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Remote metering + displacement

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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Remote metering + displacement

Unread postAuthor: polk county » Thu Sep 04, 2008 3:39 pm

I want to mount a conventional propane metering device away from the gun and on a plywood box. The propane will be fed tfrom the box to the chamber through a 3 foot fuel line. How will this effect the accuracy and precision of the propane metering device? Do I have to include the volume of the 3ft line in the total chamber volume.
Here is the chain.... cylinder >torch head >regulator >Ball valve >metering pipe >ball valve > 3 ft fuel line > 90 elbow > chamber

Someone said that fueltool accounts for displacement. I don't think this includes three feet of extra line

thanks for the help
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Unread postAuthor: ramses » Thu Sep 04, 2008 3:50 pm

you could add it to the chamber volume, but for 1/4" id hose, you're only adding .56 cu in. Odds are this won't effect accuracy at all.
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Unread postAuthor: potatoflinger » Thu Sep 04, 2008 5:03 pm

I'm not sure if it will be a problem or not, but to solve it you could make this your chain... cylinder> torch head> regulator> fuel line> ball valve> meter pipe> ball valve> 90 degree elbow. (or you could replace the elbow with a quick disconnect so that the whole thing can be taken off of the gun easily)
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Unread postAuthor: jimmy101 » Fri Sep 05, 2008 4:41 pm

ramses wrote:you could add it to the chamber volume, but for 1/4" id hose, you're only adding .56 cu in. Odds are this won't effect accuracy at all.

For 36" of 1/4" ID hose:
Volume = Pi(r<sup>2</sup>)L
Volume = (3.14159)(1/4/2)<sup>2</sup>(36")
Volume = 1.77 in<sup>2</sup> = 29cc

That's a lot of propane, probably beteeen 25% and 50% of the fuel load for a "typical" combustion gun.

You could just add that volume to the calculated volume that the meter puts out but you get an unknown amount of diffusion of the fuel that is supposed to have stayed in the hose into the chamber. So the hose is a significant factor and there needs to be a valve beteen the hose and the chamber.

A couple possibilities besides a valve;
1. Go to Mcmaster and order some very small ID steel line (capillary line). Something like an ID of 0.01" or less. Use that as the hose. The volume of gas in the capillary will be insignificant. Diffusion of the fuel (and air) in the capillary will be insignificant in a time frame of a few tens of minutes.

2. Stay with what you have but put a very small orifice at the chamber end of the hose. If the hole is small enough diffusion will be greatly reduced and the fuel that is supposed to be left in the hose will stay put. The orifice of a Bernzomatic torch head would probably work. Use the smallest ID hose you can get easily. There is really no need for a large hose ID and in this setup 1/4" is on the large side.

Any method you use will require that you thoroughly "burp" the system before the first use.
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Unread postAuthor: man_o_brass » Mon Sep 08, 2008 10:34 am

Jimmy nailed your explanation, but there's one more option you may want to consider if cost isn't a factor. You could still mount the meter pipe directly to the gun and use electrical solenoid valves run off a battery. They're not cheap, but they would eliminate any uncertainity in your metering setup. McMaster has quite a selection running between $40 and $90, but I'd have to do alot more research to figure out what's what before I could recommend one.
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Unread postAuthor: starman » Mon Sep 08, 2008 1:31 pm

Here's homedepotpro's "remote" (sort of) metering solution that I'm using. Not sure if this serves your purposes or not. It's similar to Jimmy's syringe solution.

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Unread postAuthor: ramses » Mon Sep 08, 2008 2:04 pm

ahh... sorry. I forgot about diffusion. :oops:
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