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is my valve good?

Posted: Mon May 04, 2020 7:26 am
by Backspace
hello, first time builder (and poster) here,

i'm planning out to build a spud gun using compressed air, the classic tank - valve - barrel design, but i was looking for an electrical valve so i could just push a button to shoot, but i'm having doubts. the one i have my eye on is 1/2 inch in diameter, while my barrel is 75mm. I can make it fit, but am i going to lose a lot of pressure by going so small in the valve and than expanding it back to 75mm?

thanks in advance!

Re: is my valve good?

Posted: Wed May 06, 2020 5:07 am
by jrrdw
Anytime you decrease a port size in the middle a pneumatic system, you create a choke point. It's best to match the port sizes or as close as you can.

Welcome to Spudfiles. 😀

Re: is my valve good?

Posted: Thu May 07, 2020 8:17 am
by Anatine Duo
I agree, that is a pretty extreme choke. I have used 1/2" valve on 20mm barrel. Valve has less than half the flow of the barrel, still good for velocities around 499 fps at bicycle pump pressures with light projectiles. If you want a significant portion of the velocity available then more flow is needed. If you are planning a very low velocity launch then perhaps pressure will be maintained behind the projectile

Cross sectional area of 1/2" tube (free flowing valve) is 0.196 sq inches. Cross sectional area of 75mm (2.953") is 6.844 sq inches. THe valve will have less than 3% the flow of the barrel. Seat of the pants calculations put velocity around 10 fps ;) unless extreme pressures are used.

My math isn't good so anybody is welcome to fix that :)

Just for fun I ran some specs through GGDT and projectile failed to clear muzzle unless i put 2000 psi through with giant reservoir. Interesting.

Re: is my valve good?

Posted: Thu May 07, 2020 2:54 pm
by Cthulhu
It's more work but if you really wanna go electronic you could make a piston valve that is vented with an electronic solenoid.
Also look into sprinkler valves that are modified for pneumatic action, they also work with just the push of a button, but without having to worry about batteries.