Who do you trust?

Jump to latest Follow Reply
thammer

thammer

Member
Jun 8, 2015
1,000
271
Ohio, United States
Here's two shots over 3 chrony at the same time. I am using my phone for it all so you'll see the screen shot then the pic. You'll notice the FX string shows the first shot in the upper left corner. 
1596844341_6591008995f2de9358ba5e0.38333387.jpg
1596844354_9183455955f2de94217d7a8.70483369.jpg
1596844372_12975257465f2de954edb933.83639390.jpg
1596844388_11465126985f2de964703a58.03111320.jpg




FX is coming in about 15fps lower every time. 
 
My WAG is that the Caldwell is (or Caldwells are) probably accurate (based on nothing but the history of the two units. What would be more interesting to me would be seeing a longer shot string, say 10 or 15 shots, measured across all 3 this way. The few strings I've seen reported seem to suggest that the FX may not show as "fine" a measurement of small changes in velocity and may be using a repeat or average type of display. All just a hunch, and the FX unit certainly is compact and, if easy to use, then that is a good thing.
 
Upvote 0
Welp you are shedding velocity between chronographs.

659 -> 658 -> 643

and

665 -> 662 -> 649

The difference between the shots looks like

Chron #1 665-659 = 6

Chron #2 662-658 = 4

Chron #3 649-643 = 6

So consistent ... nobody read higher than the one before it... That's good. Each of them shows a drop from the one before it... That's good.

To be honest I'd be comfortable trusting any of them. The differences just aren't that big. Change the order of them and run the test again if you want to get an idea of how much error there really is.
 
Upvote 0
Welp you are shedding velocity between chronographs.

659 -> 658 -> 643

and

665 -> 662 -> 649

The difference between the shots looks like

Chron #1 665-659 = 6

Chron #2 662-658 = 4

Chron #3 649-643 = 6

So consistent ... nobody read higher than the one before it... That's good. Each of them shows a drop from the one before it... That's good.

To be honest I'd be comfortable trusting any of them. The differences just aren't that big. Change the order of them and run the test again if you want to get an idea of how much error there really is.
Click to expand...

The FX is strapped to the second one and the first one is touching the second. Less than 2 feet between the three. 



On shot 1

Caldwell 1 = 659

Caldwell 2 =658

FX = 643

Shot 2

Caldwell 1 = 665

Caldwell 2 = 652

Fx = 649

Fx is 16fps slower than Caldwell 1 on both shots and 9-15 fps slower than Caldwell 2. 



I'll try to get a 15 shot string with the FX strapped to Caldwell 1 on video. 
 
Upvote 0
"Trust" means nothing -- what you are describing is called "Dilution of precision".

Every device capable of measuring anything suffers from dilution of precision because there is no human reproducable reference for calibration. The best would be heat death/zero entropy/0K but one cannot measure it and exist.

https://en.wikipedia.org/wiki/Dilution_of_precision_(navigation)

You are comparing an optical sensor against an RF emitter. Dilution of precision comes into play.

Don't be an appliance operator, understand and research the why :)
 
Upvote 0
I trust the Caldwell. My Caldwell reads about 2.6% hotter than my FX Chrony. I adjust the FX using the app to its max adjustment of 2% to get them close, but the Caldwell still reads half a dozen fps faster. The reason I trust the Caldwell is that over shot strings of 60+ it records more consistant data with a smaller standard deviation. The FX will throw a few higher or lower than expected velocities. Over a large enough sample size, the FX still gives a very reliable average. Its shot to shot consistency just isn't as good.
 
Upvote 0
You understand this is known as sampling bias and selection bias? Depending on the calibration reference, you're always going to have dilution of precision.

Allow me to reframe it:

"I am able to install five airgauges on my tank. One reads 4500PSI, the other 4350 PSI, the three other are liquid filled and vary +- 200 PSI."

Queue sampling of those who prefer the one that reads 4500 PSI? Queue sampling of those that prefer the lowest indicated gauge?

This thread is nonsense :)
 
Upvote 0
You understand this is known as sampling bias and selection bias? Depending on the calibration reference, you're always going to have dilution of precision.

Allow me to reframe it:

"I am able to install five airgauges on my tank. One reads 4500PSI, the other 4350 PSI, the three other are liquid filled and vary +- 200 PSI."

Queue sampling of those who prefer the one that reads 4500 PSI? Queue sampling of those that prefer the lowest indicated gauge?

This thread is nonsense :)
Click to expand...

I agree that without an instrument that is known to be 100% accurate that calibration is not possible. However, the thread is definitely not nonsense. Here is a small sample of data I collected today while testing. Both samples stay pretty close to the average. However, the Caldwell is much more consistent judging by the Standard deviation of 5 vs the FX's 10.5. 

1596855173_3431378395f2e138559ef07.74792255.JPG


Which would you trust? A gun that is consistent or one that shoots an occasional flyer/outlier? I'll go with the consistent one. 
 
Upvote 0
My personal old trusty red Chrono reads 20fps slower than a Labradar, a different Chrono, and some friends Chinese chronograph device that hangs off the barrel. 

It was really interesting to see my incredibly consistent and stable fps USFT come back reading 870 when shot over a chronograoh at last year's state ft match. That gun does 905-930 fps (885-910 on my red Chrono), trustable as lietenoutcolumbos Jesus. So that state match Chrono to make sure everybody was legal, was reading between 30-50 fps slow. 

I think there's just a little difference between fps collecting devices. I trust the Labradar and it's technology probably more than the sensor/eye setup of most other types of fps measurer. 
 
Upvote 0
This is very interesting. I had a ProCrono that started to not pick up shots to the point it wouldn’t work most of the time. When I just recently replaced it, I opted for the fx, figuring it would be much easier to use.

Just as op pointed out, the fx measured shots about 2-3% slower than the ProCrono did. This was with 3 of my air rifles and pistols shooting between 600-1000 FPS. For instance, I set my HW100 shooting AA 10.34 to shoot at 890fps. I get the fx and it’s telling me the same setup is shooting at 865 fps. Which do I believe, or doesn’t it matter as I should just be concerned about using the velocity as a reference? I also noticed that the Fx seems to have a much larger shot to shot variation.
 
Upvote 0
With a screen-based device it’s fairly obvious where the start and stop points are for the measurement. With a radar based system how do we know when the device starts tracking the projectile? Does The unit go for the first return, strongest return, or some kind of averaging? something with as low a ballistic coefficient as a pellet is shedding velocity dramatically once it leaves the muzzle. I’m guessing given the price point that the fx unit takes awhile (Distance) to get a solid reading which is most likely smoothed by averaging. LabRadar probably processes faster but again could be averaging/smoothing the data particularly for the displayed data. The detailed file data may be more interesting. With the fx fire into a dense backstop like modeling clay at various distances starting around a foot to out to where you get solid readings. The pellet needs to stay in backstop because with my LabRadar anyway it tracks Bullets going through and out the back of wood backer boards. I am thinking that the FX unit is actually tracking the pellet at a much greater distance from the barrel than the optical units are.

For A benchmark I would use an Ohler crony at maximum distance between screens with solid bracing against droop or movement with artificial illumination. Math required to calculate muzzle velocity. 
 
Upvote 0
You must log in or register to reply here.
Top Bottom
Back
Home
Forums
Resources
Menu