Home Forums Hunting & Shooting Ask The Gunwriters Chronograph calibration & accuracy

Thank you Dr. Oehler.

I always appreciate your input into all things chronograph related.

NIST had a collection of those at one time, but someone opened the container and all the bullets were quickly lost.

Nice to see you, Ken.

Although I am a new member of this site, I am a long term lurker on this and several other shooting sites. This thread motivated me to register so I could counter some of the comments I have observed here and on other sites regarding the precision of chronograph readings. I have used a Shooting Chrony for at least five years, and based on my experience its only value is to compare velocities during load work ups on a given day. I have very little confidence in the actual velocities because of the variables and inaccuracies pointed out by Denton and Dr. Oehler.

I participated in a back to back comparison of three Shooting Chronys. About 10 shots of a given load were fired from one rifle over the three units. The difference between the highest and lowest readings was around 240-260 fps. The differences were quite consistent shot to shot. The highest velocity was recorded on the unit closest to the rifle and the lowest was on the middle unit. The third unit indicated about halfway between the highest and lowest. The average of the highest and lowest velocity for each shot was in the 2900-3000 fps range. The 250 fps difference is about 8.5% of the 2950 mean velocity. Based on these results, and other uncontrollable issues pointed out by Denton and Dr. Oehler I believe we should consider the absolute values recorded by our cheap Chronys with a few granules of NaCl(salt). On a given day average differences during load work up are useable if obvious wild readings are ignored.

Spot on.

Chronys vary as well with location and also the projectile. Checking a Chrony against an Ohler is meaningless as it only applies to that day/location and projectile.

Sounds to me like something was peculiar about that middle unit. Was the screen set-up the same on all three?

I've had a couple of Chronys, and found both of them to be consistent and to give results that always seemed correct. They have never given me a reading that I thought was 250 FPS low. I'm pretty careful to always do my measurements under a very standard set of conditions.

I've heard enough of these reports that I think there are two populations of Chronys, many that work just right and some that don't work well at all. I can also tell you that the people I can reach by phone at Chrony have no technical clue at all.

Having to do all my measurements outside on a sunny day is a real nuisance. I've figured out a much better design, which I can graft onto my existing Chrony, but so far it's just more work than I can fit in.

That is why I checked them on the same date, in the same location, and often with the same projectile.
All three were very consistent.

Bit of a roller coaster ride, this. Denton and Oehler were leading me to believe in my Chrony. Dbfletch's story gives me much doubt. I think the only way for me to sort this out is to take advantage of an opportunity to compare my Chrony readings, ideally against another unit and using a reference load.

Or...I could just buy an Oehler chronograph...

Wonderful discussion fellas. I came to the right place.

Folks, when Ken did his original chronograph it was quite an insightful and nicely done piece of work.

Once someone figures out the hard stuff, copying is easy.

Today, there are lots of good chronographs built on Ken's original work and principles. I think almost all of them work with all the precision and accuracy that any of us will ever need. If you're not getting screwy answers, chances are extremely good that the answers are accurate and precise to a percent or better. Note that there are reports from several shooters here that tested two or three chronographs in series and got very consistent results. That is what I would expect.

I'm not discounting dbfletch's report of the very odd middle reading at all. Obviously, it happened and it's not happy news. But in my experience it is the exception.

My rules are:

1. Protect the chronograph from muzzle blast. Usually 10 feet or so separation is sufficient. (Thanks for the catch!)

2. Make measurements only on days when the sun is distinct enough that you can see the shadow of the tripod. Do not attempt measurements at times when clouds cover the sun. I think the Oehler has better optics, but the Shooting Chrony definitely gives lower readings with the skyscreens in place when a cloud covers the sun. I know that with some units it's claimed, and probably with reason, that overcast days give more consistent results. I think you'll find more people that follow that than my suggestion. All I can report is what I've found with my equipment, which is that sunny days give better results.

Similarly, do not make measurements when the chronograph is in the shadow of a building or canopy.

3. On the Shooting Chrony, do not use the skyscreen extension rods.

4. Make sure you set up the chronograph properly and that the battery is fresh enough.

5. Shoot straight through the chronograph, not at an angle.

Not a rule, but possibly a factor: The range where I shoot only gives the option of shooting to the east. Maybe I don't have problems because the sun is never at right angles to the chronograph.

When I do all that, I always get very consistent and believable results with my Shooting Chrony.

The best thing about a "cheap" Chrony is when you shoot it they
will take it in trade. If you don't use the skyscreens and
rely on clouds for a background you will get bit sooner or
later.
God luck!

In 20+ years I've never destroyed any chrono by hitting it with a bullet or arrow.
Funny how many shot ones I've seen pictures of though.

