chrono shootout

[taliv]

As I mentioned I intended to do in another thread, I set up my shooting chrony and CED Millenium chronos inline today and shot through them. results weren't really what I expected. Pretty much, the CED registered 35-45fps higher than the chrony all day.

A sample 15-shot group has the following
Max difference 45 fps
min difference 35 fps
Mean difference 39.5 fps
Std. dev. 3.4 fps

gives some insight though into their error margin

 

[happy old sailor]

thanks. everyone should heed this when spouting chrono results. i know i will.

 

[steveno]

difference is meaningless

 

[Kamicosmos]

I'm a bit confused.

It looks like taliv is posting the difference between 2 different Chronos, correct?

I'm not too sure what the next two posters are saying:

everyone should heed this when spouting chrono results

versus what....just believing the books or taking a SWAG?

and

difference is meaningless

Meaningless? Could you explain? Reloaders are loading for consistancy, so you want your loads to be consistant. So, difference is pretty much the most important stat your chrono gives you during load development. If your loads are more than a few FPS off, there is a problem somewhere in your process or components. If you're way off....a KaBoom or worse is waiting to happen.

 

[steveno]

there are too many variables in reloading. you could do the same test an hour later or the next day with the same loads and the next string of fire could be different. for it to mean anything you need to make these tests in a highly controlled lab like the ammo and powder companies do. you can have more than 75 fps variance just in a single load. since it was said that the chronographs were in line which one was in front and which one was second in line?

 

[stans]

Steveno makes some good points. Also, every chronograph that rolls off the assembly line may be just a little different. You could put two CED chrono's together and get very different readings. I would say the only way to determine if there is a real difference would be to get several chrono's from each manufacturer and test them all in a laboratory where atmospheric conditions can be held constant.

 

[oldfart]

Experiments like this are interesting but about all they really prove is that different chronographs give different results.
I use a cheap Chrony and it works just the way I want it to. It might not give me precisely correct velocities but it does give reliably correct results. I have a load that I've shot for a long time. It's reasonably accurate out to 600 yards and I've shot enough rounds over the Chrony to know what the MV is. In actuality, it might be 20, 40 or even 60 fps different than what my Chrony says it is, but it IS consistent and that's what I really want. Some recent experiments that I've made indicate that the Ballistic Coefficients published by some manufacturers may be so far off that any error there might be in the chronograph would be unnoticed.
I guess I started out trying to calculate my way to the 'X' ring when I really needed to shoot my way there. I still keep a ballistics program on my computer, but I only use it to see if I'm in the right ballpark. To actually find the target, I go out and shoot. Sometimes the results from shooting back up what I've calculated but most often they don't.

 

[WayneConrad]

Accuracy vs. Precision

Accuracy is how close you get to the bullseye. Or, for a chrono, how close to the actual bullet speed is the chrono's measurement.

Precision is how tight a group you get. Or, for a chrono, how close together are the numbers when the same bullet speed is measured repeatedly.

It seems that the two chronos have good enough precision that you could use either of them to work up loads, or to tell if a new load is in a rifle's "sweet spot." They differ in their accuracy however, so you cannot directly compare the numbers from one chrono to another without correcting for accuracy.

That means that if your favorite chrono breaks and you replace it, your "sweet spot" loads might measure a bit differently now. Or if you make up a new load to the same measured velocity, it might not be in your rifle's sweet spot because your new chrono, even thought it may have good precision (a tight group), might have different accuracy (be indicating a different speed).

As with all instruments, if you want to compare results from two chronos to each other, and the numbers don't differ much (is my bullet measured with my chrono 20 fps faster than your bullet measured with your chrono?), you should first calibrate the instrument against a known standard. I have no idea how one would do that with chronos, though. Chrono makers must know.

Is that a reasonable interpretation?

 

[taliv]

for it to mean anything you need to make these tests in a highly controlled lab like the ammo and powder companies do. you can have more than 75 fps variance just in a single load. since it was said that the chronographs were in line which one was in front and which one was second in line?

the chrony was closer to the muzzle than the CED. They were about 6 inches apart, and the chrony was almost at the end of its cord; call it 13 feet from the muzzle.

(btw, I did this with several different loads in two different calibers and the results were consistent)

i'm not sure i'm following what you're saying, but i disagree with what i'm reading. you might have 75 fps extreme spread on some loads. I have some loads that will shoot less than 30 fps extreme spread on 30 shot groups, and a couple loads that occasionally show single digit extreme spreads on 5-shot groups. for it to do that consistently (which it does) means the chrony must be able to record it consistently.

