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I can't get it to 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: MountainousDew » Sat Jun 13, 2009 2:21 pm

theBOOM wrote:Thanks for the fast reply... I'll try sniffing around for some screws although I don't think my dadd has any small ones :/

I know my quote is from awhile back and I know Psycix covered almost everything, I wanted to state the fact (if it is a fact) that thicker screws are more conductive... If I'm wrong disregard this post, but I remember it from a post a while back... I'll see if I can find it for reference.
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Unread postAuthor: jon_89 » Sat Jun 13, 2009 3:47 pm

So I was thinking of the syringe being the problem. My theory is if you are pulling the syringe maybe you are not get getting the right amount.
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Unread postAuthor: psycix » Sun Jun 14, 2009 5:12 am

Thicker screws have more conductivity then smaller screws, but it is not about the screws being too resistive, it is probably more about the resistance of the connections.
The screw itself may have a few hundred ohms of resistance, while the spark gap has like... millions.
Certain types of screws may have coatings interfering with electrical conductivity, also, make sure there is enough contact between the wire and the screw.
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Unread postAuthor: john bunsenburner » Sun Jun 14, 2009 9:35 am

To raise conductivity dip the screws in molten elctrical solder, if they are really crap and have very high resistance, should solve the problem(and they should still be screwable, and don't misunderstand that)
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Unread postAuthor: Biopyro » Sun Jun 14, 2009 9:51 am

I really don't think it is the screws. Compare their resistance to that of the air, and it is nothing in comparison. Either your spark gap is too large, or your voltage source is too low output.

If your combined spark gap is 3mm and the spark looks very weak, then I think you either need to just use one gap, or upgrade your sprak source.
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Re: I can't get it to fire!!!!!!!

Unread postAuthor: jimmy101 » Sun Jun 14, 2009 3:50 pm

theBOOM wrote:The total chamber length with the fittings is about 7.5 inches long the pipe that I used for the chamber was 5.5 Sch 80 600 pressure rated pipe
Also the chamber is 1 inch in diameter... I measured the volume of my cannon with a measuring cup and it took 160 ML of water and if I am not mistaken 4.2% is the desired propane ??? So I inject 3.8 ML of propane... I have tried from 3-6 ML with no success


Wait a minute. The chamber volume by water displacement is 160mL? 4.2% of 160 is 3.8? Not in this universe.

(160)(0.042)=6.72cc

BTW, the correct ratio if your fuel displaces air from the chamber is 4.0%, not 4.2%. So the correct volume of fuel is (160)(0.04)=6.4cc.

With a <1"ID it'll take a long time for the fuel to fully equilibrate in the chamber. Let it set 5 minutes after fueling, that should be enough. Or add a chamber fan. A small DC motor from radioshack would work. Or, use the vibrator motor from a cell phone. In both cases you'll have to make your own fan blade out'a something like a plastic soda bottle.

As a temporary solution, drop one or two packing "peanuts" in the chamber. After fueling invert the gun a few times so the peanut will mix the fuel.
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Unread postAuthor: theBOOM » Mon Jun 15, 2009 11:28 pm

Thanks jimmy for the correctiones.. btw guys I replace the 2 spark gaps with only 1 small powerful spark.. Even though im not sure what caused the problem in the first place... Nway im waitting on the jb weld to dry :lol:


Boom :twisted:
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Unread postAuthor: Technician1002 » Mon Jun 15, 2009 11:45 pm

john bunsenburner wrote:To raise conductivity dip the screws in molten elctrical solder, if they are really crap and have very high resistance, should solve the problem(and they should still be screwable, and don't misunderstand that)


Just a quick note on the electrical properties of screws and the relation to the currents and voltages at stake. Current is equal to the voltage divided by resistance, so as resistance goes up, the current either drops or the voltage goes up. That is simply Ohm's law.

Now, does the screws resistance matter? A BBQ grill sparker puts out less than 1 ma. (1 / 1/000 Amps) The resistance of most screws are under 1 ohm, for sake of argument lets go for worst case and call it 1 ohm. Spark voltage is maybe 5,000 volts. The voltage lost in the screw is under 0.001 volts. Coating the screw in solder does little to the spark. It's sorta like causing flooding by throwing a rock into the ocean so it holds less water. :D

Wire tables showing resistance for common size copper, nickel, brass, and iron are online.

http://en.wikipedia.org/wiki/American_wire_gauge Copper

Correction factors for other resistances including silver, iron, etc are here.
http://www.stealth316.com/2-wire-resistance.htm

It is unlikely to find a screw with a resistance over 1 ohm in any collection of metal screws. Nylon and such are another story.
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Unread postAuthor: theBOOM » Tue Jun 16, 2009 12:37 am

Well I had an electrical grill sprker and it made sparks 1 inch in length... anyway I fixed my problem and did some testing... it works pretty well :D
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Unread postAuthor: jimmy101 » Tue Jun 16, 2009 12:41 pm

Like Tech said, the gauge of the wire and/or screws makes zero difference in how the spark system behaves. Any gauge, any type of metal (Al, Cu, Fe ...) will work just fine since the resistance is dominated by the resistance of the air gap, which is several orders of magnitude greater than all the other resistances in the circuit combined.

Heck, in a gasoline engine the spark wires are purposely very high resistance (hundreds to thousands of ohms per foot) compared to generic copper wire, which is tens of milliohms per foot.

The connections between wires and electrodes don't have to be all that great either. The wires don't even need to touch the electrodes and it'll still work. Though the gap(s) between wire(s) and electrode(s) has to be figured into your total spark gap.

There are really only a couple design considerations for a spark system;

1. Total voltage of the source and total spark gap distance. Reduce the total spark gap a bit (~20%?) to account for the inconsistency (or inflated claims from the manufacturer) of the spark source. Piezo's don't generate the same voltage every click. Stun guns rarely generate the manufacturer's claimed voltage and their output will also vary from shot to shot.

2. The breakdown voltage of air is a function of the air pressure. Fortunately it's a nice linear relationship. For every increase of 1 ATM in pressure you need to double the voltage or half the total spark gap.

3. The shape of the electrode tips. The sharper the better. The breakdown voltage of air is about 3KV/mm if the electrodes have a large radius of curvature. Between sharp points (i.e., sewing needle sharp) the breakdown voltage drops to about 1KV/mm. With a given spark source you can jump a larger gap (or more gaps) with sharp electrodes than with blunt ones.

4. Sufficient insulation to keep from shorting the spark system to itself (or you :shock:). Typical wire insulation is basically useless at the voltages in a spark system, the wire might as well be bare wire for all the insulation does. To provide sufficient effective insulation, you need to keep the wires well separated from each other and anything that is conductive. How far is "well separated"? Basically the closest the wires every approach each other should be more than the total spark gap in the system.

Nothing else really matters all that much. The wire type and gauge, the material the electrodes are made of etc., as long as they are metal then they have sufficient conductivity.

EDIT: Really want to know if your spark gaps are working in the buttoned up and fueled gun? Wire the flashtube from a disposable camera or a NE-2 type (neon 120V) indicator light bulb into the spark system. The tube or bulb should be in series with the gap(s). When you click the ignition system the flash tube will show a blue spark inside. A NE-2 Bulb will blink. No spark/blink then there was no spark at the spark gaps. Both types of lights have fairly small breakdown voltages and consumer very little power from the spark circuit. NE-2 bulbs are often used as the power indicator light in older electronics. "Bus strips" with on/off switches and an indicator light generally used NE-2 bulbs. (NE-2 are no longer acceptable for use as indicator lights in safety critical applications because they eventually burn out.)
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