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Chronometer tested! (New result!)

A place to ask general spud cannon related questions.
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Unread postAuthor: psycix » Mon Jun 22, 2009 5:19 am

Yes, the marble weighted about 5 grams.


Vibration through the chrony itself would give a speed of 1300 m/s or more (if this was the thing triggering the sensors). But it didnt. The chrony gave a speed of 270 m/s, and 295 m/s when using more pressure, which is just 10% more then expected.

This is my theory on how the condensation cloud doesnt trigger the sensors (even on dry shots!). The chrony looks up into the sky, which is white (clouds) or if it is blue, you have to use the diffusers (white).
So it always looks into a bright white background and then senses a drop in lightness when the projectile passes.
The vapor cloud is semi-transparant white, which gives it about the same color/light intensity of the background. Which makes it practically invisible.

If the schelerien effect would distort the measuring, then the chrony wouldnt work as advertised: it would always give wrong readings, especially with firearms with even more power. And the chrony measures up to mach3 with no-problem.
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Unread postAuthor: Ragnarok » Mon Jun 22, 2009 6:15 am

psycix wrote:The vapor cloud is semi-transparant white, which gives it about the same color/light intensity of the background. Which makes it practically invisible.

Invisible to YOU. Water vapour is pretty opaque in the infrared region, which is why it tends to show up much thicker in pictures and videos of cannon firing than it appears to in real life - you can't see it, but cameras can see into the near infrared (and even further with a simple modification).
Similar things happen with most sensors, that they pick up a lot of infrared light. Just because it looks transparent to you, doesn't mean it's transparent to the sensors.

Also, the Schlieren effect is very different between supersonic shockwaves and muzzle blast. The first is consistent, the second is not - think of it as taking a photo through distortion glass rather than a lens.
Consistent error on both readings will average out, and the end result will be accurate. Different error on both readings won't average out.

Is it not enough for some people that chrony procedure states that the muzzle should be 3 to 5 feet from the sensors?
No, people should be able to completely disregard all procedure in the interests of a slightly higher number, then go about trying to find reasons to ignore said procedure to validate their disregard for it, then convince themselves that they've found a reason that's more right than what the manufacturers say.

I think not.

Put it this way. It's possible to work out that the aerodynamics over 1.5 metres (5 ft) on a 300 m/s (1000 fps) marble within a fast moving muzzle blast will result in velocity changes of at most ~1%.
If you're getting more than that difference between 0 feet and 5 feet, then your increase at 0 feet is due to more than lack of drag.

Seriously, just do it as procedure states. You will never find a reliable professional source who will tell you that you should chrony at the muzzle rather than at a few feet from it.
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Unread postAuthor: psycix » Mon Jun 22, 2009 12:51 pm

Very well then, I will shield my chrony with a rock and shoot a marble over it from 2 meters distance.
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Unread postAuthor: jimmy101 » Mon Jun 22, 2009 1:54 pm

psycix wrote:Vibration through the chrony itself would give a speed of 1300 m/s or more (if this was the thing triggering the sensors).

Not necisarily, it would also depend on the resonant frequence of the device. Resonance takes a few cycles to setup, that will delay the "trigger"
event to be a couple multiples of the SOS. Two resonances to build up sufficent amplitude would put the chrony reading at 1/2 the SOS in metal. Four resonances would put the Chgrony reading at 1/4 the SOS in metal over ...

I believe you need to use the diffuser when there are no clouds because the round is unlikely to actually cast a significant shadow. Dark round against a blue sky, blue sky = very little IR light. Small change in signal = no change detected. The big bright sun is blasting the detector, if the round's shadow doesn't actually fall on the detector then the Chrony won't see it. The diffuser makes the sky apear to have a constant brightness in all directions.
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