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Metering Pipe Length

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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Metering Pipe Length

Unread postAuthor: hansom72 » Tue Apr 22, 2008 9:27 pm

I just measured the volume of my combustion chamber and did the math to find the length of my metering pipe. unfortunately my chamber calls for a 12inch piece of 3/8 pipe. The problem is I already bought all 3/8 brass fittings and have lots of time and money invested and it is almost too long. I know if i used a 1/2 inch pipe it would call for a shorter length, but i would like to stick with 3/8. If I used a 6 inch piece of 3/8 pipe for a metering pipe and charged the chamber twice, would it be the same as charging the combustion chamber once with a 12 inch 3/8 metering pipe? Is charging the chamber twice a logical way to do this? Will it work?
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Unread postAuthor: LikimysCrotchus5 » Tue Apr 22, 2008 9:30 pm

Man you keep popping up topics here and there like popcorn!

you can make the meter smaller by using a higher pressure of your fuel in the meter.

This shoud help as well.
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Unread postAuthor: bluerussetboy » Tue Apr 22, 2008 10:38 pm

Use whatever size pipe you want. When it is all together either go to the link liki posted or go to burntlatke's fuel tool. Input all your measurements and like magic it will tell you what pressure your gauge should be. If your meter pipe is shorter than the gauge pressure will be higher. If it is longer the pressure will be lower. The only time you should double up on charges is in the colder weather when propane stays in a liquid state due temperatures >50F.
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Unread postAuthor: hansom72 » Tue Apr 22, 2008 11:23 pm

Burntlatke is the site I used to determine the pipe length. For the volume of my chamber at 90psi, it determined that i will need over a 12 inch 3/8 pipe or a 6 inch 1/2 inch pipe. 12 inches seems a little long right?? I am just worried about misfires and lack of performance. I would hate to build this thing and not have it work. My chamber is either going to be 24'' long or 20'' long. With a 24'' chamber, it determined i would need up to a 14 inch 3/8 metering pipe. Does this sound like I'm heading in the right direction? Or should I make a way smaller combustion chamber so my metering pipe does not have to be as long? If I did make a smaller combustion chamber, doesn't that mean less power and distance???
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Unread postAuthor: starman » Wed Apr 23, 2008 12:40 am

You should try to get your meter psi down some by using a somewhat larger meter pipe. 90 psi will be about all you can get out of a propane tank under the best of conditions.

Calculate a pipe size that would require say 40-60 psi in your meter for proper volume...or about twice your present meter pipe size. This will make it easier to use in cold weather and as your tank begins to run low.
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Unread postAuthor: biggsauce » Wed Apr 23, 2008 12:46 am

Those lengths seem reasonable. If your chamber is 20 - 24" long, why worry if your meter is 14"? You can always put some elbows in the meter and make a "u" shape if you want. No shame in a long meter, better than having a short one right? :wink: :lol: :roll:

But seriously, try bringing your pressure down from 90psi. See what it says for say, 60psi. If its cold outside, you may have trouble getting 90 psi out of your bottle. (its my understanding that propane bottles are only under about 90psi, someone correct me if I'm wrong)

To shorten your meter, go up in your diameter like you did.

I'm assuming you are using a 4" diameter chamber. You are right, theres no replacement for displacement. With a chamber that size and the right length barrel, you'll have a right nice cannon brother 8)
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Unread postAuthor: hansom72 » Wed Apr 23, 2008 7:42 am

I looked into what 60psi would do for the next size pipe which is 1/2. it calls for 10 inches. Now from all of you experienced propane injection cannon builders...is 60psi usually the pressure that you run into through out the life of the propane cylinder. I read on burnt latke that you should always get about 90psi over the life of the cylinder. Is burnt latke not taking into consideration the colder weather?? If I set my Metering Pipe length for 60psi, and then wanted to use my gun on a really warm day and the pressure was at 90psi, will that cause issues with misfires? My question here is what is the average psi over the life of the propane cylinder??? Should I go with 60psi, 70psi, 80psi, or somewhere inbetween there??
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Unread postAuthor: JDP12 » Wed Apr 23, 2008 8:58 am

i dunno, all i do is meter 28psi into my propane meter for each shot. it depends... and you shouldn't get misfires. if you're worried about the tank pressure, just use a ball valve meter and slowly open it, watching the pressure gauge, or use a regulator. the PSI for your propane meter is dependent on meter volume and chamber volume
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Unread postAuthor: biggsauce » Wed Apr 23, 2008 9:01 am

Cut out the variable and put a small regulator inbetween your bottle and meter. then set it to whatever pressure you want. I'll be running mine at about 50 for propane, but its up to you. From Home Depot, a regulator only about 20 bucks, and well worth keeping your gun consistent.
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Unread postAuthor: starman » Wed Apr 23, 2008 9:03 am

hansom72 wrote:I looked into what 60psi would do for the next size pipe which is 1/2. it calls for 10 inches. Now from all of you experienced propane injection cannon builders...is 60psi usually the pressure that you run into through out the life of the propane cylinder. I read on burnt latke that you should always get about 90psi over the life of the cylinder. Is burnt latke not taking into consideration the colder weather?? If I set my Metering Pipe length for 60psi, and then wanted to use my gun on a really warm day and the pressure was at 90psi, will that cause issues with misfires? My question here is what is the average psi over the life of the propane cylinder??? Should I go with 60psi, 70psi, 80psi, or somewhere inbetween there??


90 is a highend for all weather shooting...go with a meter that requires less as has already been recommended.

Build your gun and find out for yourself some of these things. You have plenty of info to get started.
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Unread postAuthor: bluerussetboy » Thu Apr 24, 2008 6:42 am

Have a look at this page. http://www.engineeringtoolbox.com/propa ... _1020.html

If you take the lowest temperature you will be operating your cannon at and then find what the maximum propane vapor pressure is at that temperature, just design your metering pipe around that pressure.
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Unread postAuthor: hansom72 » Thu Apr 24, 2008 11:32 pm

Alright,...I used burnt latke's fuel tool 1.5 to figure out my metering pipe length and diameter...My chamber is 2 ft long which is a little over 300 cu. in. It calls out for a 12-13 inch metering 1/2 pipe around 60-70psi... so i went and bought one tonight and put all the fittings together and the dang thing is almost as long as the chamber itself. This has to be a little excessive dont you think, or does this seem reasonable? I may just go and buy a 10 inch 1/2'' metering pipe since the fittings themselves have some volume. This metering pipe thing is a little hard for me to get the hang of.
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