My First Pressure gun ever. Over/Under Design, need help.

Yukondano2 · Sat Oct 30, 2010 11:48 am

jackssmirkingrevenge wrote:
saefroch wrote:As a general rule for pneumatics, your chamber should have 2-3 times the volume of your barrel, this is based on a generalization of efficency.
Too much of a generalisation, have a look through threads like this one.

So.. The smaller the chamber the more i have to pressurize.... but strangely enough i get more range. So.. how does that relate in ANY way to the relativity of chambers to barrels? Only issue i have with GGDT or whatever is that i don't know what to put IN IT. There's like.. wall thickness, volume... all this crazy crap. I actually have to MEASURE the wall thickness, put that in along with the other piles of measurements, then calculate?

Ragnarok · Sat Oct 30, 2010 12:19 pm

saefroch wrote:As a general rule for pneumatics, your chamber should have 2-3 times the volume of your barrel, this is based on a generalization of efficency.

No! More than 2:1 is always going to be a horrible waste. The "general rule", if there is one at all, is more like between half and twice your barrel volume.

Yukondano2 wrote:I actually have to MEASURE the wall thickness, put that in along with the other piles of measurements, then calculate?

No. This is a common mistake with GGDT.

I'd recommend pasting this file into the GGDT language folder. When it asks if you want to replace the old file, click yes. (It shouldn't break anything if you do it right.)
This changes the GGDT language files to relabel "Inner" and "Outer" diameter to names that don't actually imply the wall thickness needs to be measured.

I did try suggesting such a change to D_Hall a while ago, as it's the most common misinterpretation of GGDT, but I don't know whether he plans to make the change "official".

Yukondano2 · Sat Oct 30, 2010 12:45 pm

Ragnarok wrote:
saefroch wrote:As a general rule for pneumatics, your chamber should have 2-3 times the volume of your barrel, this is based on a generalization of efficency.
No! More than 2:1 is always going to be a horrible waste. The "general rule", if there is one at all, is more like between half and twice your barrel volume.

Yukondano2 wrote:I actually have to MEASURE the wall thickness, put that in along with the other piles of measurements, then calculate?
No. This is a common mistake with GGDT.

I'd recommend pasting this file into the GGDT language folder. When it asks if you want to replace the old file, click yes. (It shouldn't break anything if you do it right.)
This changes the GGDT language files to relabel "Inner" and "Outer" diameter to names that don't actually imply the wall thickness needs to be measured.

I did try suggesting such a change to D_Hall a while ago, as it's the most common misinterpretation of GGDT, but I don't know whether he plans to make the change "official".

So would a 2" compression chamber and a 1/2" barrel work very well, or does it begin to not work as well.

-EDIT-
New Design, dry fit. I think this will Work. I'm planning on making a brace out of wood. See pictures
http://filesmelt.com/dl/New_Gun_1.JPG
http://filesmelt.com/dl/Brace.JPG

Ragnarok · Sat Oct 30, 2010 1:11 pm

Yukondano2 wrote:So would a 2" compression chamber and a 1/2" barrel work very well, or does it begin to not work as well.

A larger chamber won't reduce performance - and strictly, increase in chamber size will always improve performance slightly.

However, after it's gone past a certain point (which is usually no more than twice the volume of the barrel*), then the performance gains will be incredibly small. All that extra volume (and, more importantly, the extra air to fill it) will just be pointlessly wasted.

* It's about a 2:1 ratio with a really good valve.
With a slower and less efficient valve (like the ball valve you're using), there just isn't enough flow to make use of all that extra air - for that kind of valve, you'd be better off with something more like 1:1 (similar volume of both barrel and chamber).

Yukondano2 · Sat Oct 30, 2010 1:13 pm

Ragnarok wrote:
Yukondano2 wrote:So would a 2" compression chamber and a 1/2" barrel work very well, or does it begin to not work as well.
A larger chamber won't reduce performance - and strictly, increase in chamber size will always improve performance slightly.

However, after it's gone past a certain point (which is usually no more than twice the volume of the barrel*), then the performance gains will be incredibly small. All that extra volume (and, more importantly, the extra air to fill it) will just be pointlessly wasted.

* It's about a 2:1 ratio with a really good valve.
With a slower and less efficient valve (like the ball valve you're using), there just isn't enough flow to make use of all that extra air - for that kind of valve, you'd be better off with something more like 1:1 (similar volume of both barrel and chamber).

I'd use something besides a ball valve.. but i don't know how. I've heard about QUEVs or QUV or QUDV or something like that... don't quite remember. Are those like.. REALLY hard to set up?

-EDIT- I've seen videos where they actually had a spring that Opened the ball valve after firing a trigger. Those would be good too if i... knew how to build one of those. Honestly i don't want to use a ball valve because its a pain to open quickly, it screws up my aim, all that.