The point of this exercise was close to what wayne was saying about accuracy/precision, except my definition of precision in this case is basically significant digits. i.e. both chronographs measure to the foot per second for velocities over 1000 fps. for velocities under 1000 fps, they measure to the tenth of a foot per second.

The RELATIVE accuracy of both chronos was pretty good (better than I expected) as the standard deviation of the difference between their results was only 3.4 fps.

That tells me that RELATIVELY, either one is within a couple FPS.

The ABSOLUTELY accuracy, however, is a total unknown. It could be that the chrony was correct, and the CED was 35 fps high. It could be that the CED was correct and the chrony was 35 fps low. It could be that the CED was 100 fps low and the chrony was 135 fps low. It could be that the CED was 18 fps high and the chrony was 18 fps low. There's just no way to discern that from this experiement because I have no means of calibrating either chronograph.

However, the relative information is still useful, as I think oldfart was trying to say. that is, if x grains of powder is consistently giving me a chrono reading 2500 fps, then x+1 is probably going to show me a bit more velocity on the chrono, perhaps 2540 fps. In reality, of course, the bullet may be going 2700 fps, and x+1 may give me 2740 fps. see what i mean?


finally, I agree with my interpretation of happy sailor's comment: if you're using velocity as an indicator for when you're getting into dangerous pressure levels, beware. calibrating your chrono would be as important as calibrating your scale when weighing powder.

 

[taliv]

one more thing about my expectations...

the sensors on the CED chrono are almost 3 times as far apart as the shooting chrony's. I expected this to give the CED a big advantage in precision, but I guess I don't have any way to validate that.

 

[Ol` Joe]

what was the velocity range of the loads used? 30 fps variation in a 1000 fps load is 3% max error for one of the machines. The differance in readings at 2000 fps of the same 30 fps is 1.5% max, acually quite close. Remember too the 45 fps max variation you found could have included one crony being 23 fps too high and the other 23 fps too low you don`t know if either reads exactly right to begin with. That would reduce the error of each crony to half of the values above.
The CED as noted with its longer screen spaceing should be more accurate if all else is equal but with out calibrating it someway we`re guessing.

 

[taliv]

mmm, good point. that is an important detail.

2506 fps was my slowest ( using .308win) to a high of 2815 fps (using .223)
Those were both CED values. The corresponding Chrony results were 2543 and 2776 respectively. (Thus, the respective "differences" are 37 and 39 fps)

and yeah, still guessing.

btw, at the same time i was doing this chrono test, i was also recording data with the pressuretrace system strain gauge. That system is also not calibrated, but the relative results (graphing pressure curves of each shot next to each other with velocities from the chronos) are interesting. once i get that sorted out, i'll post some more info. maybe a graph or two.

 

[Ol` Joe]

Taliv,
Interesting you were useing a pressure trace. I got one from the better half for Christmas but haven`t had a chance to use it yet. The job has me going 7 days a weeek and when I get a day off it`s either -10 F or the suns so bright on new snow, the lap top screen washs out and can`t be read. I`d be interested in your findings and if you have any tips for a high tech dummy I`m all ears. I`m playing, or should say planning on playing, with a 6.5x284 if I can get the right time off.

 

[taliv]

cool! PM me when you get time to work on it. we can compare notes. I got mine up and running pretty quickly, but the software seems a bit buggy. good luck getting some time off,

 

[griz]

That's pretty interesting. I remeber a writer for one of the mags set up a test much like that. My feeble memory seems to recall his data showed a bit less difference, but as with yours, shot to shot always showed the same trend on each chrono. Can you try slower ammo such as 22 LR? I'm wondering if the difference would be a precentage of the velocity or more of a constant offset between them. (slope or intercept if you are thinking in terms of calibration)

And just to make sure I'm not misunderstanding, you are saying the "faster" chrono is the 2nd in line right?

 

[griz]

Re: title

The only time I got in a shootout with a chrono I "won", but had to buy a new chronograph. :banghead:

 

[taliv]

griz, yes, the "faster" chrony was farther away (by about 6")

re: title; a nod to somebody in the other thread who suggested I shoot "over" the chronos after i mentioned i was going to shoot "through" them. yeah, i poked a nice hole in the chrony once. chrony replaced it for free. very nice of them, i'd say. it probably comes out of chrony's "entertainment budget" instead of customer service, since they probably roll around on the floor laughing at us.