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GGDT Question

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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GGDT Question

Unread postAuthor: socoj2 » Sun Sep 30, 2007 8:31 am

I am going to use the following valve in my cannon. and i need to figure out if i need to go a larger one so i am using GGDT..

A quick exhaust Valve.

Inlet - outlet - exhaust
3/8 - 3/8 - 3/8 - Co-efficient of Volume 2.98 Max CFM@100 PSI 178

WHat do i p ut in GGDT for:

Valve Type
Flow Coef %
and Seat Diameter

thanks
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Unread postAuthor: windshrike » Sun Sep 30, 2007 9:03 am

Valve type: umm.... a QEV is barrel-sealing I THINK...might want to wait for a 2nd opinion
Flow Coefficient: keep it around 50%, unless you have a huge QEV and a very tiny chamber(like a 1 1/2 valve for 8-inches of 1/2" pipe)
Seat diameter is the diameter against which the sealing face seals, so it should the diameter of where you attach the barrel(don't be fooled, actual diameter is larger than the name it is given, i.e 1/2" pipe can be .6022")
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Unread postAuthor: iPaintball » Sun Sep 30, 2007 10:03 am

Most QEVs have a flow coefficient of 45%. A QEV is a barrel sealing valve, and for a 3/8" porting valve, put 3/8" for seat diameter.
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Unread postAuthor: noname » Sun Sep 30, 2007 10:57 am

iPaintball wrote:for a 3/8" porting valve, put 3/8" for seat diameter.


I'd have to disagree with that. A 3/4" sprinkler has a 1.1" seat; I doubt the QEV has a 3/8" diameter seat.

Take apart the valve and measure th diameter of the outlet hole, from the inside, where the diaphragm seals against the barrel.
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Unread postAuthor: iPaintball » Sun Sep 30, 2007 11:38 am

QEVs are different than sprinkler valves. I know for a fact that my 3/4" qwv has a seat the same diameter as the port.
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Unread postAuthor: noname » Sun Sep 30, 2007 11:47 am

You never know unless you actually measure it, I guess. Plus, it could be a different brand valve or something. Even a 3/8" seat diameter and a 1/2" or 1/4" seat diameter could throw off the calculations.
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Unread postAuthor: socoj2 » Sun Sep 30, 2007 12:46 pm

Ok lets just assumem .375 for Seat Diameter..

Now that leaves

Piston Diam
Pison Mass
Vent Diam
Pilot Vol
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Unread postAuthor: iPaintball » Sun Sep 30, 2007 1:16 pm

Take out the diaghpragm/piston and mass it, measure the diameter, Find the port of the pilot valve, and measure the pilot area with water and dump it into a grad. cylinder to find pilot volume.
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Unread postAuthor: clide » Sun Sep 30, 2007 2:42 pm

For most accurate results you should measure the piston diameter, seat diameter, and piston mass. You can probably estimate the pilot volume by measuring it if you don't want to mess with liquid measurement. If you are using a blowgun for exhaust like most people who use QEVs then your vent diameter would probably be about .15" for a nice unmodded blowgun.

The good news is that you already have the value for the most important aspect of a valve, the flow coefficient. Go to File>Configuration and under valve flow parameter change it from Efficiency to Cv. Now you can just use the 2.98 value given by the specs in the flow coefficient box.

If you still don't want to measure then given that flow coefficient I would guess the seat diameter is around 0.5" and piston diameter is around 0.75".
For piston mass use a low value like 0.1 oz and 0.5 cubic inches for pilot volume.
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Unread postAuthor: socoj2 » Sun Sep 30, 2007 6:07 pm

Does this program actually help people?

So far there is only a bunch of guestimation of what to actually put in. and with the stuff i have put in so far Its giving me 45 FPS.. which is not accurate based on my test so far..

based on the stuff i put into the spud tech spreadsheet for chamber and barrel. Should be in the upper 200s at 100 PSi

And i am using a 1/8th Solenoid 3way normally closed solenoid to Pilot the QEV.
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Unread postAuthor: Redcoat » Sun Sep 30, 2007 6:08 pm

Yeah! Tonnes of people have told me that it's helped them.

Where do you download it from?
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Unread postAuthor: DYI » Sun Sep 30, 2007 6:12 pm

GGDT is the single most useful pneumatic launcher design tool that I have ever used. The reason that lots of info is needed is to obtain an accurate result. The result are usually incredibly accurate (except for supersonic speeds, which it is almost useless for).

You can download it from the Hall's site, but I don't have a link. Just search Google for GGDT and you should find it. Be sure to follow the instructions closely, or it won't work.
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Unread postAuthor: clide » Sun Sep 30, 2007 9:15 pm

socoj2 wrote:Does this program actually help people?

So far there is only a bunch of guestimation of what to actually put in. and with the stuff i have put in so far Its giving me 45 FPS.. which is not accurate based on my test so far..


Its a bunch of guestimation because we don't have your valve, everything that you put into GGDT you can measure and get a very accurate result... but we can't measure it for you.

If your getting strangely low results it is probably because you aren't putting in the chamber dimensions right. Inner diameter should be 0 (that box is for coaxials) and outer diameter should be the inner diameter of your chamber pipe.

GGDT is VERY accurate if you put the proper inputs in. Just a few weeks ago I built a launcher that performed within 1 fps of what GGDT had predicted.
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