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Dual ignition troubles

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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Dual ignition troubles

Unread postAuthor: niglch » Mon Jul 09, 2007 9:34 pm

The chamber for my combustion cannon is nearly complete. I am attempting to set up a dual ignition system. I was able to use a drill press to be very exact with my drilling, and the two pairs of screws are almost exactly 1/4" apart as far as I can measure without any bending of screws. I'm using a push button grill igniter as a power source.
I am able to get a very consistent, strong spark when only one pair of screws is wired together. It works every time. However, when I tried wiring both pairs of screws together, I only manage to get the two sparks about 10% of the time. Since I'm getting sparks at least intermittently, I know it is wired correctly. However, I'm afraid the two gaps are causing a lot of resistance. Since the screws are already in all of the way, should I try bending them closer together (maybe to 1/8"?) on both pairs in order to decrease air resistance? Either that or the grill igniter is just not powerful enough. Does anyone have any other suggestions on how to get this working?

EDIT: Just for the heck of it, and because I've heard of this working before, I tried briefly exhaling into the chamber before pressing the igniter. I was then able to produce a spark every time until I aired out the chamber and allowed it to dry out again. This makes sense, so I guess I could take a quick breath into the chamber before firing... lol.
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Unread postAuthor: Flying_Salt » Mon Jul 09, 2007 10:00 pm

Grill sparkers aren't known for being able to clear many large gaps. You would be better off with one 1/4'' gap or two 1/8'' gaps, which would be much more reliable. There is minor debat if multiple sparks close together help at all, but you don't really need to worry about that.

Oh and be careful drilling lots of holes in your chamber, drilling holes weakens the pipe.
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Unread postAuthor: niglch » Mon Jul 09, 2007 10:18 pm

I was careful to only drill through couplings and made absolutely sure the screws fit correctly. Hopefully this will keep problems from arising. Thanks for the warning though. If the two 1/8" gaps provide the same amount of resistance as the single 1/4" gap, this should work well.
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Unread postAuthor: Hailfire753 » Mon Jul 09, 2007 10:26 pm

Ok, the same tjing happened with my gun, until I fixed it. Are you sure the wires are insulated? Are the screws on the outside covered? If they are not, cover them in ELECTRICAL TAPE. I originally covered the outside of the chamber in duct tape, but the spark jumped right through the tape and into my arm.

You may not feel a shock on the outside, but if the screws are not insulated, you are still acting as a ground to the spark. Make sure the grill clicker is covered too. Basicly, cover any exposed wire that is outside of the chamber.

Edit- I also have 2 spark gaps. one is about 1/4 in, the other mayby 1/6. Pretty reliable. Also, don't bend the screws if you can help it. This makes them really hard to remove or ajust later. If you need a smaller spark, wrap some aluminum foil over the screw tips.
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Unread postAuthor: psycix » Tue Jul 10, 2007 6:52 am

Maybe test it in the dark
You might see a spark jumping THROUGH insulation of wires!

Also, the total distance of the gaps should be the same distance as if you had only one huge gap.
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Unread postAuthor: dongfang » Tue Jul 10, 2007 2:00 pm

Hi,

Yes, insulation can be a problem! Thin PVC hose around the wires is good.

Resistance doesn´t matter here. The combined length of the gap does. Also, pointed electrodes (pointing towards each other) are better than blunt ones.

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Soren
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Unread postAuthor: niglch » Fri Jul 13, 2007 9:40 pm

Edit- I also have 2 spark gaps. one is about 1/4 in, the other mayby 1/6. Pretty reliable. Also, don't bend the screws if you can help it. This makes them really hard to remove or ajust later. If you need a smaller spark, wrap some aluminum foil over the screw tips.


Would using nuts on the end of the screws work or would they be too blunt and too likely to just fall off? I might try you aluminum foil suggestion though and see how it works.
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Unread postAuthor: pyrogeek » Fri Jul 13, 2007 10:22 pm

You could do what I did, and make a spark strip that uses welding tungstens as electrodes. I did that on my friends spud gun I built for him. It ended up being quad spark with a 150K volt stun gun. I drilled the holes through a fitting, ran wires from those to the strips which were mounted to piece of PVC solvent welded to the inside of the chamber. It was sweet.
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Unread postAuthor: FiveseveN » Sat Jul 14, 2007 9:46 am

Here's another trick: sharpen the ends of the screws (mind the threads). It will act like a funnel for the electrons, accelerating them. Should allow you to use a slightly larger gap.
Though someone here said one doesn't really need sparks longer than a couple mm. I tend to agree, especially for multiple ignition points.
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