Chrony calibration

[mrgeoff]

I have used 2 types of high quality chrony recently, F1 and a Skan.

they are 30fps different. Dosnt sound a lot, but I set my rifle up on the lower reading one at 11.2 ft/lbs, and if i take a reading on the higher reading one, just 30fps higher reading means just over 12 ft/lbs....

This isnt good, not knowing which one to trust, although i know all but 1 of my rifles were set up on the one which reads higher fps, so its all good, but is this common???

[Rich]

Periodically we send our Skan away to be calibrated against a known standard. We get in return a certificate which has a table that gives the maximum discrepancy at various velocities. For example it might say, 1.5 fps margin of error at 800 fps.

[what barn door]

There are a couple of things you can do with the skan depending on situation.

I sometimes pop mine down to a local RFD who uses skan, we will shoot through them both at the same time, one in front of the other, and expect pretty much identical results.

You could try to get a shooting buddy to shoot his rifle through your chronos, providing he knows what his is shooting at and then decide which is most accurate.

HTH,

Ian

[Artfull-Bodger]

My skan and F1 were within 10fps , I say were as the Skan died, just packed up no worky!
Annoying

[Archer50]

Periodically we send our Skan away to be calibrated against a known standard. We get in return a certificate which has a table that gives the maximum discrepancy at various velocities. For example it might say, 1.5 fps margin of error at 800 fps.

Where do you send it, Rich? I've tried for some time now to find out what commercial chronos are calibrated against and hit a brick wall. Is there a true standard?

Alan

[Rich]

Back to Mike at Skan.

[krisko]

very simple, you need a third.

[MartynB]

I think the Police use Skan's or at least used to

[Robet]

My combro and my "polish" chrono were within 3fps last time I checked

[krisko]

i doubt that anyone can calibrate a chrony properly only the manufacturer,
basically you would need one known to be good. With this is IMPORTANT: at least 3x-10x better precision/accuracy to the one being calibrated. This is to minimise uncertainty.

I doubt there is one such out there. Even skans are not that good. So you can only compare to another one, if they are very different, you need to get a third and bin the one which is way off.
or best is to bin them all and get a life

[Archer50]

i doubt that anyone can calibrate a chrony properly only the manufacturer,
basically you would need one known to be good. With this is IMPORTANT: at least 3x-10x better precision/accuracy to the one being calibrated. This is to minimise uncertainty.

I doubt there is one such out there. Even skans are not that good. So you can only compare to another one, if they are very different, you need to get a third and bin the one which is way off.
or best is to bin them all and get a life

That's what I thought - 'calibration by consensus'. It depends what we are using the chrono for, but I have thought for a long time that, if we want an accurate velocity reading, as opposed to tracking consistency and variation, comparisons are vital: I have seen so much disagreement between well known, carefully set up chronos run under identical conditions. I also think we need to recognise that most of the units we have access to are pretty low-end consumer products, and that even with laboratory standard systems, the degree of care needed to set them up and the possibilities for error are enormous.

Alan

[Rich]

Ask yourself how can a chrono go wrong and give an erroneous result?

In essence a clock starts ticking when a pellet (or its shadow) passes one sensor and stops when it passes another sensor. Speed = distance divided by time. The distance is the space apart between the fields of view of the sensors, and the time is the number of ticks of the clock, converted into seconds once you know the frequency of the clock. The higher the frequency the more precise the result, potentially.

The clock will be quartz crystal controlled and it's very unlikely for that frequency to drift. The sensors are fixed to the chassis of the chrono so it's still pretty unlikely they will move, unless the kit has been dropped.

Battery voltage ought not to be an issue, as it's either enough to drive the electronics or it isn't; it's a digital business.

Inconsistency in use and the human factor are substantial issues, though.

[Archer50]

I certainly take your point, Rich and I'm sure that a lot of problems are down to human factors including poor understanding of the statistical effects of small groups and tolerances. But I have set up a Combro, a Skan and a Caldwell (with IR light source) in a line, using a spirit level and a laser pointer to ensure alignment, and despite using weighed pellets, they disagreed by up to 30 fps on some shots. And this was just one test of many, using 7 chronos of 5 different types - none of the results were much better.

Chronos are pretty well essential for any serious air gunner, but I for one will never regard them as anything more than indicative.

Alan

[Rich]

Yes, I was really talking about one chrono moving, rather than several differing from each other. For example, if one has the sensors actually a millimetre further apart than the spec, due perhaps to an assembly fault, then it will give apparently slower readings as the transit time is longer. If the expected base was say 100mm and the actual was 101mm, it's OK to say it reads 1 percent slow, which at the figures we usually work with is 7 or 8 fps.

If the positioning of the sensor is such that its "beam" is not truly square to the chassis (I know it's not a beam as such but it makes the point) then another error comes in, and then that isn't a fixed error as it depends on how far the shot is off the axis.

[RobF]

Some chronos make their own light, others don't.

How does the shadow on a cloudy day differ from a sunny day?