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what size qev

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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what size qev

Unread postAuthor: jsefcik » Wed Jan 29, 2014 7:25 pm

This is probably a useless post but i have a 3/4 and 1 1/2 qev,,,and i got a 1 1/4 barrel for d batteries


Which qev would be more ideal?

I was thinking the 3/4 cause its small but convient a d battery doesnt weigh much ,,and i think the 1 1\2 is over kill

Opinions???
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Re: what size qev

Unread postAuthor: jackssmirkingrevenge » Wed Jan 29, 2014 7:38 pm

jsefcik wrote:over kill


What is this strange expression of which you speak :?
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Re: what size qev

Unread postAuthor: jsefcik » Wed Jan 29, 2014 8:04 pm

I just think its to much
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Re: what size qev

Unread postAuthor: jackssmirkingrevenge » Thu Jan 30, 2014 4:45 am

The larger one is slightly too much, but the 3/4" one is far too small, especially if you're not using high pressure.
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Re: what size qev

Unread postAuthor: jsefcik » Thu Jan 30, 2014 6:57 am

For the 3/4 id certainly use @ least 100 psi
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Re: what size qev

Unread postAuthor: jackssmirkingrevenge » Thu Jan 30, 2014 7:52 am

You'll get reasonable performance, but remember that while it might be half the diameter, the smaller valve will be about 4 times less effective.
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Re: what size qev

Unread postAuthor: Dig_Gil » Thu Jan 30, 2014 3:11 pm

JSR has it right. IMO, the larger the better, but if it feels too bulky relatively to other parts of the gun it will... erh... be too large.
A case example: My marble shooter has a valve with an outlet 1/2 inch wide. I would like it to be larger (maybe 3/2 inches), but then it wouldn't fit in the other special valve I made which in turn needs to be sized to the caliber of the marbles (so it can't be larger; it actually could, but it would create too much dead volume, that is, space where air will stall and not propel the marble).
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Re: what size qev

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Re: what size qev

Unread postAuthor: jsefcik » Thu Jan 30, 2014 8:44 pm

Dig makes sense,,,i have marbel barrel using 1/2 pvc it acutally sucks,,they get stuck in the barrel 9 out of 10 times,,ive used my 3/4 barrel and had better luck,,id like to find really good ammo for a 1/2 barrel i was using steel slugs but im running out lol

Was using lead balls but i lost my molds :(
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Re: what size qev

Unread postAuthor: Dig_Gil » Fri Jan 31, 2014 12:24 am

Can't you make a mold by casting plaster around one of the marbles? After testing if that marble will fit into the barrel, obviously. Remember that marbles are made as toys, so their manufacture isn't much of a precision work.
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Re: what size qev

Unread postAuthor: jackssmirkingrevenge » Fri Jan 31, 2014 6:44 am

jsefcik wrote:Was using lead balls but i lost my molds :(


It could have been worse, you could have been using lead molds and lost your balls ;)

Darwin might have had a thing or two to say about that actually...
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Re: what size qev

Unread postAuthor: Ragnarok » Fri Jan 31, 2014 3:36 pm

Dig_Gil wrote:but it would create too much dead volume, that is, space where air will stall and not propel the marble).

Possibly, but you may also be looking at it with an overly simple view of dead volume.

Dead volume can actually be beneficial. The valve is generally the main flow restriction - under these circumstances, the dead volume acts as something of a "capacitor" to store up some of the air coming through the valve.

This does reduce the initial pressure of course, but as the dead volume is generally less of a restriction, its stored air can allow pressures to be higher further down the barrel - this is the point that the projectile's picked up speed and is producing the rapidly expanding volume that needs higher flow to keep pressurised.

~~~~~

As for choosing a valve, the accelerations involved are generally an important factor in how much flow is necessary.

For an example, GGDT predicts the 230 gram steel rod I used here as having a muzzle velocity of 67 m/s. Halving the valve diameter to 11mm gives a prediction of 60 m/s - about 90% of the velocity and 80% of the energy.
The 8 gram darts from these tests gives predictions of 265 m/s and 185 m/s under the same circumstances - this time about 70% of the velocity and 50% of the energy.

It's fairly simple. If the projectile's going to give you a long time to fill the space behind it, you don't need to fill the space behind it as fast.

For a D cell and 100 psi... well, that's never going to be setting speed records. Unless you're planning on also using considerably lighter projectiles than D cells, I'd say that using the 3/4" QEV won't hurt the performance much. Save the 1.5" sucker for a bigger project.
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Re: what size qev

Unread postAuthor: jsefcik » Fri Jan 31, 2014 10:41 pm

Makes sense,,,the 1 1/2 qev is set up mainly for golf balls and i got the 3/4 for free so might as well have some fun with it,,pvc is cheap,

Thats why i was wondering which one to use
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