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Fueling problems (manometer)

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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Unread postAuthor: wyz2285 » Fri Jan 25, 2013 7:07 pm

Well I know it's 3.2% per volume, I have done the math myself during chemistry class... So are.you telling me I just have to 3.2% x pressure = fuel pressure ?
Edit: ok, I'm good now, I got it :lol: stupidly simple.
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CpTn_lAw wrote::D "yay, me wanna make big multishot pnoob with 1000 psi foot pump compressor using diamond as main material. Do you think wet bread make good sealant? " :D
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Unread postAuthor: MrCrowley » Fri Jan 25, 2013 7:16 pm

I would just substitute the 3.2% for propane in the equations from 'Part 2' in the Hybrid Fueling 101 thread.

So...

VP = (100*10)/0.958-(100*10)
VP = 44 cubic inches
PC = 0.44*14.7
PC = 6.44 psi

would become (by subtracting 0.032 from 1):

VP = (100*10)/0.968-(100*10)
VP = 33 cubic inches
PC = 0.33*14.7
PC = 4.85PSI

Thus; 1x mix of butane = 0.485PSI

I assume that's how one would calculate it anyway. Remember that the n mix pressure of butane is dependant on atmospheric pressure so substitute the 14.7PSI for your local atmospheric pressure.
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Unread postAuthor: mark.f » Fri Jan 25, 2013 7:16 pm

The math for butane works out to be 0.47481 PSI/mix.

Basically, it works like this... simplified.

14.7(1+0.0323)-14.7

You can substitute 0.0323 for the stoichiometric fuel/air ratio for any fuel and get the PSI/mix for that fuel. For propane, it works out to 0.6174 PSI/mix. For propylene, it works out to 0.68649 PSI/mix, etc., etc.

EDIT: Crowley, I have some problems with that method. If SB15 dropped by that would be great. :P

Basically, SB15 is treating stoichiometric ratios like this... 4.2% of the total composition is propane, with 95.8% of the total composition being air.

When, in fact, doing stoichiometry, you do dimensional analysis, and find that 4.2% of the air you're using in your reaction is the amount of propane to react completely with that air... so 4.03% of the total composition is propane, and 95.97% of the total composition is air.

I wish SB15 would pop by... I believe we talked about this in spud chat one night.

Although, in practice, the difference isn't that noticeable, as you, SB15, DYI, and a few others have more than proven. :P Not trying to be a know-it-all or anything.
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Last edited by mark.f on Fri Jan 25, 2013 7:24 pm, edited 1 time in total.
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Unread postAuthor: wyz2285 » Fri Jan 25, 2013 7:20 pm

Yay, thank you ^^
@MrC: what? The atmospheric pressure is different from your side of the world.than my? :lol:
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CpTn_lAw wrote::D "yay, me wanna make big multishot pnoob with 1000 psi foot pump compressor using diamond as main material. Do you think wet bread make good sealant? " :D
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Unread postAuthor: SpudBlaster15 » Fri Jan 25, 2013 9:18 pm

Mark is correct, the equations in my thread use the fuel percentage as a fraction of the total molar quantity of gases, when it should actually be a fraction of the vessel volume. I've been meaning to update the thread for at least 2 years now, but other things have shifted my focus away from spudding.

You can still use my equations, but to obtain exact results you will need to use 4.03% for propane, 3.10% for butane, 4.46% for propylene, etc.. In any case, it's not likely to make much of a difference, as discrepancies between calculated values will almost always be offset by practical measurement error.

Also, good day to all SF members, it's been a while. :wink:
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Unread postAuthor: CS » Sat Jan 26, 2013 9:20 am

@mark.f

I'm pretty sure my adjustment for altitude is the unaccounted variable. Here is my math.

x=propane (psig)
y=manometer height difference (inch)

sea level wrote:Propane 4.2% for 14.7psia
4.2/100=x/14.7
x=.6174psig

.6174/.0361
y=17.1024930748
y=17.102


990m altitude wrote:990m altitude=12.7578psia
4.2/100=x/12.7578
x=.5358276
x=.536

.5358276/.0361
y=14.8428698061
y=14.843


*does not include SB15's "it should actually be a fraction of the vessel volume"

In other news...
Added 2x worth of propane, then the manometer slowly recedes back to zero. Then I added ~13psi, 2x worth of air, click the ignition and boom!

In essence this incurable leak adds this phantom variable I'm not accounting for. Next chance I get I'm adding an air meter, same construction as a propane meter, so that I can speed the process. AKA the propane/air mixture can leak equally. It is a mess, amid my ramble I'm realizing no one is reading this far so this is end of this post...
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Unread postAuthor: jrrdw » Sat Jan 26, 2013 4:04 pm

no one is reading this far so this is end of this post...


I am. So are you counting down the leak time then firing? Equal shot regularly...
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Unread postAuthor: MrCrowley » Sat Jan 26, 2013 4:12 pm

wyz2285 wrote:@MrC: what? The atmospheric pressure is different from your side of the world.than my? :lol:
Haha well I just meant if you weren't at sea level. Even though I'm at sea level, atmospheric pressure still fluctuates about 0.5PSI between the average of summer and winter months. Day-to-day it's very consistent but the difference between the record high and record low is more than a whole PSI in Auckland (which would throw off a 10x mix by up to 10PSI).
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Unread postAuthor: wyz2285 » Sat Jan 26, 2013 4:22 pm

You know what? I'm going to stay with my syringes, I can't find some decent gauge and portable gas(any kind) with threads that I can use to build a meter isn't sold here anyway :l
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CpTn_lAw wrote::D "yay, me wanna make big multishot pnoob with 1000 psi foot pump compressor using diamond as main material. Do you think wet bread make good sealant? " :D
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Unread postAuthor: CS » Tue Jan 29, 2013 12:51 pm

25.6 inch height difference = .925 psi propane
15 psi air

Hail to the potato launcher god(s)! It works every time!

Also made a projectile out of 3/4" pipe. Gnarley looking, but cheap.
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Attachments
IMAG1074.jpg
Gnarley! Found a use for pvc shavings. Soaked the inside with primer and glue and shoved the shavings in.
IMAG1073.jpg
Bent after heating with propane torch.
IMAG1072.jpg
Cuts with band saw in 3/4" pipe.
IMAG1075.jpg
Interesting. Filled with yellow food dye and foolishly open the propane valve letting the air in shooting most of the yellow water across the shop. Then I added the green dye to bring the water level back up, and this happened. The two stay surprisingly separated.
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Unread postAuthor: jackssmirkingrevenge » Tue Feb 19, 2013 4:29 am

I hadm issed this working, good job :)
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