Home Forums Hunting & Shooting Ask The Gunwriters Chronographs

I have two chronographs. I'll leave the brands out to keep biases to a minimum for now. The reason for getting a second was simply to get more readings in less than perfect lighting. The new one is great for getting readings. Very rarely misses a shot reading. I did notice that all my loads seemed a bit slower though. Same rifle, same powder, etc. In fact many were loaded the same day awhile back and thus have been shot through both chronos. So, today I set them up and shot my bow through them. They were set up such that one was inside the other to make the readings the same. The new one read slower every time. I averaged it to 1.5% slower. For rounds shooting 2700ish that would equate to ~38FPS. That is in the ballpark of what I was seeing.

Is that typical of chronographs?

Are you sure the problem is that one is reading slower and not that one is reading faster

It's all relative. I am certain that one is reading slower than the other and the other is reading faster than the other. No idea which is right.

With the same brand of rifle, same brass, same bullets, same powder, same primer the faster chrono readings very closely matche the loads published by JB in his article on loading for the 338 FED.

Buy a brick of x500 22RF target ammunition, use it to 'calibrate' the chronograph before each session by firing say 5-10 shots. That way the 22RF can be used as a reference for whether lighting conditions on that particular day were conducive to high or low readings.
Cheers...
Con

+1

I like the .22 idea for understanding day-to-day variability. Now I just need to find a way to determine which is closer to correct.

Trying to get a grip on this but if I have it right you have 2 clocks on ones about 1.5% slower than the other and it's doing that consistently right? Thinking if it's that big a deal that you must be into shooting tween 500-1K a lot?

Gotta ask, what is it about the 30-40 fps diff tween the two that's so important to you?

Shoot the rig to those long ranges and make your notes and roll on.

Best of luck to ya

Dober

Definitely not suggesting it's a big deal. Really, it might make a half a bullet width difference at 100-200 yds! More of an annoyance. Really just more curious if there is an easy way to resolve which is correct. If I could shoot at 500 or beyond easily (East coast sucks in that regard) I would probably care less about a chrono. Reality trumps a chrono every time.

True that it does.

I get intrigued by the day to day diffs on how clocks can read. I think, mainly due to different light conditions?

Dober

Been my experience. I figure you really can't trust chrono results or group size unless you have repeated the process a half dozen times.

Yes, lighting is the most important parameter for proper skysreen function. It does make the design of skyscreens a challenge.

We regard the use of a .22 (or even an air rifle or BB gun) to be a reasonable test of chronograph function. We do not consider it to be a reliable calibration.

If you want a definitive test of accuracy, you will be testing chronograph "pairs" from different makers. If the two units of a pair agree, you have a chance of accuracy. If the units don't agree, you know that at least one is wrong. It is a pain to set up six or eight chronographs to measure the velocity of each shot, but we know of no better way to do a test.

KenO

Thanks Ken. Has it been shown that sun angle changes readings? In other words, will a chrono consistently read differently when the sun is directly overhead vs. low in the sky. Assuming it gets readings of course.

Yeah, how come we can get fast or slow readings on certain days? And what about the screens, how important are they on bright or dark days?

Thx
Dober

Over cast days are the best for accurate chronograph readings

Buy a third chrono, shoot over all three and take an average.

I've read that before, but I don't understand how accuracy would be impacted.
I see how it could impact getting a reading, but if it gets a reading why would accuracy be changed?

The box has worked best for me in bright days. Still like overcast the best also but the box works on bright days.

The box? What is?

Lighting conditions do make a difference. With more diffuse light, such as you get when the sun is behind a cloud, the image of the bullet on the photosensor is not as sharply defined, and that does affect the point at which the electronics think the bullet passed.

The other thing that makes a difference is the height of the screens. If I put the extenders on my Chrony, it drops the readings about 25 FPS.

Most people seem to like to do their chronographing on a cloudy day. I guess I'm the one that's different. I do all my chronographing during the middle part of the day, with diffusers in place, on days when the sun is distinct enough that I can see the shadow of the tripod. Since barrel temperature is a major factor, I fairly often strap a thermocouple to the barrel, just forward of the receiver.

The other thing to be aware of is that Ken Oehler says that there is about 1/8" uncertainty in the effective distance between the photosensors. I suppose this relates to getting the photocell and the lenses perfectly square with the frame of the chronograph.

I have no trouble at all replicating results from day to day.

This is the innards of a sensor from a CED M2 chrono:



The sensor is looking for the flicker of a shadow (green line), which tells it a bullet just whizzed past. If the sun is low, its light can bounce off the sides (pink line), then strike the sensor. The passing bullet doesn't interfere with this light in the least, which makes it all the harder for the sensor to detect the shadow.

Provided you have a truly accurate BC (and what many manufacturers publish is not truly accurate), you can use something like JBM's ballistic calculator (online) to "reverse-engineer" MV from BC, zero range and bullet drop at range (preferably at least 2x the zero range). That should give you a better idea of which is the more nearly accurate.

Using an IR light source solves about 90% of all consumer chrono problems.