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First Timer w/ a metering system??

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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First Timer w/ a metering system??

Unread postAuthor: q » Thu Mar 06, 2008 2:41 pm

Whats up folks. Newb here.


Questions I have:

1. I made a metering system exactly like what spudtech guy uses with 6" of 3/8 brass and 2 ball valves with a regulator etc. My combustion chamber is 24" of 3"sch40 pvc. How many charges of my propane meter do I need for this. I have my regulator set to 50psi. I have not shot the thing yet but it seems to me that the metered section of propane that spits out when i tested the thing is not that much??? I am pretty sure I went a little overkill with the combustion chamber in length but I can always fix that.
2. I sure as hell dont want to kill myself with this thing so what precautions do you all suggest.

I can post pictures later on. This is my first spudgun i made with a metering system on it. I have made plenty of them with just the normal cleanout cap on the end so bear with me here in my newbness.

Any help is appreciated.

thanks!
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Unread postAuthor: SpudFarm » Thu Mar 06, 2008 3:01 pm

just tweak the regulator until it works with a clean blue flame :)
if you post pics it will be to big help for us (and you) :D
and that thing will not blow up just with propane.
is your pipe and fittings rated?
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Unread postAuthor: q » Thu Mar 06, 2008 3:06 pm

yes the pipe I bought has a pressure rating i believe it says 330psi on the side of it. As far as the fittings, I am not sure if they are pressure rated.

I will try and get some pics up tonight.

what do you mean tweak the regulator till it gets a blue flame????


also, and this is probably a stupid question but is it best to have everything capped and plugged off and then push the spud into the barrel to create pressure and then inject the propane or should you not have that extra pressure and push the spud in with the cleanout cap unsrewed.

again, thanks and I am trying to find as much info as I can out here.
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Last edited by q on Thu Mar 06, 2008 3:09 pm, edited 1 time in total.
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Unread postAuthor: Fnord » Thu Mar 06, 2008 3:08 pm

You need to inject 4% of your chamber volume worth of propane.

Having 14.7 psi in the meter will put one meter-volume of propane in the chamber. 30 psi will put 2, etc. Let's see if you can figure this out :)


2. I'd fill the cleanout cap with hotglue or epoxy to strengthen it. They have blown out in the past.
Also, you want to avoid liquid propane running into the meter. put a loop in the hose or something.
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Unread postAuthor: BigGrib » Thu Mar 06, 2008 3:12 pm

http://www.advancedspuds.com/SpudToolonline.htm

this here has a propane meter calculator, just plug in all your measurements of your cannon and for your meter and it will tell you everything you need to know
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Unread postAuthor: Jared Haehnel » Thu Mar 06, 2008 3:27 pm

If you put too much propane, it won't fire..... to little it, won't fire
You don't have to worry about it exploding in your face if you get the measurement wrong. A much more likely case is that it just won't fire... in which case you have to vent the chamber and make an adjustment...

Good luck 8)

Edit: When propane burns at an ideal mix it burns a nice blue flame. Think about the propane stoves you see. If the spud guns fires right it shouldn't be much different. I.E no orange flame.
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Last edited by Jared Haehnel on Thu Mar 06, 2008 3:38 pm, edited 1 time in total.
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Unread postAuthor: q » Thu Mar 06, 2008 3:28 pm

_Fnord wrote:You need to inject 4% of your chamber volume worth of propane.

Having 14.7 psi in the meter will put one meter-volume of propane in the chamber. 30 psi will put 2, etc. Let's see if you can figure this out :)


2. I'd fill the cleanout cap with hotglue or epoxy to strengthen it. They have blown out in the past.
Also, you want to avoid liquid propane running into the meter. put a loop in the hose or something.


I used the calculator thing in the above post and that was easy enough.

