Chrongraph Accuracy

[jps2486]

I realize we use chronographs to measure velocity relatively from one load to another. But how accurate are these instruments? If you are trying to measure absolute velocity, does your chrono's accuracy plus or minus 10 percent, 5 percent, or?? When I was a test engineer, we considered the accuracy of cheap instrumentation plus or minus 10 percent and was unusable. And, our instruments had to be calibrated on a regular schedule whether we used them or not. I can't figure out what kind of calibration standard a chrono would use?

 

[cem]

"Chrono" is a pretty generic term in this time of rapid change in technology. I don't know much about the newish radar systems. Bryan Litz has written about radar for the interested shooter and I suppose others have written as well. I understand radar units specified +/-0.1% of claimed precision. I suspect but do not know that all the three or so brands meet their claimed precision. I suspect but do not know that differences side by side are as much due to differences in triggering and so where measured as due to instrumental errors.

Ken Oehler has written a good deal on older designs both on discussion boards and in magazine columns though I have no idea how to access writings scattered over many years and different sites.

There is the question of where the measurement is taken and correction for muzzle velocity and consistency of where the measurement is taken from one setup to the next for most people on a temporary installation on public range by contrast with a permanent installation on an industry test range.

Some error arises from clock speeds and when the distance traveled in one click is meaningful reported speed will not be continuous. This shows up especially in drag racing today where the steps in reported speeds for the fastest dragsters are several mph apart. Clock speed may vary with temperature and power quality. The Accutron watch people offered a high accuracy version that averaged three crystals to guarantee better timing accuracy.

A much greater error for traditional chronographs is screen spacing where in the field measurements are a significant portion of the total space between start and stop sensors.

This from Ken Oehler on his 35P consumer model chronograph.

CRYSTAL OSCILLATOR FREQUENCY: 4.0 MHz oscillator for 0.25 microsecond time resolution.

ACCURACY: The expected error on any one shot depends on both velocity and screen spacing. Typical errors are shown in the table. The table assumes good light conditions, dark bullets, exact screen spacing, and shooting through the center of the Skyscreen III window. Actual errors under your shooting conditions will be reliably indicated by the PROOF CHANNEL®.
1 FT 2 FT 4 FT 8 FT
1000 FPS 5 fps 3 fps 1 fps 1 fps
2000 FPS 10 fps 5 fps 3 fps 2 fps
3000 FPS 16 fps 8 fps 4 fps 2 fps
4000 FPS 21 fps 10 fps 5 fps 3 fps
You will typically see the above errors as differences between the primary and proof channels of a Model 35P as you shoot under "normal" conditions.

For my purposes checking the oscillator frequency for time resolution in a metrology lab is pretty much a waste of time as I do not expect the same numbers in the field and the values that can be checked in a metrology lab don't include the errors in field set ups. I do try to use a fresh battery when I really care about the range session.

 

[Jager]

Pretty much what @cem said. I'll just emphasize that chronograph alignment is crucial anywhere that dead-nuts accuracy is desired. Doppler chronos in particular suffer from high susceptibility to alignment errors and sloppy session-to-session placement. But even excellent optical chronographs can show variance if sky screens are not placed precisely, each time.

But, yeah, inherent accuracy of high-end chrono units are quite high, with specs from Garmin, LabRadar, and Oehler in the 0.1 - 0.5% range.

 

[xr650rRider]

 

[Fuj]

I realize we use chronographs to measure velocity relatively from one load to another. But how accurate are these instruments? If you are trying to measure absolute velocity, does your chrono's accuracy plus or minus 10 percent, 5 percent, or?? When I was a test engineer, we considered the accuracy of cheap instrumentation plus or minus 10 percent and was unusable. And, our instruments had to be calibrated on a regular schedule whether we used them or not. I can't figure out what kind of calibration standard a chrono would use?

I own a Lab Radar and Oehler. Tested against each other was a 20 fps
spread, max. A buddy of mine several years back bought one of the
first Magnetospeeds and we tested that against the lab Radar. We got
about a 15 fps avg spread. All three chrono's averaged under 1.5 %
when testing at 2800 fps.....Close enough to test true velocities to suit
me......

 

[ELR LVR]

My Competition Electronics Chrono seems to register slow for some reason
No matter how I set it up (correctly, incorrectly, canted, straight, correct distance etc)
I was wondering the same type of thing if these instruments need to be occasionally calibrated
Not to mention if electronics have a +/- 15% value,
(when I was a Tech - resistors, capacitors etc all had a tolerance value, military components usually having the tightest specs)
This being the case, then should also the outcome of the total sum of all parts. Who knows how accurate they really are.
I tend to stick to my dope charts and hardly use the chrono
Maybe we need to go Davinci Old school and use a weighted pendulum
-----------------
Meaning
If I have a Bullet with a BC of .600 and it takes 2.0 mils to hit dead on at 600 yds
Then the velocity should be obvious
Then - if I use that to create a drop chart and I need 7.0 Mils to hit at 1000 yds
That should confirm my velocity is correct if BC is true to its claimed value.
After doing this, it doesn't matter what the Chrono says.

