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Cheap Chronograph 
PostPosted: Tue Dec 18, 2012 1:58 am Reply with quote
Slavia
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A chronograph usually works by photocells timing the passage of a pellet's shadow over a fixed distance. I had read about using sound to do the same thing - the time between the initial report and whacking the target, all over a fixed distance. I finally got around to trying it, and it works.

SETUP: The equipment was simply my laptop with a $5.00 Radio Shack microphone. Many laptops these days come with built-in microphones, but mine is relatively old. I measured out 33' on the basement floor, muzzle to target. The microphone was placed perpendicular to the pellet's path, halfway down the range (16 1/2'). It takes some time for the sound of the discharge to reach the microphone, so the computer trace is delayed. If the microphone is placed at the halfway point then there is an equal delay after the pellet hits the target - the two cancel each other out. The target was a pine 1"X6" so it would make a little noise.


PROCEDURE: I used the simple sound recorder that comes with Windows. Microphone gain was set at maximum, "microphone boost" was turned on, and sampling was set at 16 KHz, 16 bit mono, 31 KB/Sec. Punch the record button, walk back to the bench, fire, walk back to the computer, and hit the stop button. The (5) recordings were saved to disk as .wav files.


ANALYSIS: I used a waveform analyzer called Audacity. It's a free download:
http://audacity.sourceforge.net/download/
I cropped out the silent parts of the .wav files until I could see just the two events - discharge and smacking the target. Each of the discharge traces consisted of two spikes. I'm interpreting that as noise in the action, followed by either the piston slamming home or the pellet leaving the muzzle. In any event, I used the second spike for measurement. The sound of the pellet hitting wood was more definite, so I used the beginning of that series of squiggles. The software gives a time stamp for each part of the recording, so by subtracting I got time in flight over the 33' course:


RESULTS: The test was done with a Crosman Titan .22 and Crosman Premier Hollow Points. The mean for the five shots was 766 F.P.S., with S.D. = 13.6.
Data from last summer using a Chrony F1 was a mean of 741 F.P.S., S.D. = 7.6. The microphone method was only 3% off from the Chrony F1.

ERROR: Imagine an old geezer crawling around on the basement floor with a yardstick - that's me. This was just a quick and dirty proof of concept, but a tape would have been better. Some kind of fixture or plumb line to consistently locate the muzzle would be in order. There is also probably some error in my interpretation of the graph and my (visual) location of the starting and ending points.

CONCLUSION: As a means of estimating kinetic energy for hunting purposes I think this would be close enough. As a tool to evaluate modifications It would also work - there it's not so much the actual value as the incremental change. For ballistics it would depend upon how persnickety you were. Chairgun says that the difference between 741 F.P.S. and 766 F.P.S. (with a 25 yard zero) would result in a 0.35" difference in P.O.I. at 50 yards.

So, all in all - it's a cheap, usable alternative to commercial chronographs.

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PostPosted: Tue Dec 18, 2012 11:15 am Reply with quote
rsterne
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According to ChairGun, using a BC of 0.020 and a MV of 766, the velocity at target is 714 fps, so the average velocity would be 740 fps.... Sounds BANG ON (pardon the pun) to me....

Bob

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PostPosted: Tue Dec 18, 2012 1:22 pm Reply with quote
Slavia
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Quote:
Sounds BANG ON (pardon the pun) to me

Who'd have THUNK it.
Anyway, that brings up a good point. What I did was simply calculate distance per unit time, which assumes a constant velocity. It would be better to use Chairgun and fiddle with the muzzle velocity until time in flight equals the measured elapsed time.

It also highlights a drawback of this method. The photocells in my F1 are only separated by about a foot, so a velocity drop between them isn't very critical. With the microphone you have to get good physical separation between the gun and target in order to get good visual separation between the blips on the graph. On the other hand, getting closer to the sound sources would probably give more definite spikes. There's probably a balance that needs to be worked out experimentally.

A side note: This isn't my original idea. If you Google chronograph+microphone you will find zillions of references. There is even a software package:
http://talonairgun.com/softchrono/SoftChrono_beta1.ZIP

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PostPosted: Tue Dec 18, 2012 3:16 pm Reply with quote
Alstone
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Nice work Slavia, a lot more intresting than using a chrony, so the next time you shoot the guts out of your F1 you now have a back up.

Al

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PostPosted: Tue Dec 18, 2012 4:56 pm Reply with quote
Slavia
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Quote:
the next time you shoot the guts out of your F1 you now have a back up.

Sort of - I don't have a printer installed on the laptop for the targets I have stored there, and the display is getting a little beat up...

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PostPosted: Tue Dec 18, 2012 10:45 pm Reply with quote
AirGunEric
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3 per cent seems pretty fair for a rather "primitive" setup using a $5.00 microphone. Any thoughts as to whether or not this 3% could be reduced even further by using a different shooting distance and/or a more sensitive microphone?

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PostPosted: Wed Dec 19, 2012 9:54 am Reply with quote
Slavia
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Quote:
Any thoughts as to whether or not this 3% could be reduced even further by using a different shooting distance and/or a more sensitive microphone?


Microphone Sensitivity - It seems like the visual spikes on the graph are plenty visible, so more probably isn't better. The SoftChrono program I mentioned can be adjusted for muzzle and target "trigger" levels. It scans constantly, and ignores noise under a certain level. Those trigger levels are superimposed on the graph so you can set them higher than the noise your feet made shuffling back to the bench, but lower than the shooting event.

I did my tests in the basement at roughly 10m (33'). Outdoor shooting with more background noise could be a different story.

Different Shooting Distance - This could make a difference. Shooting closer to the microphone would result in a louder "trigger" at the microphone, and it would be easier to set up if you didn't have a longer tape measure. I think it's not so much a question of the actual distance, but rather the precise determination of total range and accurate placement of the mid point. Shooting over 10m had the spikes well separated on the graph, and at 5m they would still be visually separated enough to be useful.

SoftChrono (which is a free download) has a number of features that duplicate what I did with Audacity, but simplifies them. There are the trigger levels I mentioned. You can shoot a shot string without leaving the bench, because the program is constantly scanning for spikes of a certain amplitude and paired closely enough to be the shooting event. It ignores spike pairs closer than what would correspond to 1200 F.P.S., and farther apart than 500 F.P.S. (Adjustable - those are the defaults.)

SoftChrono allows you to enter a "retained velocity" percentage to approximate muzzle velocity from average velocity over the course. It also allows you to enter an "offset" value to calibrate it relative to a light gate chronograph. It also has a huge font on the display so you can easily read it from a distance.

Both Audacity and SoftChrono showed a double spike at the muzzle end - I used the second, and SoftChrono uses the first spike to cross the trigger threshold. I'm still thinking about that, but any difference could be easily tuned out with the offset value in SoftChrono.





Quote:
a rather "primitive" setup using a $5.00 microphone

Well, yes and no. It is primative in the sense that you can't just shoot through it right out of the box, like a light gate chronograph. Once you get the numbers right and tape marks on the floor I think it would be a good alternative. Even if you don't own a laptop you could probably borrow one easier than borrowing a chronograph.

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PostPosted: Wed Jun 12, 2013 9:16 pm Reply with quote
Robw
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I use a very similar program on my Android phone called ChronoConnect lite.
You can ring a little more accuracy out of it if you set it to feet as opposed to
yards and to set it to around 5-7 feet. It was not far off from my radar chrono.
Definitely useable for tunning if great care is given to accurate
measurements of device to target, device to projectile path, and muzzle to
target distances are adhered to.

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PostPosted: Thu Oct 15, 2020 5:43 am Reply with quote
hojoos
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