-EDIT2-
I don't see how the air can be wasted. If the entire diameter of the barrel is filled with ammo (Spud) then how can air escape? Would the air like come in late, after the ammo has left the barrel?

Technician1002 · Sat Oct 30, 2010 1:15 pm

Are you trying to say QEV (Quick Exhaust Valve) or QDV (Quick Dump Valve)? They both work well.

Yukondano2 · Sat Oct 30, 2010 1:17 pm

Technician1002 wrote:Are you trying to say QEV (Quick Exhaust Valve) or QDV (Quick Dump Valve)? They both work well.

Uhm.....
Which is cheaper, and which is easier to set up/use? REALLY all i care about. That and efficiency of course. If i had known this was inefficient i wouldn't have bought it :l I guess I'll knew better for next time.

-EDIT-
I MIGHT be able to buy some 1" pipe so that the barrel and the chamber aren't as different. I would be at a loss of some bushings.. but they cost 99 cents so, meh.
Only issue is that i would need a new valve, but seeing as how I kind of want to get my hands on a QDV or QEV probably doesn't matter.
-EDIT2-
Thanks for your help so far guys. I've learned quite a bit about Potato guns so far and i know there's much more. I'm definitely sticking around these forums. Thanks for the welcome!

Ragnarok · Sat Oct 30, 2010 1:37 pm

Yukondano2 wrote:Are those like.. REALLY hard to set up?

QEVs are pretty easy to set up. QDVs are custom made, so they're not so easy - not a good choice for a first project.
(Efficiency between the two is comparable.)

One thing to say is that QEVs are usually only found on internet stores and can be slightly costly as valves go. Not prohibitively so, but they're not the cheapest option.

Would the air like come in late, after the ammo has left the barrel?

In simple terms - yeah, that's close enough.

Yukondano2 · Sat Oct 30, 2010 1:43 pm

I've been looking into sprinkler valves and using those for a pilot valve and such. Not sure i understand it sadly, and it seems to involve modifying it. How would that work?

Gun Freak · Sat Oct 30, 2010 1:48 pm

Modifying a sprinkler valve simply means adding a pneumatic valve to the top as opposed to using the solenoid to vent the pilot.

Yukondano2 · Sat Oct 30, 2010 1:50 pm

Gun Freak wrote:Modifying a sprinkler valve simply means adding a pneumatic valve to the top as opposed to using the solenoid to vent the pilot.

Yeah i still don't understand. I watched this video
http://www.spudfiles.com/forums/the-spr ... 16786.html
I don't see how that relates to adding a valve.. or how in the process i would do that. I'm kinda noobish so I wish i didn't have to do this. If only i could get a QEV.

Gun Freak · Sat Oct 30, 2010 1:55 pm

You don't need a QEV to mod a sprinkler. Drill a hole in the top, tap 1/4" threads and screw in a blowgun and bam, modified spinkler valve.

Yukondano2 · Sat Oct 30, 2010 2:00 pm

Gun Freak wrote:You don't need a QEV to mod a sprinkler. Drill a hole in the top, tap 1/4" threads and screw in a blowgun and bam, modified spinkler valve.

Well thats really descriptive (sarcasm)

So, blowgun is a what? Garden hose nozzle? I don't want to screw into the valve and then screw it up. (Crap that was a pun)

Anywho, Can someone link me to something a wee bit more descriptive? I'm searching around to find something.

Gun Freak · Sat Oct 30, 2010 2:04 pm

Youtube video:
http://www.youtube.com/watch?v=A3EOdNP6Iag
http://www.youtube.com/watch?v=qY8c8iybpwA
Examples:
http://www.spudfiles.com/forums/my-firs ... 21882.html
http://i979.photobucket.com/albums/ae27 ... 0_4011.jpg
http://www.spudfiles.com/forums/4-shot- ... 21284.html

Yukondano2 · Sat Oct 30, 2010 2:12 pm

Gun Freak wrote:Youtube video:
http://www.youtube.com/watch?v=A3EOdNP6Iag
http://www.youtube.com/watch?v=qY8c8iybpwA
Examples:
http://www.spudfiles.com/forums/my-firs ... 21882.html
http://i979.photobucket.com/albums/ae27 ... 0_4011.jpg
http://www.spudfiles.com/forums/4-shot- ... 21284.html

I just finished watching it. Thanks.
Yeah That looks a wee bit tough though. Sprinkler valves look pricey. 13 bucks or so it says on the site though, Hm.
-EDIT-
Is it really necessary to put Teflon tape on the valve? Because if i ever make another one i might want to be able to remove the blowgun to put it on other valves.
-Edit2-
I noticed they didn't say anything on the site about shipping and handling. Do they not charge?