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Just built first potato gun and it won't fire.

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: saefroch » Tue Oct 11, 2011 10:50 pm

Flint sparkers are definitely inferior. Electronic ignition is much more controlled and produces higher temperatures.
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Unread postAuthor: jimmy101 » Thu Oct 13, 2011 7:23 pm

saefroch wrote:Flint sparkers are definitely inferior. Electronic ignition is much more controlled and produces higher temperatures.
I don't think there is any data to support electronic ignition producing higher temperatures. Do you mean spark temperature or combustion temperature? Spark temp is largely irrelevant, you just need roughly 0.5mJ in a sufficiently small volume to ignite 1x propane in air. Putting in a spark with 100 times that amount of energy doesn't significantly change things. A spark is a spark. Besides, I would WAG that a flint sparker actually releases more energy than does a typical piezo sparker.

Flint is really only inferior in two ways that I can see;
1. They don't work when wet and therefore can be a problem with spray-n-pray guns. (The obvious solution of putting the sparker in the clean out cap is an extremely bad idea.)
2. They require all the capabilities of one hand to operate. Leaving only one hand to do all the work of holding, aiming, steadying and absorbing the recoil. (Piezo's can be easily built into a handle so that only one finger is needed to actually fire the gun. That leaves 7 fingers, two thumbs and two palms to support the gun.)
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Unread postAuthor: saefroch » Thu Oct 13, 2011 7:43 pm

I was just thinking about plasma versus burning metals, but I suppose in a highly localized environment they could definitely produce the same temperature at the metal-oxygen interface.

Isn't "would WAG" redundant? Unless I found the wrong definition...

EDIT: I shall now attempt to answer the issue of flint sparks vs electrical arc with SCIENCE!

<sup>1</sup>/<sub>2</sub>mv<sup>2</sup>=<sup>3</sup>/<sub>2</sub>k<sub>b</sub>T
T=(<sup>2</sup>/<sub>3</sub>K<sub>e</sub>)/k<sub>b</sub>
Onlineconversion.com says that 14.534eV, the first ionization potential of Nitrogen is 2.3286*10<sup>-18</sup>J
T=(<sup>1</sup>/<sub>3</sub>)(2.3286*10<sup>-18</sup>)/1.3806503*10<sup>-23</sup>

Giving me an almost unreasonable result of a whopping 56,220.K.
White black body radiation is about 7,000K.
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Last edited by saefroch on Thu Oct 13, 2011 8:13 pm, edited 1 time in total.
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Unread postAuthor: mattyzip77 » Thu Oct 13, 2011 8:12 pm

jsefcik wrote:
mattyzip77 wrote:try a flint sparker from a camping lantern, like 4 bucks at walmart, but a grill ignitor is 100 times better and should work!!!



matt i beg to defer, i think the flint sparker is better, i went through lots of the bbq igniters and still love my flint sparkers


Well apparently, or, as usual, you were doing something wrong yet again. I have had the same bbq ignitor in the first combustion I ever made, and it still works after hundreds of shots and pounds of potatoes!! 8) 8)
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Unread postAuthor: Crna Legija » Fri Oct 14, 2011 12:02 am

mattyzip77 wrote:
Well apparently, or, as usual, you were doing something wrong yet again. I have had the same bbq ignitor in the first combustion I ever made, and it still works after hundreds of shots and pounds of potatoes!! 8) 8)


+1, iv never had a piezoelectric igniter fail in any application from spudguns to BBQ they seam to last for ever
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Unread postAuthor: sprayandprayboy » Fri Oct 14, 2011 2:26 pm

+1, iv never had a piezoelectric igniter fail in any application from spudguns to BBQ they seam to last for ever

my last one only lasted for like 100 clicks

Edited by jrrdw, fixed quote tag.
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Unread postAuthor: jrrdw » Fri Oct 14, 2011 3:45 pm

Kdmann64 wrote:Yeah my spark sucks, it's jumping all around and not going the way it is supposed to


Try: http://ultimatespudgun.com/30kv-igniter-p-219.html

or

http://ultimatespudgun.com/electrodes-chamber-p-273.html
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Unread postAuthor: jsefcik » Fri Oct 14, 2011 3:56 pm

jrrdw wrote:
Kdmann64 wrote:Yeah my spark sucks, it's jumping all around and not going the way it is supposed to


