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My New DIY Chronograph!!!

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Unread postAuthor: tansit234 » Tue Dec 25, 2007 2:29 am

Well, it looks like I'm going to have to build that chrony now. I've thought about it but you actually did it :D . Looks great, oh and try hooking it to a stop watch or something with one shot switch circuits (think engineer's mini notebook).

Edit: Ok, I've looked at Jimmy's page and I must say you are thorough. Anyways everything is straight forward aside from ROF equation and some of the peaks look equal height when they should be different yes? might be me. Also I noticed that the baseline changed during one of those tests so that might be it.

I would think ROF would be # of BBs divided by average time between the first peaks. Your equation boggles my tired mind, I've been working too hard on my rifle. Also, you could turn off one sensor to get a ROF sample only, but you would want to bypass the phototransitor and not the resistor to keep voltage levels the same I think, but if you bypass the resistor and phototransitor entirely it could raise the output of the detector left standing. I could be wrong but would that work? Does it even make sense?
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Unread postAuthor: jimmy101 » Tue Dec 25, 2007 2:00 pm

cwazy1 wrote:ahh, nice design, but i do see one problem that could occur: if you are to use a very high pressure source that creates a vapor plume after the shot, that could easily skew your results.

Never had any problems with vapor plumes, course the only pneumaitcs I've chono'd have been charged to just 120 PSIG.

A plume really shouldn't be much of a problem since it is transparent enough to sun light. The plume will just make the peak tail in the recording, which is not a big deal.

What kind of BB gun fired at 1100 FPS?

And the speed of sound is not "1116.44". The temperature dependence and other factors makes the speed of sound change by a fair amount. You certainly can't quote the speed of sound with 6 significant figures. A 20F change in temperature gives a ~15 FPS change in the speed of sound.

I wonder if the crack you heard was one of the cells breaking?
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Unread postAuthor: jimmy101 » Tue Dec 25, 2007 2:28 pm

tansit234 wrote:Well, it looks like I'm going to have to build that chrony now. I've thought about it but you actually did it :D . Looks great, oh and try hooking it to a stop watch or something with one shot switch circuits (think engineer's mini notebook).

People have used a hacked digital stopwatch as the timer. You need one that records to at least 1 millisecond resolution or, if you use a generic digital stopwatch that records to 0.01 second, then you need to increase the distance between the gates to several feet to get reasonable accuracy.

tansit234 wrote:Edit: Ok, I've looked at Jimmy's page and I must say you are thorough. Anyways everything is straight forward aside from ROF equation and some of the peaks look equal height when they should be different yes? might be me. Also I noticed that the baseline changed during one of those tests so that might be it.

The peaks should be different heights but it looks like there is some differences between the two detectors, or the path of the individual BBs is different, that is offsetting what the resistor network was supposed to do. Turns out it doesn't matter since there is never more than one BB in the trap at any given time.

tansit234 wrote:I would think ROF would be # of BBs divided by average time between the first peaks. Your equation boggles my tired mind, I've been working too hard on my rifle.

Yep, that would be another way to calculate the ROF, and would give a slightly different value. My way is just the number of BBs divided by the time. The bit of fudging (i.e., #BBs-1) just takes into account that the first BB marks time zero and really doesn't count. It really doesn't matter all that much since a typical BBMG, that uses a cloud or vortex, has a pretty variable ROF.

tansit234 wrote:Also, you could turn off one sensor to get a ROF sample only, but you would want to bypass the phototransistor and not the resistor to keep voltage levels the same I think, but if you bypass the resistor and phototransistor entirely it could raise the output of the detector left standing. I could be wrong but would that work? Does it even make sense?

Don't know why you would want a ROF only sample since you get good ROF data along with the muzzle velocity data. But, if you want ROF only;

If you use two phototransistors in series then you would want to bypass one of them for a ROF only sample.

If the two detectors are in parallel then you would want to disconnect one of them for a ROF only sample.

If you want, you can omit the resistors and just use the phototransistors (or photodiodes, or photovoltaics, or CDS cells). You need to limit the amount of light that reaches the detectors so that they are not saturated. The easiest way I've found to do that is to just mount the detectors behind very small holes. For phototransistors the holes probably are best if they are no more than 1/16"D. Photodiodes, photovoltaics and CDS cells are much less sensitive and probably should be used with larger holes.

BBs are tricky ammo to chrono with this method since they are so small. Things are a lot easier if the barrel is greater than 1/2" or so. With a 2" barrel just about anything should work.

People have also just used a microphone to record the sound of the round leaving the barrel and the sound the round makes when it hits a target. Indeed, there is a stand alone app that tries to do just that called SoftChrono.
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Unread postAuthor: cwazy1 » Tue Dec 25, 2007 3:16 pm

this is the bb gun i was using
http://www.spudfiles.com/forums/hpvr-first-copper-rifle-cannon-t12266.html its mine. i added a qev to pilot the piston and i addeda scope...havent updated pics yet.

it was a .12g 6mm airsoft bb.

yes the vapor plume adds insane fluctuations after the actualy projectile, but as i have chenged my design a bit, there is more of a gap between the projectile and the vapor plume. . . but this problem only occurs when im shooting a tiny projectile (bb) because the peaks are soo small that you need to be certain.

well nothing physically broke when i shot the gun, the thing just stopped working and my LEDs broke .

i need to up the pressure to 150 and see if i can get a 100% super sonic shot rather than being on the line of fluctuation (~1150+/- 50)
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Unread postAuthor: jimmy101 » Tue Dec 25, 2007 3:42 pm

Cwazy1, ahhhh, I thought you were shooting a generic BB gun (like a Daisy) :oops:
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Unread postAuthor: cwazy1 » Tue Dec 25, 2007 5:54 pm

hehe nope, those are too bland for my taste =P DIY all the way!
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Unread postAuthor: tansit234 » Tue Dec 25, 2007 10:03 pm

Ok thanks jimmy101, that minus one was confusing the heck out of me, now I know. Also, I didn't think it would be possible to have more than one in the trap with the fps we're playing with. Now I know I have to make this chrony along with a buffer for my grandmas tv remote (a dumb IR rebroadcaster, the radioshack ones don't work)
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