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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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Unread postAuthor: natas » Wed Apr 04, 2012 5:42 pm

ok thnx for the advice, it will be followed :-)

what about the use of burst disc, like a few layers of blue 3M paper masking tape, just to get a litle more punch ?

is there a simple way to predict how much pressure a burst disc will add ?
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Unread postAuthor: Fnord » Wed Apr 04, 2012 5:51 pm

Burst disks are fine, just add layers until you don't get any additional power out of it (or until it doesn't pop on ignition. A burst disk which remains intact won't hurt the rest of the gun, provided you used proper fittings and solvent welding. Avoid cleanout caps; they tend to blow out).
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Unread postAuthor: natas » Wed Apr 04, 2012 6:17 pm

for me... a burst disk not popping sounds like a bomb.... why is that not dangerous ? the explotion is simply not powerfull enoughto blow thw pipes ??
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Unread postAuthor: Fnord » Wed Apr 04, 2012 6:54 pm

Propane and air will only generate a theoretical ~120psi in a perfectly sealed container. Pretty much any pressure rated pvc gun should handle that with no trouble. As long as you don't use exotic fuels or add extra oxygen, the chances of a failure are pretty much zero. If you have to drill holes in it, do it where the fittings overlap and be sure your pipe is not cell-core . It'll say so on the side.
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Unread postAuthor: natas » Thu Apr 05, 2012 1:21 am

thank you fnord, very usefull information :-)
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Unread postAuthor: Technician1002 » Thu Apr 05, 2012 11:58 am

To prevent making a bomb, be sure the burst disk is weaker by far than the chamber. If this is true, then a fail to burst disk will also fail to burst the chamber.
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Unread postAuthor: natas » Fri Apr 13, 2012 4:29 pm

when making a fuel meeter, is there a meeter/chamber ratio that is better easyer to use ?? , have tried to find something about it, but dont seem like anyone have bothered with that. it probably dont matter as long as the meeter is big enough for filling only once pr. shot.

or is there a psi limit for meeters ?, or a prefered psi pressure ? atm i have 350 ci chamber :-)

paul

edit : the project i nearly done now :-)
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Unread postAuthor: jackssmirkingrevenge » Sat Apr 14, 2012 3:08 am

You want a pressure close to what the minimum your regulator can supply in order to make the most of it.
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Unread postAuthor: natas » Sat Apr 14, 2012 12:12 pm

ah.. yes of course :-).. but is it so that the less pressure u have, the more gas is left inside the measuring tube ? or are we talking about marginals not worth calculating ?
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Unread postAuthor: King_TaTer » Sat Apr 14, 2012 3:06 pm

natas wrote:ah.. yes of course :-).. but is it so that the less pressure u have, the more gas is left inside the measuring tube ? or are we talking about marginals not worth calculating ?


Not sure what you're asking here but it seems like you're worried that if the meter is too large it might throw off your mix? If so, count the volume of your meter as part of your chamber volume in order to compensate.

In other words, JSR means that the larger your meter, the lower pressure you will have to use to fill your chamber, which translates to more shots per propane tank (as pressure decreases nearing tank depletion).
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Unread postAuthor: Gun Freak » Sat Apr 14, 2012 3:36 pm

That doesn't make sense. If you have a 20mL meter at 1 bar, that's the same amount of gas as a 10mL meter at 2 bar. You can use lower pressure, but to make up for the lower pressure you need a higher volume which ends up amounting to the same amount of gas, assuming a constant mix.
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Unread postAuthor: King_TaTer » Sat Apr 14, 2012 3:45 pm

Gun Freak wrote:That doesn't make sense. If you have a 20mL meter at 1 bar, that's the same amount of gas as a 10mL meter at 2 bar. You can use lower pressure, but to make up for the lower pressure you need a higher volume which ends up amounting to the same amount of gas, assuming a constant mix.


I'm aware of this. I probably should have elaborated that if he wants to get the most out of his tank, without having to meter multiple times when tank pressure gets low, use a larger meter volume. Alternatively he could swap between meter pipes when the time comes.
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Unread postAuthor: natas » Sat Apr 14, 2012 5:28 pm

sorry, bad formulated question. ill try again :-)

when releasing the gass from the measuring pipe, and in to the chamber, there will be gass left in the measure pipe. the bigger chamber u have, the bigger % of the measured gass remains in the pipe.
so with this in mind, will higher pressure and smaller measuring pipes give more accurate results ? :-)
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Unread postAuthor: jackssmirkingrevenge » Sat Apr 14, 2012 5:35 pm

In your calculations, you're supposed to calculate the percentage based on the total volume of chamber+meter, otherwise your mix will be off. When you take this into account, the size of the meter relative to the chamber is not really relevant to accuracy.
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Unread postAuthor: King_TaTer » Sat Apr 14, 2012 6:59 pm

I would have to agree that the volume of the meter should be taken into account no matter what. On the other hand let's not forget about what causes inaccuracies in practice, not on paper.

1. Chamber and Meter irregularities. Sure you can do a fluid measurement but how sure are you that it's accurate?

2. Common regulators are not perfect, the output pressure tends to fluctuate every time you recharge the meter.

3. Gauges are categorized by their degree of accuracy. Whats the accuracy of your gauge? Is it +/- 5% or .25%?

All these factors can throw off what your calculated mix should be, and what it actually comes out to be. I'm sure there's a slew of environmental factors I'm not mentioning but the point remains. Some of these problems can be resolved with money, some not. If you're able to get a consistent (not necessarily ideal) mix every time, the only thing left to do is run tests with a Chrony and adjust you regulator to find what mix yields the fastest average velocity.

Please feel free to correct or elaborate on this.
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Last edited by King_TaTer on Sun Apr 15, 2012 5:19 am, edited 1 time in total.
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