How accurate is a Chronograph? (Chrony)

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FunGun

FunGun

Member
Jun 21, 2015
1,020
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OKLAHOMA, United States
Hey guys,
I posted a thread over in the PCP airgun section with my "out of the box" FX Streamline .25 shooting 25.39gr JSB's and 33.95gr JSB Heavies MKII pellets. Didn't even give it a second thought as to the speeds I was getting off my Shooting Chrony Alpha Master Chronograph. I got a comment about how fast my velocities were as compared to the advertised velocities from FX Airguns.

So my question to you guys is this. How do we know how accurate our chrony really is? I got mine from PyramydAir and they advertise the chrony to be "...more than 99.5% accuracy." I don't doubt what they say, but is there anyway to verify that our chrony is still accurate? Just wondering what your thoughts are on the subject. Or if you have had any experience with Chrony's going haywire?

PY-A-1335_Chrony_AlphaMaster_1335.jpg
 
FG, I also gave this some thought when I got my AW Monday. The FPS velocity was a bit higher (average 856) than I had read. Then I started weighing pellets. They were off up to a full grain +or -. So then country boy logic stepped in and said SO WHAT! in the end its POI that matters at a given yardage. As some know I sorted 1400 .25 JSB's 25.39's with a spread of .1 +and-.I don't think at 50 yards? its going to make a difference. Or even at 100yards.
My opine

 
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I fully agree with you 2D1C! The poa vs poi is what matters in the end. I am just trying to explore the possibility that our chronys are or are not repeatable and reliably accurate between different chronys. I don't have two chronys to compare to see if mine is comparatively accurate, it does seem to be consistent and somewhat predictable. I am looking for some input on how we can use some other tool or method that might help to calibrate the chorny. Or if someone has two different chrony's and can share their experience as to whether they are comparable in their measurements. It would be nice is there was some industry standard way of calibrating, like maybe there is some method, like using a strobe light over the sensors to get a particular velocity?

BACKGROUND: My FX Streamline .25 is new out of the box and shooting 25.39gr JSB pellets at an average of 945.4fps. No big deal to me, but it was pointed out that FX advertises the gun to shoot 890 fps. This just seems out of sorts that my unmodified streamline would be shooting 55fps (6%) over the advertised velocity. So I'm trying see if the chronys we use are accurate. Or at least get some opinions from folks that might have two or more? Or know of some way to calibrate the chrony? Just looking for some insight that might have some experience in this issue. thanks.
 
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As mentioned earlier a chronograph uses electronics along with a program to calculate velocity. That program uses the preprogramed distance between the sensors and the information the sensors provide. Each sensor records the shadow of a projectile as it passes over it.

If distance between sensors or the shadow of the projectile is altered from a lighting source not being directly over each sensor the velocity can be incorrect.

I've owned three chronographs with the first two being the Chrony brand and my current one is the first version of the Caldwell Ballistic chronograph. The first Chrony brand was one of the first versions to come out and used some sort of fiber shroud around each sensor capped with a white plastic hood and you shot thru an oval opening in each shroud so the projectile was in an area it expected it to be in. I only used it outside with powder burners. The second Chrony brand was a newer version that had a light kit on it primarly for use indoors with my new airgun hobby. I've used the newer Chrony far more than the first and have found sensors are held in location mechanically and can loosen up changing the distance. I also found lighting can affect how the shadow is seen by the projectile as it passes over the sensors. My Caldwell chronograph uses IR lighting and I've had not false reading yet (love them). I understand they sell just the IR lighting system if you are interested.

On another forum a poster known as 'Steve from NC' posted how a single light bulb positioned in the center of both sensors affect the velocity by altering how the shadow was cast on the sensors. Since Photobucket changed there linking policy I doubt the examples can be found but I did find a link to a rather long article that explains a lot about chronographs and may be worth the time to read. I only skimmed the article myself but page 3-4 shows the affect of distance between sensors.

link:
http://www.appliedballisticsllc.com/Articles/ChronographChapter.pdf

Without using a second chronograph to compare velocities the ChairgunPro program might be worth investigating. Assuming the input information is correct it should provide some comparison data. I've always found it to be very good but one of the hardest input values to provide is the distance between the scope and the barrel.

It sounds like a very nice rifle you have there...

 
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Just want to throw this into the mix. Gave this some thought last night after I posted as to how I could test my chrony. Ok, I'm not saying it would test it throughout its velocity range but it should work as a poor mans test rig.
Here we go.
Gravity is my friend. I did the calculations, I won't bore you with the details, but if I can drop a golf ball from approximately 5meters it will accelerate at -9.8m/s toward the earth due to gravity. After falling 5meters the calculated speed (foregoing the impact of air) the ball would have a velocity of 32.2ft/sec. The minimum velocity my chrony will measure is 30ft/sec so I think I will set up this experiment if I can talk my wife into climbing on our roof in this 100 degree Texas weather....I will let you know how this goes!
 
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