I would still like to understand what is being calculated here. I got 169.56ci for my volume which is slightly lower than what that thing calculated. 4% of that is 6.78ci. now that i got to this point i am trying to figure out your one meter-volume part.........ahhh my brain...haha


edit: also could anyone answer the other questions i asked in my second post....thanks...sorry i edit'd them in :oops:
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Unread postAuthor: Jared Haehnel » Thu Mar 06, 2008 3:35 pm

If it I got it right (Sometimes I don't) your meter volume is suppose to be 6.78in3 at atmospheric pressure (14.7psi)

So take the dimensions of the meter pipe you have now and figure out if you need to add more to get the desired volume or cut it down sum...which is probably they case...


Edit: When you push the potato in with the cap unscrewed you create added pressure in the chamber which will basically give you a hybrid. Yes you will get more power from it as long as you give it a little extra propane to account for the extra air that is compressed in the chamber. As long as your PVC and fittings are sch40 you should be OK.

My first spud guns were a section of 2" PVC with a drain cap. I stuck the potatoes in the end of the barrel then sprayed in the hair spray, put the cap back on and used a ram rod to shoved the potatoes to the end. A small grill starter was the igniter. Effectively this was a small hybrid and it launch the spuds pretty near 75 yards. It wasn't very reliable though... but it worked.
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Last edited by Jared Haehnel on Thu Mar 06, 2008 3:46 pm, edited 1 time in total.
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Unread postAuthor: SpudFarm » Thu Mar 06, 2008 3:42 pm

what do you mean tweak the regulator till it gets a blue flame????


also, and this is probably a stupid question but is it best to have everything capped and plugged off and then push the spud into the barrel to create pressure and then inject the propane or should you not have that extra pressure and push the spud in with the cleanout cap unsrewed.

you try different pressures in your meter to find out what works...

then if you got a cleanout cap you want to take the cap of then put the spud in (if not the propane amount can be of by the different pressures generated by the air in the barrel) you put the cap on load with propane and ignite:)
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Unread postAuthor: q » Thu Mar 06, 2008 10:18 pm

well, here are some pics of my gun.....lemme know what ya think.

i tried it out and 3-4 shots of the propane results in a pretty nice shot :D

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Unread postAuthor: TurboSuper » Fri Mar 07, 2008 6:08 am

That's a nice cannon!

You may wanna extend the barrel a bit, though. :D
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Unread postAuthor: Jared Haehnel » Fri Mar 07, 2008 7:33 am

That is a nice cannon....I agree a longer barrel and you'll get a much better performance. I bet it one heck of a noise maker 8)
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Unread postAuthor: DYI » Fri Mar 07, 2008 10:51 am

To finally give a complete answer:

If your meter is SCH 40, and the dimensions you gave, its volume is 1.145ci.

If your chamber is 170 ci (you'll need to do a liquid volume measurement to be sure though), and your chamber is airtight, then you need ((170/95.8)x100=177.453 - 170 = 7.453 ci of propane), to account for the fact that the chamber would be slightly pressurised (0.644 psi more actually) after the propane was injected.

If you need 7.453ci of propane going in the chamber, and have 1.145ci of meter volume, you need (7.453/1.145=6.509 x 14.7psia = 95.685 psia of fuel. Subtract 14.7 from that for psig, and then add 14.7 to account for the propane that will remain in the meter.

So, once again assuming that all of your volume measurements are accurate, you need to charge the meter to 96 psig.
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Unread postAuthor: jimmy101 » Fri Mar 07, 2008 11:48 am

DYI's numbers look about right, given the uncertainty in the volumes of the chamber and meter, and the accuracy of the gauge.

Do you have a chamber fan? The DWV Y with the sparker is going to be slow to mix without a fan. Since you have to get a pretty good mix at the spark gap for the gun to fire you may have some problems without a fan.

The barrel looks way too short (or the chamber way too big). If that's a 1.5" barrel then it probably should be about 4 times longer than it is.
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Unread postAuthor: BigGrib » Fri Mar 07, 2008 12:36 pm

Math Hurt Head..............................................
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Yea, that's definitely going to get you at least a tazer.

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