Piston or no piston

[ralphd]

I am a long time combustion fan but now I am considering going pneumatic, well at least for my next launcher. I would appreciate some advice from you pros out there. One point in particular is the use of a piston. Is it better or just a demonstration of building skill. I see that the use of a sprinkler valve is used as well. I would also appreciate any location of tested ratios or figures. There are a ton for combustion. Any info helpful.

[Gippeto]

Opinions vary widely on the whole C:B issue with pneumatics.

The best solution is to play with chamber volume in ggdt. This will allow you to get satisfactory power, and minimize pumping.

I would suggest 0.5:1 to 1.5:1 as being in the best range of compromise.

Piston valves are not about any demonstrations. They simply provide a better valve, and despite some assertions to the contrary, are quite easily built.

Understand how and why a piston valve works, and you're most of the way there.

Build it neatly, with the principles in mind and it WILL work.

I've never bought a sprinkler valve, never used one, and don't intend to.

BUT;

Judging by the "how to mod a sprinkler" type threads, there isn't much to them. Should be an easy valve to get working. (<60 minute launcher)

[jonnyboy]

I would guess that if you had a 3/4 inch sprinkler and 3/4 piston the piston would beat it but by how much I do not know I've never built a sprinkler cannon before.

Your going to invest alot more time into a piston valve than a sprinkler valve so if this is just a weekend project go for a sprinkler valve they offer decent performance but the come with a pricetag.

[john bunsenburner]

i think pistons are cheeper over all...especially if you mod the sprinkler to use a pilot.

[roboman]

i think pistons are cheeper over all...especially if you mod the sprinkler to use a pilot.

It all depends on the materials used, valve size (larger fittings aren't cheap), and pilot valve. You can pick up a 1" sprinkler at Home Depot for ~$10, and a blowgun is only ~$3 at Harbor Freight Tools. Of course, you can build much larger piston valves that have higher flow rates, but they are more expensive than sprinklers.

[john bunsenburner]

a piston needs a rubber sealing surface that like .50$(more per mat but you can make lots), then 20bucks fro 250ml of casting resin, that will make like 100pistons so .20$. And then .50$ per O-ring and 3$ for a pilot: 5.20$ per simple piston...

[jonnyboy]

Well its hardly fair to compare valves that are different sizes. It's like comparing the size of a turtle to a giraffe. So lets compare a 1 inch sprinkler and a 1 inch piston valve. Assuming all the fittings are a dollar a piece you still come out on top. Last time I was at home depot the 3/4 fitting were about 30 cents so it will probably be less.

[john bunsenburner]

and remember the better performance of a piston, you have to add that ontop of the sprinkerl...

[Ragnarok]

One point in particular is the use of a piston. Is it better or just a demonstration of building skill.

It's a bit of both, but mostly that it's just that much better.

In general, a piston valve of the same seat diameter has roughly twice the flow that the sprinkler valve has, and generally a slightly faster opening time.
That doesn't however translate to twice the power I'm afraid, but it does make a big difference.

Should be an easy valve to get working. (<60 minute launcher)

Not forgetting the 24 hour PVC cement drying time - so less build time, but eventually, you won't be using the launcher any quicker.

However, after the basic parts of the build are finished, it will likely require less fettling than a piston valve, so it will be less effort.

[ralphd]

This is some good stuff to start with. I just jumped in with my combustion and ended spending more time & money with all the mods to get what I finally wanted. Actually, I still would like to do another mod or two but it's too loud & heavy now. My friends say it's as loud as my 12 gauge. By the way, how loud are pneumatics. I do realize that releasing compressed air in an instant isn't silent but compared to a combustion. I watched a bunch of videos on YouTube but couldn't tell.

[roboman]

It all depends on the efficiency and chamber size of the cannon. If there is just enough air to fill the barrel, then there won't be much left over to make noise.

(Taken from the Wiki)