreens but it'd be a trivial additional error compared to what you get out of one of those anyway and such a wind would likely knock the thing over.

 

[Engineering101]

DartonJager

Ballistics Guy is sort of correct but not entirely. I designed and built my own Magneto Speed style chrono including 25,000 lines of assembly language programming and I've used an optical chrono for many years until I upgraded to the Lab Radar so I know a few things about chronos. The 8-bit processors referenced by BG don't effect accuracy but the clock rate at which they run sure does as this determines the error in responding to the optical inputs. In my chrono I used a 100 MHz clock (10 ns clock period) to minimize that error.

Enough background - about your question, it is pretty easy to calculate the effect of misalignment on chrono error with some simple math. If your chrono is not aligned with the bullet path, the bullet has to travel further between sensors than the physical length of the chrono. So anything that causes such a misalignment be it static (you set it up crooked) or dynamic (the wind blew it momentarily crooked) will cause the chrono to read slow. A chrono can never read fast due to misalignment since it reads fastest when it is perfectly aligned and you can't get any more aligned than perfect. It takes about 10 degrees of misalignment in both pitch and yaw to make a 3,000 fps bullet read as a 2,900 fps bullet. The roll axis has no effect. Ten degrees is a lot of misalignment and is pretty easily spotted with the naked eye however 100 fps is a pretty big error so smaller errors would not be seen with the naked eye which is why I always used a laser bore sighter to align my rifles to my optical chrono. Bottom line - any misalignment will result in a slow reading no matter what caused the misalignment.

 

[Shootin4fun]

+1 to E101's post. After 8 years of shooting over a Pro Chrono, same basic design as the Caldwell, I got a Magnetospeed V3. There is a world of difference. You will see significant variations using the 2 sensor optical design, waste ammo, and scratch your head. I couldn't figure out why I couldn't consistently get single digit SDs when using the Pro Chrono, even with loads that performed very well out to 500 yards (.3 MOA). Once I started with the V3 it all became clear. However the V3 does seem to alter the POI (lowers it) and group size on a sporter contour barrel, which is all I own. So my advise is, toss the Caldwell and save up for a V3. You can always sell it for at least 80% of what you paid. They never go on sale.

 

[Golovkin]

Here is an interesting read - http://www.appliedballisticsllc.com/Articles/ChronographChapter.pdf

 

[huntaxhunta]

These issues are why I switched over to the Magneto Speed Chronographs.

I have not had any issues with mine with the exception of Cliff breaking the strap assembly. It was one of the very first units though and they sent an upgraded model gratus.

I have never missed a shot reading on my magneto speed, and what little variation there may be from barrel taper is negligible in my applications. For most folks it won't matter a hoot. I'll take that over losing half my shoot string due to wind or passing cloud cover. As far as accuracy of the readings go, any system is a leap of faith that it's calibrated correctly and accurate. If you are consistent in your test protocols and setups, you'll get a consistent reading and that's what we're after.

POI shifts are inherent, but again I can live with it. The ease and consistency make it worth it. Once I get a load narrowed down, then I'll shoot for groups. It's a great setup.

 

[jimbires]

I've read some guys just put a bubble level on the chrony , I wasn't convinced that's what was needed since I could be shooting up or down hill . I put tape on the uprights . the red tape is all the same height . I put white tape on the front ones so I could tell the front uprights , from the rear uprights , when looking through the scope . what I did then was set up the rifle on a target . I placed the chrony in front of the rifle 12 feet , any closer I'd blow the sky screens off with a couple rifles .with the scope on lowest power I would look through the scope at the chrony uprights , and move the chrony to make it true with the scope reticle .being mounted on a tripod I could move it up / down , tilt front / back , tilt side to side . the red tape was the height I aligned the reticle with to shoot also . I've went to a labradar .

 

[Howland]

...but the clock rate at which they run sure does as this determines the error in responding to the optical inputs. In my chrono I used a 100 MHz clock (10 ns clock period) to minimize that error.

So is @BallisticsGuy right when he says the timing alone can contribute +/- 3% to the measurement uncertainty? When trying to measure in the 3,000 fps range, that's 90 fps right there before taking into account any other inputs to the budget!

If so, which brands/models have the most accurate timing? It sounds like you actually built your own.