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Pop-off / Safety Valve to empty a chamber

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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Pop-off / Safety Valve to empty a chamber

Unread postAuthor: Strigol » Fri Mar 27, 2009 5:35 pm

Working out an idea in my head and wanted some advice before I'd built anything

Working on a piston, air flow would be coming in through a schrader valve

Could you use a Safety valve as a sort of semi-auto device?

Fill the chamber to the safety valves pressure. Safety valve blows, piston comes back to the epoxy stops, sealing off the valves and firing my projectile.
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Unread postAuthor: jrrdw » Fri Mar 27, 2009 6:05 pm

No i don't think it will work because after enough air escapes through the pop off it will close back up. May not be enough to acuate the piston.

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Unread postAuthor: ramses » Fri Mar 27, 2009 6:15 pm

I agree, but with proper choice of tee and barrel (using a bigger tee than normal) a lower pressure drop would be necessary for actuation. The popoff on my 150psi compressor closes at roughly 50psi. Using that example, the piston would start to close when the chamber reached 50psi. This would have the advantage of retaining some air in the chamber for the next shot. at 100psi, a minimum C:B ration would be 1:1.

I would be more concerned about low flow through the schrader.

the piston would need to be very tight, maybe with a check valve integrated into the leak hole type thing.
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Unread postAuthor: jackssmirkingrevenge » Fri Mar 27, 2009 7:01 pm

jrrdw wrote:No i don't think it will work because after enough air escapes through the pop off it will close back up. May not be enough to acuate the piston.


I think it will ;)
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Unread postAuthor: Strigol » Fri Mar 27, 2009 10:20 pm

Thanks for the link, big help looking at it

I had one more question though, anyone know how fast a standard pop off valve would exhaust at ~90psi
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Unread postAuthor: jackssmirkingrevenge » Sat Mar 28, 2009 12:09 am

Strigol wrote:I had one more question though, anyone know how fast a standard pop off valve would exhaust at ~90psi


It depends on the size of your chamber.

Have a look here for my experiences on the subject.

I was fine with a homemade pop-off but when it came to using a standard one (check the end of the thread) it turns out the pilot volume was too small for it to work effectively.

My best success so far was using a pop-off valve directly as a firing valve.
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Unread postAuthor: psycix » Sat Mar 28, 2009 3:50 pm

jackssmirkingrevenge wrote:
jrrdw wrote:No i don't think it will work because after enough air escapes through the pop off it will close back up. May not be enough to acuate the piston.


I think it will ;)

I'm very sure it will.
More information on this within a few days... :D
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Unread postAuthor: Strigol » Sat Mar 28, 2009 5:13 pm

Once again thanks for the links

If I ever get a day off work, I'll be attempting my build finally
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Unread postAuthor: jonnyboy » Sat Mar 28, 2009 8:06 pm

The schrader valve may not be enough to seal the piston. I would look into building a snap valve as a pilot btb built one for his qev+pop off gun.
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Unread postAuthor: Mitchza89 » Sat Mar 28, 2009 11:14 pm

Pop off valves are still the best actuator I've ever used. I took the spring out of it and stuck on a sear to hold the pin of the pop off valve closed. As soon as you rip out that sear, the pop off valve I could say had 'explosive' air flow. I'm sticking one on my new marble piston gun now :)


...And yes, I achieved semi-auto on the same gun using a pop off valve. It took me a lot of mucking around with the piston, pilot and spring but I got it. Except I preferred the 'no spring and sear'.
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