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Anyone wanna hop on aim and Discuss my failings at GGDT?

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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Anyone wanna hop on aim and Discuss my failings at GGDT?

Unread postAuthor: socoj2 » Sat Oct 06, 2007 8:38 pm

Seriously, by my calc my 3/8s QEV should be able to flow 128 Cubic inchs at 100 psi in 25 m/s

Using another spreadsheet for chamber to barrel volume i come up with 270 FPS given this ratio.

Yet GGDT is sticking me at 120 fps max

Hell i have even



Information below

20" long 2" SCH40 Barrel
15" long 2" SCH40 Chamber

3/8x3/8x3/8 QEV from mcmaster part #6646K11
C/V of 2.93
SCFM@100 PSI is 178

Piloted by a 1/8th Solenoid

it triggers almost instantly when i throw the valve, yet i dont want to put this thing together to find out i need a different valve. so i cant really test it.

Ill be online for another 2 hours, if anyone wants to hit me up on AIM

SOCOJ2
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Unread postAuthor: clide » Sat Oct 06, 2007 9:25 pm

I get the same thing, so I don't think you are doing anything wrong with GGDT. Its just that the valve has very little flow for a 2" barrel (less than half the flow of a sprinkler).

Why you got higher on the spreadsheet I would guess is because it doesn't take into account valve flow which has a significant affect on performance.
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Unread postAuthor: socoj2 » Sat Oct 06, 2007 9:33 pm

based on some math, a 3/8ths QEV will still be able to dump 3 times as much volume as whats int he chamber in 25 ms though.

THis is not a huge ammount of air to move.

what is the ratings on a 3/4 of 1 inch RaidBird?
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Unread postAuthor: clide » Sat Oct 06, 2007 11:15 pm

GGDT is very accurate. I have yet to see a reason to doubt it except at very high velocities.

As for the time it takes to move the volume in the chamber, the SCF part of SCFM means cubic feet at standard pressure so being at 100 psi you would have 6.8 times your chamber volume as your volume at standard pressure. I did a rough calculation (that could be wrong) and got about 62 ms to empty the chamber. Not to mention that flow decreases as pressure decreases.

The sprinkler valves that I have measured (1") have a Cv between 8 and 9. I would expect a 3/4" to have about half of that.
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Unread postAuthor: noname » Sun Oct 07, 2007 12:19 am

clide wrote:The sprinkler valves that I have measured (1") have a Cv between 8 and 9. I would expect a 3/4" to have about half of that.


Why? The valves are EXACTLY the same in every way, except for the fact that the thread size is different.
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Unread postAuthor: clide » Sun Oct 07, 2007 1:00 am

noname wrote:
clide wrote:The sprinkler valves that I have measured (1") have a Cv between 8 and 9. I would expect a 3/4" to have about half of that.


Why? The valves are EXACTLY the same in every way, except for the fact that the thread size is different.


Not on an orbit watermaster. The top cover and diaphragm are the same, but the lower portion is smaller and has smaller porting. Just went down and measured it, but it was larger than the .75" like I figured it would be. I would expect about 72% of the flow of a 1" valve now that I've measured it.

Not sure about other sprinkler valves.
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Unread postAuthor: jackssmirkingrevenge » Sun Oct 07, 2007 1:54 am

I would say GGDT is being realistic, the porting on a 3/8" valve is minute compared to a 2" chamber - we're talking about a flow area difference of 2855%
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Unread postAuthor: noname » Sun Oct 07, 2007 10:01 am

Really? I always thought they were. I guess I'd never had 2 of them taken apart side by side before.
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