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Meter Pipe

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Meter Pipe

Unread postAuthor: Joshatdot » Sat Dec 12, 2009 3:04 am

I've decided to convert my Spray-n-Pray to measured propane. I've read up abit over at Burnt Latke. Once I mod my chamber it should be about 230 in^3. According to their calc, I need 8.773in length of 3/8 pipe for a 4.03% mixture. But I am wondering if that takes into effect the reducer pieces and/or not the T for a pressure gauge.

I've made a mockup meter pipe, but don't have any way of accurately measuring its volume.

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Re: Meter Pipe

Unread postAuthor: D_Hall » Sat Dec 12, 2009 3:19 am

Joshatdot wrote:I've made a mockup meter pipe, but don't have any way of accurately measuring its volume.

I don't believe you.


1) Fill meter pipe with water.
2) Dump water into a measuring cup or similar measurement device.
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Unread postAuthor: Joshatdot » Sat Dec 12, 2009 5:17 am

key word accurately .. all I got is like a cooking/baking cup, and it's in OZs :( . Plus I don't have a pressure gauge yet in the "T" to measure it.

I'll check around to see if I can find a diff measuring cup with mL, good thinking! I don't know why I didn't think of that before.

edit: Found a different one in the pantry!. Mmmmmm, I am only getting about 50mL of water from this. I though I would be getting close to the 151.9 mL that I need, according to BL.

edit2: No wait! I was reading BL wrong, I only need 24.8 mL @ 90 psi in the meter pipe! which will expand to 151.9 mL @ 14.7 psi!
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Unread postAuthor: SpudBlaster15 » Sat Dec 12, 2009 5:54 am

Just a tip; if you live in a climate that sees the ambient temperature drop below 'normal' values (70 degrees F or so) for any significant period of time, don't design the meter around the vapor pressure of propane at room temperature. If you do, the meter will not be able to reach the proper pressure during cooler weather, and you will need to use multiple lower pressure injections to create a stoichiometric mixture of air and fuel. This is a pain, and can potentially increase the error margin of the meter. I usually aim for an operating pressure of about 50PSIG, which allows me to properly charge the meter at temps down to a few degrees below freezing (IIRC). Any colder than that, and it's going to be pretty uncomfortable to stand outside and fire more than a few shots anyway.
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Unread postAuthor: Joshatdot » Sat Dec 12, 2009 6:49 pm

I got it down to about 29~30 mL, I'll just have to adjust the PSI alil down.

How does the quick disconnect stuff work? just regular 1/4" air fittings?
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Unread postAuthor: McCoytheGreater » Tue Dec 15, 2009 3:45 pm

How does the quick disconnect stuff work? just regular 1/4" air fittings?


Yes. I'm not sure if it's fine or coarse thread, though. PTFE tape is a must (but you knew this). Also, don't rule out the possibility for using barb type connectors. If you're worried about the extra fuel in the pressure gauge, try this, have the pressure gauge at the propane tank instead of on the meter pipe, though I see you already have the T in place for it.
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Unread postAuthor: Joshatdot » Fri Dec 18, 2009 12:29 am

How do you keep the quick disconnect/air fittings air/gas tight? I know the female air fitting is air tight, but how about the male fitting? It's just a metal tube with 1/4 NPT & air fitting, I don't see how it will be air/gas tight.

I guess you'll have the male on the chamber and the female on the meter pipe, and leave it on the chamber when fireing...firing?

edit: what's the EZ'est way to measure my chamber? I don't think our juice pitchers have mL measure marks. I re-calculated my chamber, and would guess now it is about 213.628 in^3 (about 3500.73571 mL)
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Last edited by Joshatdot on Fri Dec 18, 2009 12:58 am, edited 1 time in total.
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Unread postAuthor: McCoytheGreater » Fri Dec 18, 2009 12:53 am

You can do that. that's pretty much how it works anyway. If you're worried about hose coming off of those barb fittings don't, they regularly can hold up to 150 PSI easily. Our large chamber gun is going to have the male fitting mounted on the chamber with the female fitting connected to our feed hose via a barb fitting so propane doesn't leak out of the hose. We've done it before, and I'm pretty sure it'll work for you. If you're asking about how the Air fittings stay airtight, they use an O-ring to seal the male to the female. When the male is disconnected it's a lot like a pop exhaust for combustion guns. It works for Air tools and they don't leak (much anyway) Besides, the propane is going to be valve controlled in a leak proof sysyem anyway (using PTFE tape on every joint of course, even the air fittings). Hope this answers questions.
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Unread postAuthor: Joshatdot » Fri Dec 18, 2009 1:09 am

Thanks McCoy, I just looked at another thread, and I like this idea or quick connect meter pipe. I might have a go at it after I get this all working.
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Unread postAuthor: Joshatdot » Fri Dec 18, 2009 3:54 am

Poops! I think I narfed me cannon!

I wanted to fill some voids in the chamber (the front handle right now). So I filled it with GREAT STUFF expanding foam. And just now saw that it is flammable at 250*f when cured.

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Unread postAuthor: jimmy101 » Fri Dec 18, 2009 1:36 pm

I wouldn't worry too much about the foam burning. If the gun is fueled properly and works like it should, the gases in the chamber are only above 250F for a fraction of a second. Besides, isn't temperature so much that ignites things it is heat. There is fairly little heat in the combustion gases.

I would worry more about the foam getting compressed and absorbing some of the energy in the chamber.
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