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update page 2 how much fps you think !!!

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: inonickname » Wed Aug 19, 2009 8:27 am

Jack, I'd have thought a marble/granite block? Only saying because from experience, a lot of perfect calibrated level surfaces for mills are marble/granite or similar.
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Unread postAuthor: jackssmirkingrevenge » Wed Aug 19, 2009 8:30 am

inonickname wrote:Jack, I'd have thought a marble/granite block?


It has a granite face but that's mounted on a substantial concrete block.
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Unread postAuthor: jimmy101 » Wed Aug 19, 2009 1:18 pm

Ragnarok wrote:If you need 0.001 gram precision, expect serious money. Also, expect it to fluctuate any time anyone moves anywhere within the room.

Usually, any scale that is accurate to 1mg or less will have an enclosure. In addition, it'll usually be mounted on something massive, like a 4" thick by 2'x2' hunk of marble. With that setup it'll stabilize almost instantly when a mass is placed on the pan and wind currents (people walking about, the rooms AC etc) won't affect it.

The standard analytical laboratory scale is accurate to about 0.01mg. It'll always have an enclosure. Usually the maximum weight is about 30g for 0.01mg resolution, ~500g for 0.1mg resolution. Here is a photo of an analytic scale used in thousands of labs around the world. Relatively cheap ($2500~$5000) for a lab grade instrument and very robust. Much cheaper and much more robust than the older designs that used actual weights and a ruby knife edge as the fulcrum.

The mid sensitivity scales are typically +/- 1mg and will usually also have an enclosure, though the enclosure is often much simpler than with the precision scales. Mass limits of 500g or so are common.

Micro scales will go down another order of magnitude or so in senstivity compared to analytic scales. Figure 0.001mg or so but with a fairly low maximum weight, perhaps as little as a gram.

It is pretty rare to need accuracy and resolution below about 0.01mg. A single small grain of salt weighs about that much. A moist finger print weighs more than that so you must be careful in how you handle the sample and it's container. At the 0.01mg scale things like moisure absorption from the air start to become a problem, the sample will slowly drift up in weight as it picks up water, or drift down in weight if it is moist.

For a spudder, a good balance would be one of the cheap digital postal scale type electronic top loaders. Harbor freight has a couple for $20:
http://www.harborfreight.com/cpi/ctaf/d ... mber=93543 with 0.1g resolution and
http://www.harborfreight.com/cpi/ctaf/d ... mber=95364 with 1g resolution.
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Unread postAuthor: Technician1002 » Wed Aug 19, 2009 2:00 pm

jackssmirkingrevenge wrote:
Technician1002 wrote:The fact that it made a hole the shape of the projectile indicates fairly slow speeds. The impact shock wave energy distorts the impact site at higher speeds as high amounts of energy is transfered in the impact.


I disagree there, the more the hole resembles the projectile's shape, then the faster it has to be going. Increasing velocity decreases the time which the target material has to distort and absorb the impact so the projectile simply punches straight through.

In this case however, we are talking about a water-filled container, so there are other forces at work here, by virtue of an incompressible fluid which is being displaced by the travel of the projectiles.

Image


The faster you push an object through water the higher the energy transfer to the mass that must be moved out of the way of the projectile. A high speed projectile doesn't simply "punch a hole" in the water and leave the water still. The water displaced by a higher speed projectile receives more kinetics energy than from a slower projectile. This high energy water is moving with lots of kinetic energy. This transfer if energy to the fluid is why bullets don't travel very far when shot into water. This explosive force makes cans explode at higher speeds. The faster the projectile the higher the impact forces and the higher the energy transfer.

This Mythbusters episode covers the fact that the high speed high energy bullets lost lots of energy passing through water.

http://www.tv.com/mythbusters/bullet-proof-water/episode/433141/recap.html?tag=episode_recap;recap

There is a transfer of energy. It goes somewhere. At higher speed the results are explosive.
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Unread postAuthor: adem70 » Wed Aug 19, 2009 2:57 pm

I don't think it was going very fast, if you watch closely you can see it bouncing off the can going to the left, its a really quick metallics blur. The screw never even goes into the can, let alone knock the board down. The water/can knocked over the board.
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Unread postAuthor: far_cry » Wed Aug 19, 2009 6:33 pm

adem70 wrote:I don't think it was going very fast, if you watch closely you can see it bouncing off the can going to the left, its a really quick metallics blur. The screw never even goes into the can, let alone knock the board down. The water/can knocked over the board.


i think you are right (not about the I don't think it was going very fast) ,there is a point in your words

soon i will post a video shooting water full can (in the center)


:D

edit ::
i watch the video more carefully and i notice that the screw hit the flat wood before the can

see

Image

Image


i need to adjust the sight
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Last edited by far_cry on Sat Aug 29, 2009 3:27 am, edited 1 time in total.
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Unread postAuthor: c11man » Wed Aug 19, 2009 7:00 pm

oh thats how it hit low and exited high on the can. so you were prolly shooting higher fps than we had guessed
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Unread postAuthor: dewey-1 » Wed Aug 19, 2009 7:14 pm

How could that hole be caused by the shot when it is there before you even fired?

Or am I not seeing something correctly?
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Unread postAuthor: far_cry » Wed Aug 19, 2009 7:28 pm

witch hole did you mean ?if you talk about the one in the thick mdf board .yes it was a hole but the screw damage the wood also


here what happens exactly

i shoot the screw too low ,and he hits the thin board"there is a hit in the middle of the circle in the picture the second one " where the can sitting
and jumps off a little bit and hit the can in the bottom left side, and finally he hits the mdf wood .the thick one

the screw did not go through the can at all.because of this there was no shock waves in the water that rip open the can
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Unread postAuthor: dewey-1 » Wed Aug 19, 2009 7:42 pm

Your red circle should be up further or smaller so you don't include that knot hole or maybe just use an arrow.

After I ignored your red circle , I saw it right away.

Now I understand!
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Unread postAuthor: adem70 » Wed Aug 19, 2009 8:40 pm

yeah, that makes sense. It probably is shooting higher then expected then.
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Unread postAuthor: far_cry » Fri Aug 21, 2009 8:15 pm

i made another test

in the video you will see that if i shoot the screw in backward will do more damage

distance was 6 meters ,and the camera post was near the shooting
look at the first shot ,red bull can get shot with the screw tip .and the other 2 shots with the head(different damage)

[youtube]http://www.youtube.com/watch?v=8iZWmeInDeE[/youtube]
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