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Old 03-17-2013, 07:01 PM
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Default Chronograph Question

How do you calibrate a chronograph to give "accurate" velocity readings? I have never seen calibration rounds for sale, and the only known standard I have is way too fast (3E10 cm/second) to calibrate with. Besides, a photon won't trip the sky screens and it's too hard to capture a single photon.

Do you guys just use a commercially manufactured round, shoot several times and correct for SD, Extreme Spread, etc? I have found way too much commercial ammo is optimistic in its stated velocity, and the same holds for my reloading manuals. Or am I looking for perfection when a "my loads are as fast as commercial stuff" is all I really need.

My scale weights, my calipers and micrometers are traceable to NIST Standards, but I can't find any way to do the same for my chronograph.
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Old 03-17-2013, 07:49 PM
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Default chrono calibration

This is something I too have thought about. But, I guess I'm not motivated enough to dig in.

One thought I have had was to probe the circuitry of my chrono to discover the sensor outputs. Using a O'scope connected to the chrono, fire a round and see if the chrono timebase is close to that of the scope.

I already haul too much stuff to the range.

Has anyone else thought about this along these lines?

For now I just fire a 22LR round and see if it is close to what the container label states.
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Old 03-17-2013, 09:44 PM
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I have never calibrated a chrono and neither has anyone I know. I thought that was done at the factory before it is certified for distribution.
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Old 03-17-2013, 10:06 PM
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So what would you calibrate there ? As I understand how it works : the distance is known, device measure time between 2 sensors, calculates speed. What can be calibrated, time measurements ?
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Old 03-17-2013, 11:55 PM
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Quote:
Originally Posted by JDBoardman View Post
How do you calibrate a chronograph to give "accurate" velocity readings?
Two chronos and a ballistics program.

I have a MagnetoSpeed chrono that attaches directly to the rifle barrel. So it is measuring velocity beginning ~3" from the barrel.

The start sensor for my CED M2 is 10' from the muzzle.

When used together on the same shots, the two measurements correlate well. But the difference between them is greater than expected velocity drop over 9' 9". So, they are giving slightly different results.

My conclusions are that the MagnetoSpeed reads a bit high and that both are accurate and useful for the intended purpose, certainly as accurate as our powder scales are lol.
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Old 03-18-2013, 07:35 AM
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When people talk about calibrating their chronograph, they are usually talking about simply shooting a load of known velocity over the sky screens to ascertain that the set up is the same as usual. The "calibration" rounds are usually a pellet from known lot though a spring loaded gun or a .22LR from a case or carton that the shooter keeps for that purpose. If the readings are close to what the norm is, the chronograph is considered calibrated.

PACT claims that virtually all complaints of slow chronographs are either operator error, or unrealistic expectations.
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Old 03-18-2013, 07:48 AM
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Quote:
Do you guys just use a commercially manufactured round, shoot several times and correct for SD, Extreme Spread
That would be a complete waste of time, since as soon as you shoot the "standard round" in your gun, you have no idea what the speed should be. It is a great shock to new chrono shooters to find out how much speeds differ from one gun to another.

The chrono circuit calibration is easily checked on a bench to insure the circuits are right, and you can't check it by shooting, other than for gross failure---which is usually a setup problem.

I agree with PACT that the accuracy of the electronics is the least of your worries, and controlling the lighting, insuring the skyscrren spacing is correct, and keeping the muzzle blast out of the screens is your real problem. I helped one fellow who swore his chrono was no good, but he was too close and just starting the timing with the muzzle blast.
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Old 03-18-2013, 10:25 PM
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I expect that all chronographs use a quartz crystal as a reference for their time base. An el-cheapo (under $1) crystal will have an error of only +/-100 ppm (parts-per-million) over a -10 C to +70 C temperature range. A 100 ppm error is 0.01%. At 3000 fps, this is an error of 0.3 fps! So unless there are other errors, the time base is itself is plenty accurate.

It's also possible that there are other sources of error, like variable thresholds in the light sensors, dirt on the lenses, incorrect sensor spacing, etc., that could cause significant errors.
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Old 03-21-2013, 12:09 PM
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This is an interesting question and nice to see each persons observations. I have never thought about calibration of a chronograph, but my observations over the years would make it impossible to calibrate with a "standard load".
I agree that to check to see if your chronograph is in the ball park of correct would be to purchase 22lr target ammo that has the speed on the box.

The problem with that and other standard rounds- The rimfires appear to have a larger variance than centerfire rifle rounds as you heat of the barrel. The bench rest rimfire shpoters know that you have to heat of the lube in the bore before good groups (same point of bullet impact) becomes the norm. Do this some times over a chronograph - the velocity of those first 5 to 10 rounds are never the same and can be a long way from those groups shot later as the lube warms up. Also one gun does not produce the same velocity as another due to bore dimensions and barrel length, chamber specifications, etc. Then you get into temperature variations in both air and the ammo, which causes different velocities. Point is - impossible to have a standard round to check your chronograph. Which brings up the question - why does it need to be calibrated ?