 

[Longrange57]

I own a Lab Radar and Oehler. Tested against each other was a 20 fps
spread, max. A buddy of mine several years back bought one of the
first Magnetospeeds and we tested that against the lab Radar. We got
about a 15 fps avg spread. All three chrono's averaged under 1.5 %
when testing at 2800 fps.....Close enough to test true velocities to suit
me......

Same experience! We had Oehler, Lab Radar, Garmin, and Magnetospeed across 4 different rifles and never saw a spread larger than 11 fps from high to low on each shot. If you average 20 shots from each one, they are very close. Mag Speed on barrel slightly changes POI. For me the Garmin is easy and probably as accurate as you can get with what we have today.
BTW, the Garmin works great with air rifles as well!

 

[Krogen]

It's been said before, but I think it's worth repeating when contemplating accuracy. Certainly, there's native accuracy of the device. However, it may not be known where the chronograph declares its measurement.

Optical chronographs are easy, they measure where you place them. The original Labradar gives you the option to select measurement distances. You can pick a distance and hope the Labradar does what it claims. Garmin et al don't tell you the distance. How far downrange is the measurement taken? We dunno. Do they attempt to correct back to muzzle velocity? We dunno.

So there's more to "accuracy" than the obvious. Simply comparing readings among test articles is interesting, but can't identify a more accurate chronograph.

 

[charlie b]

I also worked in an industry where absolute accuracy was important. Expensive equipment calibrated regularly was essential.

Chronograph? Unless you really need to compare your stuff to that done by someone else, why worry about it? Consistency is the key. I don't care if it reads 10 or even 100fps different than 'standard'. As long as it is consistent it suits my needs. +/- 5fps is good enough but +/- 1fps would be great. The nodes don't really care if they are 3000fps or 2950fps. I just like to know where they are so I can replicate the performance at some other time.

External ballistics programs still work ok to get on paper. DOPE is primary ballistics data.

 

[RegionRat]

Using an optical chronograph for comparison requires the user to remember to account for the velocity-decay offset to the muzzle.

Using a hobby Doppler radar also requires the user to understand how the algorithm extrapolates back to the muzzle if hairs are being split over the calibration question.

Some of these offset differences need to be kept in the perspective of how much decay there is from just a few yards. The Magnetospeed sits right at the muzzle and doesn't have an offset but still has an overall calibration question.

There are few paragraphs on the topic of chronograph velocity and calibration in one of the Bryan Litz books as I recall.

I can't figure out what kind of calibration standard a chrono would use?

There is no standard definition because there are many types of ballistic chronographs.

Sky screen types are clock based, but the optical effects add uncertainty on those trigger signals. The Oehler was considered a gold standard because it used three screens and they are typically spread many feet apart. That gives the user/algorithm a red flag warning when the differences don't make sense. Even then, the user had to account for the distance from the muzzle to the screens.

Doppler is frequency domain and red-shift, so they are a combination of time base and frequency-based calibration, plus the uncertainty of how EM reflections can distort.

 

[neckturner]

But how accurate are these instruments?

Labradar LX, like other radar units, claims +/- 0.1%, so at 3000fps your margin of error is +/- 3fps.
On the Labradar V1, Litz had some very complimentary things to say about it in his books, musing that it could replace the Oehler 35 as the reference standard.
In practice, with Labradar-derived BC and FPS, i get first-round 1MOA hits at 600+ yards.

 

[jpjulian]

I was going to start a new thread on Garmin accuracy........... I discovered a glitch today with my Garmin which does affect its accuracy. I verified the results twice. I usually set the Garmin along the left leg of the bipod. I have been getting some very high ES & SD's which made zero sense since I am loading with an AT4. By very high I mean an ES of 100-150 fps & SD of 30+ to 50+ fps. There was a friend shooting some prone who had his Garmin placed similarly but may 3-4" behind the leg of the bipod. I moved my Garmin some 6" rearward and the ES for the same load, same box, was in the low teens and SD was single digit. I moved the Garmin directly under the barrel between the legs of the bipod and ES was in the teens & SD in the single digits. I believe muzzle blast from the brake was causing my incorrect readings. The Garmin was placed correctly in relation to the muzzle per setup instructions. It was definitely not placed too close to the muzzle.
Apologies for the hijack.....