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Measure projectile velocity

A place to ask general spud cannon related questions.
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Unread postAuthor: clide » Thu Sep 06, 2007 2:49 pm

paaiyan wrote:i think if you only use one mic it would be a good idea to have the target only a short distance away from the muzzle, otherwise the split second it takes for the sound to reach the microphone might come into play and throw off your figures.


As long as the mic is halfway between the muzzle and the target, the speed of sound won't matter because it is the same delay between hearing the gun fire and hearing the projectile hit the target and they cancel each other out.
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Unread postAuthor: paaiyan » Thu Sep 06, 2007 3:16 pm

Well, that's true. I must not have been thinking straight.
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Unread postAuthor: jimmy101 » Fri Sep 07, 2007 10:38 am

clide wrote:As long as the mic is halfway between the muzzle and the target, the speed of sound won't matter because it is the same delay between hearing the gun fire and hearing the projectile hit the target and they cancel each other out.


Exactly. Besides, it isn't all that hard to correct for different distances between the mic and the target and muzzle. If you are recording a quite gun you might need to move the mic closer to the muzzle so it will be loud enough. The correction is just to add the distance from the mic to the muzzle divided by the speed of sound (~1100fps) to the muzzle sound and add the distance from the mic to the target divided by the speed of sound to the target sound.

If the mic to muzzle distance is 6 inches then the time delay is (6")(1"/12')(sec/1100')=0.45mSec. If the mic to target is 5 feet then its delays is (5')(sec/1100')=4.5mSec.

Ain't physics and math cool?
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Unread postAuthor: meatballs » Fri Sep 07, 2007 3:08 pm

i think that spudfiles should just rent chronographs so people can measure their exact muzzle velocities without so much difficulty.
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Unread postAuthor: CannonCreator » Sun Sep 09, 2007 1:39 am

I wonder how the boards real cronos completely work, theres some pretty crazy electronics out there.
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Unread postAuthor: Redcoat » Sun Sep 09, 2007 1:54 am

why not just try this, though it would have to be in a relatively open area.

http://spudcom.com/forums/showthread.php?t=45
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Unread postAuthor: jimmy101 » Sun Sep 09, 2007 10:02 am

CannonCreator wrote:I wonder how the boards real cronos completely work, theres some pretty crazy electronics out there.


"Real" shooting chronos are pretty simple. A single chip computer with optical gates (like phototransistors).

You could probably build a nearly exact duplicate for less than $100. Start with something like a Parallax Basic stamp (or any other single chip computer) for $50, add the small dot matrix display for ~$20, a pair of phototransistors for the detector and a 9V battery and you're done

I'm pretty sure that that is basically what a Chrony and the paintball and Airsoft chronies are.

The only thing remotely challenging is that speed is the reciprocal of the time the round takes to pass between the two gates. The calculating a reciprocal is not one of the basic math functions that can be done with just a couple resistors and capacitors. If you don't mind doing the reciprocal yourself then all you need is a timer accurate to perhaps 10 microseconds and the detector gates.
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