Playing with 2 chronies
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Playing with 2 chronies
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Topic: Playing with 2 chronies (Read 1326 times))
jengineer
Shooter
Posts: 90
Playing with 2 chronies
«
on:
November 16, 2012, 12:46:00 AM »
Because one is not enough
And why you need to swap them mid string
One of the threads was talking about chronographs and do you really need them. I posted why stop at one get two. It you want to zero in on ballistic coefficients you will need to know how much the pellet slows down between a given distance., such as measuring the loss of velocity between 10 yard to 25 yards. If you go deep then you will also need to accurately measure atmospheric conditions; pressure altitude, relative humidity and temperature. My PCP is unregulated and there is what is called a sweet spot in the muzzle velocity curve. If you shoot in that sweet spot your pellet point of impact (POI) is relatively consistent. If you have a regulated PCP or springer then your muzzle velocity is flat. There are methods for determining the sweet spot without a chony but it is easier with one. That is where I started. Engineer wise most of what we do could be done without a chroy but the engineers in us want to look at umbers and tie things down. Thus we are not satisfied with the generic BC found in various publications and software. Bring on the chony.
So here is the rub. Your measurements are only as good as your tools. You can get chronies for as little as $60 or upwards of $500. Me I have a Champaign desires but live in a beer budget reality. Thus I have two semi low grade machines made by the same company. In the calculation pellet BC you need an uprange (UR)and down range (DR) velocity. There are a few conversion factors that get rolled into the equations. It is best to have the two chronies with a distance between them at 20-80 yrds, and your muzzle velocity less than 950 fps
BC = (DR_yrd - UR_yrd) / (Ln (UR_fps / DR_fps)*8000) or
BC = (DR_ft - UR_ft) / 24000 / Ln (UR_fps / DR_fps) or for the number cruchers
BC_True = BC / (1 + (Temp_F - 59) / 540 + Elev_ft / 26000).
There will be scatter with the data and one way to smooth this out is to do a shot string of 10 to 20 shots.. Unless you have two of the high end chronies you should switch chrony position mid way through the shot string. And this is where I really meant to go with this post. I am shooting 0.22 Falcon 13.5 grain domed pellets (I think they are the JSB Diabolo Exacts). At the muzzle they should lose about 3 feet per second (fps) in 3 feet or 1 fps per ft. the units are about 1 foot in length so when placed nose to butt they should have a bout a 1 fps difference between them. So the two units were bought about a year apart number 1 being bought first.
UR Error range
Unit UR DR delta UR -1% +1% DR -1% +1%
1 803 795 8 795 811 787 875
1 815 808 7 807 823 800 889
1 805 796 9 797 813 788 876
2 812 815 -3 804 820 807 897
2 812 810 2 804 820 802 891
2 799 804 -5 791 807 796 884
2 811 814 -3 803 819 806 895
I do not know what happened with shot 5 but it is what it is.
Now if all things were equal the delta between the UR and DR should be 1 or at the most 2 fps difference. So what happened? The unit is missing Accuracy. And this is where the other $400/ unit goes. Accuracy. The folks in Rockford Ill have stated that the accuracy of the unit is “+/- 1% of the measured velocity, or better”. I would not put any faith in the marketing term “better”. This accuracy resolves into about a 16 fps spread on a 800 fps shot. So shot 5 could be an outlier or it could be within the accuracy of the units.
So is one of the units biased? probably. Are both biased? probably. Have I spent way too much time trying to figure out which one is worse and yeas I have spent a full afternoon with over 200 shots at various deltas only limited by my back yard. The problem is that I do not have the tools to calibrate the units the National Bureau of Standards in Boulder Colorado so I do the next best thing to take the bias and errors out within reason.
Shoot one half of the string with unit 1 UR and one half of the string with unit 1 DR. Shoot multiple strings in the sweet spot and then the averages wok it out. Two strings of 10 seems to work in my backyard.
Jengineer
Edit: typos
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Mark 611
GTA Senior Contributor
Posts: 8044
Yoda!!!!!!!!!!!
Real Name: Mark
Re: Playing with 2 chronies
«
Reply #1 on:
November 16, 2012, 08:11:43 AM »
I have the same thing happen between mine and my brothers chrony! I have a Beta and he has an Alfa and when we shoot right next to each other with our PB's theirs about 60fps difference between the 2 chrony's, IMO their not calibrated the same or the eye's are off line just a little! just my thoughts! also a chrony is a must have tool for the serious shooter!
«
Last Edit: November 16, 2012, 08:13:51 AM by Mark 611
»
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jengineer
Shooter
Posts: 90
Re: Playing with 2 chronies
«
Reply #2 on:
November 16, 2012, 11:07:30 AM »
Mark - agree that the chrony is one of the tools that you need. I knew the limitation when I bought them but sometimes it doesn't get rammed home that the inaccuracies may have an impact on your results. "Your results may vary, results not typical". With the use of one chrony you are working with a built in bias which ok as long as that tool is consistently used in all your calculations and you make your adjustments based on it. Take your PCPC that you have tuned to the ragged edge of a limit for a competition and you could get burned. It is one reason why you will tune a 20 FPE limit match to 19 or less so you don’t get disqualified. It all boils down to knowing how to work with the limitations of the tool. Dad always used quality tools. When he went cheap I usually heard about it. Limitation of consumer grade chrony for BC work is to average out some of the error or to lessen its affects. Problem its stack up tolerance. Each gauge you use adds to the error. And down the rabbit hole I go. For what we do the Alpha Beta and CED are more than adequate because in reality the tools are used to get you onto the paper but the time behind the trigger is what gets you dialled in.
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ean
Plinker
Posts: 112
Please, Lord, do NOT let me grow up!
Real Name: Ernie
Re: Playing with 2 chronies
«
Reply #3 on:
November 16, 2012, 12:28:03 PM »
Before I got my Beta Master--you guys are getting to know me, so yeah, it came from a garage sale--in doing my research, I noticed that one brand (can't remember which, but most of you prolly know), used 3 sensors to give 2 readings. If the two readings did not correlate, it threw an error. It was a bit more expensive, but it had the same advertised half percent accuracy as the chronys. Given that, what is the point? Catch the occasional insect crossing a sensor at just the correct time?
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Gordonville, TX
Ernie
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Playing with 2 chronies