Some interesting points being brought up here. I wonder if anyone can explain an odd occurrence that I've witnessed with my chronograph.

When shooting a string of shots over my chronograph using any one of my rifles and a single load recipe, I've witnessed the velocity of the successive shots fall progressively lower than the recorded velocity of the first shot. As an example, my first shot might register a velocity of 2,850 fps, the 2nd shot might be 2,830, the 3rd might be 2,815, the fourth 2,805.

I haven't tried to shoot a string of more than 3 or 4 rounds to see if recorded velocity continues dropping.

Am I the only one to ever experience this?

The screens were the same on all three units. Since the "third" unit recorded velocities halfway between the highest and lowest, it isn't logical to conclude the lowest readings were the ones in error. There was at least 4% difference between the "third unit and the two extremes. This comparison occured because the other two shooters were trying to develop loads to obtain a certain velocity with low ES and SD for 1000 yard competition. In addition to the three unit comparison, they fired their rifles over each other's Chrony and observed that the differences in measured velocities were attributable to the Chronys, not the rifles and loads.
The Chrony is marginally acceptable for load development comparisons on a given day. If one desires enough accuracy or precision to have faith in the absolute numbers a chronograph with longer screen spacing and a "proof" channel, such as the Oehler 35P is the one to use.

Ken Oehler;
I'd like to say a quick thank you to you and denton for the excellent and most insightful posts.

I've "known" I wasn't getting good readings on some days, but never put together the pieces of the puzzle as to why until now.

A tip of the hat to Matt as well for the suggestion of a control load. I happen to have a 100 pack of CCI Green Tag and and old Brno No. 4 that will do very nicely for that task.

Thanks again to you all and good luck on your upcoming hunts.

Regards,
Dwayne

Then as far as I can tell, you did everything right. I'm stumped. Perhaps you had a bad unit.

Addendum to previous rambling post: Common practice would be to use something like a Schmidt trigger to "square up" the bullet shadow pulses to start and stop the counter. In my mind, that should make the circuitry immune to variation in light. Nevertheless, lighting conditions do make a difference.

Wouldn't the old fashioned printed circuit paper screens provide nearly perfect and consistent start and stop pulses?
Super inconvenient and likely expensive but breaking a wire to to start and stop the timing circuit would be immune to lighting conditions.

Cheaper and easier would be to use artificial lighting like a LED array with the sensors shielded from ambient light. You'd have consistent lighting conditions. But I think ambient light is fine for most of us as a practical matter.

Denton, I traced the photo transistor conditioning circuit on my pro Chrono. It uses a rather clever but not unique circuit with discreet parts to keep the trigger pulse within range of the MCU port. Not exactly out of someone's app note that's around here somewhere but close. (Yup, repair job after wounding it. )

Paper screens have their own problems as do "shorting" foil screens. Doppler radar has its problems and spark photography is 'way too expensive. Using a Schmidt trigger to square up the screen response still doesn't tell you where the tip of the bullet is at the instant of trigger. (Parkinson has a law of "Compensating errors won't.") Until I learn better, I'll stick with the 1/8 inch uncertainty we've observed over the years. If someone says that 3/16 inch or 3/32 inch is a better guess, I won't quibble.

Let's just say that until you've used two or more chronographs to measure the velocities of the same shots, and then try to get the chronographs to agree under all conditions, you've missed much frustration. Measuring velocities is simple compared to ballistic coefficients and pressures, but it's all a challenging game.

Ken O

I bet!!

What struck me as a likely source of inaccuracy when repairing mine was tolerance differences of (discreet) components in the sensors and conditioning circuitry. If there's any factory calibration its hidden in the firmware.

Now the optics elude me.

That's probably where I'll end up.

There is a somewhat more elegant solution that I've dreamed up, but it may not be worth the effort. It would, however, give a nice repeatable input pulse.

3M makes tape with embedded corner cube reflectors. You've seen those.... directs light back to the source. They even have types "tuned" to various parts of the spectrum.

So instead of skyscreens, you make covers with the special tape on the bottom side.

Right next to your start and stop photocells, you place a laser diode, pointed up. Just above the diode you put a cylindrical lens to spread the laser beam into a slim fan. The special tape directs this back to the close neighborhood of the lens, i.e., the photocells. Then you slap a narrow bandwidth optical filter over the photocells.

Now you have a system that is pretty much immune to ambient light, and which will give very predictable response.

But, as I said, it's probably overkill. The LED array will probably work as well as anything there is.

Oh, hey, for overkill that's nothing. Dreamed up a scheme using microwave transceivers, the old Gunnplexers, to measure velocity by doppler. Sounded great until I got to the signal processing part though it might be doable with current DSP chips if you really know your onions.