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Propane meter question.

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Propane meter question.

Unread postAuthor: TomMazzone » Fri Feb 18, 2011 9:48 am

So I think I have down the concept of the propane metered spud gun. I understand that the optimal combustion environment for propane is 4% gas to oxygen volume in the comb. chamber. My question is this. When doing the math to figure out how much propane you need to meter for your specific set up (comb. chamber size, etc.) what is most important? From what I've read it appears as if the propane meter size (pipe diameter and length) doesn't matter as long as you know you're filling it with 4% of your combustion chamber volume. How can you regulate this? Do you need to adjust the setting on your pressure regulator / PSI indicator, or do you need to adjust the size of your meter pipe?

I've seen guys online selling propane meters that claim to work with any combustion chamber. This is confusing to me since they have no idea what size / volume your combustion chamber actually is, therefore do not know 4% of it. This leads me to believe that the meter pipe dimensions are insignificant and that the pressure regulator / PSI indicator does all of the "work" here. Or is both size and pressure essential factors in the equation?

Thanks guys, I'm new here, so I hope I'm not repeating a question that's been asked a thousand times!

Best,
Tom
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Unread postAuthor: Crna Legija » Fri Feb 18, 2011 10:37 am

im not 100% sure what the max pressure propane can reach before becoming a liquid but that the main factor on size of you meter pipe as i understand. so you cant get say 400psi of propane in a tiny pipe and let expand to the amount you need so you have to make it bigger and use 50-100 psi.
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Unread postAuthor: Technician1002 » Fri Feb 18, 2011 10:54 am

-_- wrote:im not 100% sure what the max pressure propane can reach before becoming a liquid

That is dependent on the temperature. You want the meter pipe to have a pressure lower than the boiling/condensing point of propane. If propane at room temperature boils at about 100 PSI, then a meter pipe at 50 PSI will have only vapor in it.
Image

Using this graph you can see if you try metering propane in your Meter Pipe at 20 F it will condense in the cold pipe and fill with liquid (assuming the bulk tank is warmer to supply the pressure).
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