Tuning CHRONOGRAPH ACCURACY ?

[Airgun Al]

Is there a method to verify a particular chronographs accuracy?

How do you know that the chrony you are using is accurate, no matter how fancy or expensive it is ?

Many chrony's are adjustable, so that you can equally match another chrony, but how does a person correctly establish and verify the accuracy to begin with?

 

[steve-l]

First off, accuracy is not important. The chronograph however, must be consistent. Then you tune to optimum operation and record that number. Most chronographs use sunscreens with a known distance between the two. They measure the time the projectile takes from the first to the second screen. These are the most accurate. You now also have others that use doppler radar that calculates by comparing the echo return frequency to the transmitted frequency. These are more sophisticated and most expensive, but are NOT as consistent as the old fashioned screen type. They might be accurate sometimes, but usually not in the price range they are sold at. The screen type is not only consistent, with a dual trace oscilloscope you can easily measure the time difference between the 2 screens and then with a little arithmetic, calculate the physical speed of the projectile. That allows you to compare your result to the speed display in order to validate accuracy. These type chronographs cost around $65 on AliExpress. I say again, accuracy is not important.

There are some chronographs that can calculate the ballistic coefficient. These can cost around $1,000 by calculating multiple echoes. To do that with a sunscreen is simply use two chronographs. One at the muzzle and a second at the target. At $65, the solution is cheap and accurate.

 

[GeneT]

I disagree that screen type chronos are the most accurate, or that you can verify them with an oscilloscope. The only thing you are verifying is the calculation of speed, not the measurement or the setup. The largest source of error in optical chronos is screen spacing - partly in how accurately the user spaces the screens and partly how accurately the sensor is placed within the sensor housing by the manufacturer (usually not well on $65 units - *this* you can verfiy with an oscilloscope by simply moving an object through the optical 'gates' while accurately measuring its displacement. Oehler makes the best optics I've seen and they're compatible with most other modular chronos. Additional error is added (regardless of chrono type) by poor alignment of the chrono with rifle and target. Horizontally AND vertically.

It's *very* difficult for a typical user to accurately verify a chrono. Probably the best that can be done outside of a test lab is to pay careful attention to setup and compare with other chronos (that are equally carefully set up). I have found very good correlation between Oehler, CED, Labradar (original), and Garmin. Three Pact chronographs were consistent with each other but all reported ~1% higher than the aforementioned chronos, even when using Oehler screens.

GsT

 

[JimD]

I have two functional inexpensive chinese chronographs. I bought a clamp on first and still have it and use it occasionally. I prefer the tripod mount because it does not shift my point of impact. I like to shoot targets while testing velocity. I recently tested the two chronographs versus each other. I had the clamp on installed and shot through the tripod mount a few inches away. The biggest difference I got was about 1% of the ~800 fps velocity - less than 10 fps. Most were about half that. The tripod mount I prefer was always a little higher than the clamp on - another reason to like it.

The comment on alignment is interesting. It makes sense to me that it would be true. I do not see how you could misalign the clamp on but the tripod can be twisted relative to the axis of the bore which may affect it's readings. Maybe I will test that sometime. What I think of as an old fashioned chronograph (which I had until I shot it) has much longer spacing between the sampling points so the misalignment could be greater.

I've said this before but I don't believe it makes much difference what my absolute velocity is. I want to know if moving the hammer spring changed it and by how much. Or how much did a regulator change affect the velocity. Or ambient temperature. None of this requires me to know exactly what the velocity is, but it does require that a difference be the gun and/or pellets and not the chronograph. The consistency of my two and the fact that one was always a little higher than the other suggest to me they are working OK.

 

[NateChrony]

Full disclosure - i make chronographs, and I have independent certification from the National Measurement Institute to claim that mine "is accurate".
Vid here of me banging on:

There's no way of knowing, unless the manufacturer has a process in place to calibrate the chronograph against a known/traceable chronograph - which is what I do. I have a set of master chronographs which were lab verified (above), and then each & every chronograph is mounted directly to the master chronographs (spacing is ~60mm apart). Even the way its mounted is CRITICAL to achieving reliable & accurate results.
(I could be wrong) but the only two I know of with some form of certification/calibration is the SKAN & NateChrony.

You also need to trust the manufacturer - do they actually do the calibration process? It adds a good amount of time - aka $$$ to the cost of manufacturing. Do they do it - or just say they do (i.e. marketing).

GeneT is bang on the money.
I spoke with a guy here that makes ballistic materials - he's got a $100k chronograph he gets calibrated yearly, really interesting process. Even then - there's stuff that wasnt accounted for in the manufacturers process!

I wouldnt worry much - ask the manufacturer, get a quality device.
Technically its not hard to achieve accuracy - its costly to achieve accuracy.

Another thread here:

How accurate is your chronograph?

A chronograph Is a device that measures speed. Most all measuring devices or tools can be checked or calibrated in some way by using what’s called a standard that’s traceable to NIST (The National Institute of Standards and Technology) My question is how do YOU know if your chronograph is...

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