Try: http://ultimatespudgun.com/30kv-igniter-p-219.html

or

http://ultimatespudgun.com/electrodes-chamber-p-273.html

those electrodes are crazy expensive, its cheaper just to use bolts and on the outside use ring terminals with spade connectors to connect to the stun and and just solder on qucik connects to go to stungun
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Unread postAuthor: Goats spudz » Fri Oct 14, 2011 4:38 pm

jrrdw wrote:
Kdmann64 wrote:Yeah my spark sucks, it's jumping all around and not going the way it is supposed to


Try: http://ultimatespudgun.com/30kv-igniter-p-219.html

or

http://ultimatespudgun.com/electrodes-chamber-p-273.html
copper rod? or may be gold leaf on bolts or even carbon electrodes
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Unread postAuthor: jimmy101 » Fri Oct 14, 2011 6:13 pm

saefroch wrote:I was just thinking about plasma versus burning metals, but I suppose in a highly localized environment they could definitely produce the same temperature at the metal-oxygen interface.

Isn't "would WAG" redundant? Unless I found the wrong definition...

EDIT: I shall now attempt to answer the issue of flint sparks vs electrical arc with SCIENCE!

<sup>1</sup>/<sub>2</sub>mv<sup>2</sup>=<sup>3</sup>/<sub>2</sub>k<sub>b</sub>T
T=(<sup>2</sup>/<sub>3</sub>K<sub>e</sub>)/k<sub>b</sub>
Onlineconversion.com says that 14.534eV, the first ionization potential of Nitrogen is 2.3286*10<sup>-18</sup>J
T=(<sup>1</sup>/<sub>3</sub>)(2.3286*10<sup>-18</sup>)/1.3806503*10<sup>-23</sup>

Giving me an almost unreasonable result of a whopping 56,220.K.
White black body radiation is about 7,000K.
Nicely done. Only one problem. Temperature isn't the key quantity, energy is. You need ~0.5mJ in a sufficiently small volume for ignition. Whether that energy is present as a high temperature in a very small mass (plasma in air), or a much lower temperature in a much higher mass (small shard of iron heated to incandescence) ...

Some rough measurements I did years ago suggested that a typical BBQ piezo puts out 5mJ or so, roughly 10x the minimum needed ignition energy for propane in air (1x). I suspect a flint sparker puts out quite a bit more energy than that. I'm sure your fingers put a heck of lot more than 5mJ into the sparker when you operate it.

Back to temperature vs. energy. Which would you rather be hit with: a small stream of air at 1000F or a drop of oil at 300F? The oil will do a heck of a lot more damage than the air since the heat capacity, hence the energy, is much higher in the oil than in the air.
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Unread postAuthor: jrrdw » Fri Oct 14, 2011 7:06 pm

those electrodes are crazy expensive


$21.99 + shipping is cheep for a 30Kv spark circuit. No more misfires...
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Unread postAuthor: saefroch » Fri Oct 14, 2011 7:38 pm

I respectfully disagree. I think that temperature is the key quantity. Even using the term "a sufficiently small quantity" seems to undermine your own argument, since the temperature of x moles of matter with y joules of heat added is determined by the ratio y/x. As y/x gets larger, higher temperature, and ignition. y could be an arbitrarily non-infinite number, but with a large x, no ignition. Once the temperature spikes, the flame front is created because the temperature is great enough to surpass the activation energy on a molecular level of multiple other sets of reactants.

I think the fact that we're comparing white-hot Cerium (since I bet those igniters use ferrocerium, not flint) to a nitrogen plasma seems to confuse the question because both a sufficient temperature to begin a chain reaction/flame front.

I think your metaphor is irrelevant (though I totally understand the issue, having been hit quite a few times by very, very hot jets of air).
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Unread postAuthor: jimmy101 » Sat Oct 15, 2011 10:48 am

I guess we are going to disagree then. Though clearly both 7K degrees and tens of thousands of degrees are more than adequate to ignite propane in air. Indeed the autoignition temperature is probably well below 500C. But it is possible to create an electric spark that cannot ignite propane in air.

Folks have tried using a small DC motor as an ignition source. You can see sparks created by the commutator brushes as the motor runs but, as far as I know, nobody has ever been able to actually use that as an ignition system even though it has been tried. So it appears that even tens of thousands of degrees are not necessarily sufficient, which leaves energy density as the key metric.
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