IMHO- the chronograph is used to 1) develop loads with a low SD for the target uses (high SD - big group, low SD, tight groups, always). 2) verify velocity so you can uses a ballistic table for drop and wind drift and calculate terminal energy for those inquiring minds that want to know. 3) know the velocity of your load so you can discuss with other shooters on what they are doing with a certain powder and bullet.

Now for the last question, why does anyone care on the velocity of factory ammo vs factory propaganda? I have been reading shooting magazines for over 50 years now and all I have every read for 50+ years is that factory published specs are more that what people see in real life situations. If you load does what you want it to do, then who cares what the speed of a factory round produces? M2CW. (Due to inflation, that is now worth on one cent.)
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Old 03-21-2013, 03:50 PM
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For most users, chrono calibration in the normal sense is impossible. About the only way it can be done is to use a laboratory certified chronograph for comparison, and how are you going to do that? As earlier stated, using a factory load having a published velocity is a waste of time, as whatever test gun that Remington, Winchester, etc. used to get their velocity is not your gun. As stated, the most valuable information you can get from your chrono is a general idea of velocity, the change in velocity with different bullet weights and types, and different propellant charges, and the standard deviation of the MV of your ammunition.

With some basis in statistical theory and also from personal experimentation, the average velocity and SD obtained from firing 5 shots provides about as good data as anyone needs. More than that just wastes ammunition, but if you feel 10 or 20 shots is a better sample size, have at it.
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Old 03-21-2013, 05:15 PM
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To truly calibrate your chrono, you would need to have an object of known velocity pass through the sensors.

You don't have an object of known velocity.

If you did, you would then need a way of adjusting your chrono.

You don't have one.

If you had several objects, each with different known velocities, you could perhaps develop a formula to adjust your chrono's results.

Since you don't have one such object . . .

Calibration rounds? They'd have to come supplied with the gun that shot them for timing.
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Old 03-22-2013, 09:27 PM
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Do you calibrate your watch or car speedometer? Sometimes you have to live with "close enough". It can be tough.


Upon further thought, just read the directions.
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Old 03-30-2013, 10:38 AM
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Many years ago I was told by Oehler to use a spring powered air gun to check speed, as they are very consistent.

I have an old Daisy cheapo lever gun that has run right at 248-250 fps for 25 years.

When I really want to be sure of the chrono results or check my setup I run 10 BB's over the screens. When I see the 248fps I know I'm good to go.
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Old 03-30-2013, 10:59 AM
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Velocity drop over 9'9" is an interesting concept. I'll have to check that out!
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Old 03-30-2013, 12:27 PM
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Quote:
Originally Posted by Engineer1911 View Post
Do you calibrate your watch or car speedometer? Sometimes you have to live with "close enough". It can be tough.


Upon further thought, just read the directions.

Find the Cop that gave you your last speeding ticket and says we have it on radar. Ask him to go to the range with you and point his radar gun at the bullet.

I use a 22 LR. If it is close to what the box says between 500 and 1200 fps I guess its accurate.

Good question though, I guess I just go by what the manual says it should be (test gun results not a test barrel of 12" or something) and if it is close I am OK with it.
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Old 04-01-2013, 03:21 AM
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JDB, I've been chronographing for 35+ years now and gotta admit, I never lost any sleep over trying to "calibrate" a chronograph. I just follow the manufacturer's instructions in initial set up and all goes well. I have had no reason to distrust the manufacturer's factory calibration. They all seemed to give velocities that seemed reasonable to me for the load, barrel length, etc. being tested. If there is some anomaly in the results, I have found it most always related to sky conditions, not using the shades over the screens, etc. The problem quickly becomes apparent and I never tried to get into the electronics to recalibrate anything. Now once on a lark, a friend and I set up our chronographs in series, his an Oehler, mine a PACT. Velocities never varied more than one or two fps between the two. Close enough
for me...
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Old 04-01-2013, 11:56 AM
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Quote:
Originally Posted by JDBoardman View Post
How do you calibrate a chronograph to give "accurate" velocity readings? I have never seen calibration rounds for sale, and the only known standard I have is way too fast (3E10 cm/second) to calibrate with. Besides, a photon won't trip the sky screens and it's too hard to capture a single photon.

Do you guys just use a commercially manufactured round, shoot several times and correct for SD, Extreme Spread, etc? I have found way too much commercial ammo is optimistic in its stated velocity, and the same holds for my reloading manuals. Or am I looking for perfection when a "my loads are as fast as commercial stuff" is all I really need.

My scale weights, my calipers and micrometers are traceable to NIST Standards, but I can't find any way to do the same for my chronograph.
You don't. Just like you don't calibrate a ruler.
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