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Quick question, first post!

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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Quick question, first post!

Unread postAuthor: xopti » Fri May 14, 2010 11:14 am

I am going to build a advanced spud gun. I will be using metered propane. Most guns I see on here have a psi gauge built into the regulator (i.e husky regulator), but then they throw on a extra gauge on the meter pipe. If you already know the psi post regulator, why waste the money for a gauge on the meter?

Thanks,

Alex
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Unread postAuthor: Technician1002 » Fri May 14, 2010 11:52 am

Many regulators with small diaphragms have poor repeatability. Many regulators have a pressure gauge that reads the maximum pressure it can put out.

Using a lower pressure gauge on the pipe permits better accuracy. For example, the gauge shown above may read to 100 PSI. Using a 30PSI guage on the meter pipe will make it much more easy to set a precise 25 PSI if your meter is using lower pressures.
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Unread postAuthor: mark.f » Fri May 14, 2010 1:07 pm

You could also replace the gauge on the regulator with a lower range one (the gauges on the regulators are just normal 1/8" NPT ones), in that vein.
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Unread postAuthor: Moonbogg » Fri May 14, 2010 7:18 pm

Some people don't use regulators, only gages. They turn the knob until the meter reads correctly, close the knob and shoot.
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Unread postAuthor: xopti » Sat May 15, 2010 9:41 pm

Okay thanks guys. I am going to add a T of the second ball valve with a secondary pressure gauge.

Now I just need to figure out how big to make the chamber. I want the C:B to be at least .9 with a 1.5" X 72" spud barrel. That would give me a chamber volume of 125 ci. I will have a golf ball barrel, and a tennis ball barrel.

Now the question I have is, what is the optimal chamber size for the highest internal pressure possible?

I read somewhere that with a advanced spud using only metered propane, you can achieve up to 120 psi. I guess what I'm getting at is should my chamber only be 125 ci, or would a bigger chamber with a 1.5" x 72" barrel be more powerful?

Here are some pics of my build so far, enjoy!

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Unread postAuthor: MrCrowley » Sun May 16, 2010 12:31 am

I wouldn't trust a DWV cleanout cap on an advanced combustion. There was a news story of one flying off and hitting a man in the chest, stopping his heart and killing him.

Perhaps you should use pressure rated pipe, at least instead of the cleanout adapter. If you don't know how to identify pressure rated pipe, read this.

You might get better performance with a slightly larger chamber. Try use the program HGDT to figure that out. Google HGDT if you don't know what it is and you should find the creator's website which also explains how to use it. I know with short barrels, a chamber that size is enough, but with a 6' barrel, you might be able to go bigger and still get noticeable improvements in performance.
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Unread postAuthor: starman » Sun May 16, 2010 1:40 am

Agree with Crowley on the cleanout cap...dump it!! It WILL blow out. Build yourself something that is much more substantial and will allow you to open for venting. Here's an example....

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Here's the link to HGDT.
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Unread postAuthor: xopti » Sun May 16, 2010 8:13 pm

Thanks for suggesting that program, it really helped! I'm going with 175 ci, 3000 ml.

About the DWV end cap.. I have a Lowes, Home Depot, and Menards near me and I am still having trouble finding the PVC fittings I want. Any recommendations on where to find PVC, brick and mortar or online?
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Unread postAuthor: Gun Freak » Sun May 16, 2010 8:17 pm

you couldnt find a cap big enough?
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Unread postAuthor: Technician1002 » Sun May 16, 2010 8:31 pm

Look for commercial outlets instead of the big box stores. Fine where the local contractors and plumbers shop.

I used these guys for some parts. It is where I got the 2.5 inch stuff for the soda can and tennis ball barrel
http://www.ferguson.com/index.shtml
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Unread postAuthor: McCoytheGreater » Mon May 17, 2010 2:01 am

MrCrowley wrote:I wouldn't trust a DWV cleanout cap on an advanced combustion. There was a news story of one flying off and hitting a man in the chest, stopping his heart and killing him.


Now come on, Crowley. Plenty of noob spudders used DWV caps on their first advanced guns so lets not scare this one out of a good time. Do be careful, though, and make sure it's not sewer/drain fittings. DWV holds up much better than sewer/drain fittings. It's not recommended, but it works.



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Unread postAuthor: Crna Legija » Mon May 17, 2010 3:02 am

im pretty sure that sewer/drain fittings are dwv

that would be the drain in drain, wast, vent
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Unread postAuthor: mark.f » Mon May 17, 2010 3:28 pm

Actually, sewer fittings are an entirely different beast than SCH-40 DWV fittings. They are thinner, and the cleanout plugs thread all the way into the female adapter (flush).
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Unread postAuthor: Technician1002 » Mon May 17, 2010 4:20 pm

My Cellular Core DWV test cannon uses a 2 inch cleanout (2 inch) and it does blow out sometimes. This is a replacement from the original Sched 40 PVC female adapter and plug that shattered. Due to the short acceleration distance, it smarts when the valve blows out, but there has never been any serious injuries. The valve is foot operated. I would prefer a blow out onto my foot instead of in my face.

For a combustion, I highly recommend to never place body parts behind them. The forces go up 4X from 2 inch to 4 inch. I would not want to get hit with a plug from a 4 inch blow out. PVC does not prevent failures. Treat both as if they will fail.

ABS is flexible enough to simply screw the lid back on. PVC is brittle and you will need to sweep up the broken parts and try again.
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Unread postAuthor: xopti » Fri May 21, 2010 1:07 pm

Okay guys, thanks again for all of the valuable information. I finally got all of the parts and I have started to assemble. I am about to glue the chamber and will be going with 175 ci, 3000 ml, do you think this is a good size? Speak now or forever hold your peace!
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