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Ultra-Easy Alternate Ignitor? Possible cartrage use!

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: Fnord » Sat Aug 25, 2007 4:09 pm

For an extremely thin graphite filiment, you can use some of the .3mm graphite that they sell for mechanical pencils. Maybe even sanding it with some fine grain sandpaper would be possible.








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Last edited by Fnord on Sat Aug 25, 2007 7:52 pm, edited 1 time in total.
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Unread postAuthor: joannaardway » Sat Aug 25, 2007 4:43 pm

You ideally need a shorter, fatter length of lead.

All the electrical energy used is essentially becoming heat - this will be less than 5% efficent for light emission.
Power is found by V^2/R. As you can see, a lower resistance (shorter lead length) will equal more power.
Also, given that the lower mass of graphite will also mean that for each calorie of heat, the temperature will rise twice as much.

So, the rate at which the temperature rises is inversely proportional to the sqaure of the length of the lead, assuming a constant diameter. And a lower surface area also means less rate of heat loss.

As for power, a conventional alkaline battery will seldom supply more than an amp of current at a maximum except if it's brand new, and usually only half that after a while
However, some NiMH, all NiCd and most NiFe rechargable batteries can supply many tens of amps even when they are nearly flat, in spite of their lower voltages due to a much lower internal resistance.
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Unread postAuthor: jimmy101 » Sun Aug 26, 2007 5:20 pm

Joanna: Shouldn't that be a shorter thinner lead? A fat conductor has lower resistance than a thinner one (assuming same material and same length). The fat conductor will draw more current and dissipate more power but it has a higher mass so the temperature of the conductor rises slower.

A fat conductor will need to dissipate a lot of power before it gets hot enough to ignite anything. A thin conductor will draw less power and get hotter faster.

A 9V battery will heat a piece of 000 steel wool to incandescence almost instantly. The same battery won't heat a bolt, the same length as the steel wool, to incandescence. The bolt will get hot but the battery will go dead long before the bolt gets to incandescence.
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Unread postAuthor: joannaardway » Mon Aug 27, 2007 5:06 pm

jimmy101 wrote:Joanna: Shouldn't that be a shorter thinner lead? A fat conductor has lower resistance than a thinner one (assuming same material and same length). The fat conductor will draw more current and dissipate more power but it has a higher mass so the temperature of the conductor rises slower.

The diameter isn't really of huge consequence. The resistance is proportional to the mass, but the surface area increases less, so the real gain comes from a slower loss of heat, which then produces a slightly higher equilibrium temperature.

With a 9V battery, the problem is the internal resistance. Try the steel wool and steel bolt idea with a low resistance car battery, and the same incandesance will arise from both, as long as the wires you are using are of a reasonable gauge.
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Unread postAuthor: jimmy101 » Mon Aug 27, 2007 6:18 pm

joannaardway wrote:The diameter isn't really of huge consequence. The resistance is proportional to the mass, but the surface area increases less, so the real gain comes from a slower loss of heat, which then produces a slightly higher equilibrium temperature.

With a 9V battery, the problem is the internal resistance. Try the steel wool and steel bolt idea with a low resistance car battery, and the same incandescence will arise from both, as long as the wires you are using are of a reasonable gauge.


Not to "bicker and argue about who killed who ..." but the resistance is not proportional to the mass of an object. It is proportional to the cross sectional area of the object and the length of the object. Two objects of the same material and equal mass can have different resistances. A short fat wire will have a lower resistance than will a long skinny wire of the same mass.

The issue here is trying to get high enough temperature (not energy or heat) to ignite a gaseous fuel. A thin wire can be heated to incandescence by a cheapo 9V batter supplying 5A or so. A similar length of 1/4 bolt can be gotten just as hot but it is going to take a car battery putting out hundreds of amps to do it (actually even a car battery won't do it for a 1/4 bolt, it would probably take a couple thousand amps.) So, as an ignition system you want the thinnest wire you can get so that you can use the wimpiest (and cheapest, and safest) power supply possible.
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Unread postAuthor: joannaardway » Tue Aug 28, 2007 5:59 am

jimmy101 wrote:Not to "bicker and argue about who killed who ..." but the resistance is not proportional to the mass of an object. It is proportional to the cross sectional area of the object and the length of the object.

I was referring to it solely in terms of diameter - I thought that was clear, apologies if it wasn't.
Clearly if the length is changed, then that will affect it differently, but the resistance is inversely proportional to the square of the diameter, and the mass is proportional to the square of the diameter.

These two things cancel each other perfectly, so it is correct to say that at least for something of the same material and length, the resistance and mass if multiplied together would always reach the same product.

The issue here is trying to get high enough temperature (not energy or heat) to ignite a gaseous fuel. A thin wire can be heated to incandescence by a cheapo 9V batter supplying 5A or so. A similar length of 1/4 bolt can be gotten just as hot but it is going to take a car battery putting out hundreds of amps to do it (actually even a car battery won't do it for a 1/4 bolt, it would probably take a couple thousand amps.) So, as an ignition system you want the thinnest wire you can get so that you can use the wimpiest (and cheapest, and safest) power supply possible.

I was talking in fairly general terms. If the supply has no internal resistance, the fatter bolt will reach a higher final temperature, because of it's lower mass to surface area ratio.

When there is an internal resistance - as there is in all supplies, by nature - then there will be an ideal diameter of 'filament', but that will not be a case of narrowest diameter is best. In the case of a 9V alkaline, then if you can get a very narrow lead, say one for a mechanical pencil, then that might well be best, but a really fine lead would never reach incandesence because it would radiate heat too quickly to ever do that.

But a 9V alkaline is not a good choice - I have never seen one supply anywhere near 5A except when brand new, and that only lasts a few minutes. After a little use, if coerced, half an amp maybe. If you can only use a 9V for a few minutes before it's useless, then that's going to get expensive fast.
A 12V NiCd pack is a far better choice, because those have a hell of a lot of oomph, and I've measured outputs from them in excess of 20 amps. Repeatedly reusable, and just as safe, as they can't be shorted by accident - which is fairly common on 9Vs really.
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Unread postAuthor: jimmy101 » Wed Aug 29, 2007 12:05 pm

OK, Joanna, just to through another curve ball at ya....

The resistance of a conductor increases with temperature, this is the most fundamental definition of what a conductor is.

This property is what makes a light bulb work. Without this property the filament in a light bulb would overheat and melt. (Semiconductors have the opposite temperature dependence, that is why an LED must have a current limiting resistor, as a LED heats up it's resistance drops, it draws more power, it gets hotter, repeat, until it eventually melts.)

Wires are conductors so they do the same thing as a light bulb filament. At room temp they are low resistance, as the wire heats up it's resistance increases. Some conductors, below a critical diameter, will heat up to incandescence and then stabilize (that is what a light bulb does). If the wire is too thick then it can't radiate heat fast enough and it will melt.

Or, if the tempco is wrong, even a thin wire will melt before it reaches the temperature where the resistance stabilizes.

So, you need to take into account the geometry of the conductor, it's intrinsic resistance, tempco, melting point, air flow around it, ambient temperature, ...

Turns out, tungsten and it's alloys and nichrome (alloy of nickel and chromium, IIRC) have suitable properties for use as filaments. Nichrome also has the nice property of being stable to oxidation and corrosion even when it is hot enough to glow.
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Unread postAuthor: joannaardway » Wed Aug 29, 2007 3:10 pm

jimmy101 wrote:The resistance of a conductor increases with temperature, this is the most fundamental definition of what a conductor is.

Actually, I did take that into account with my maths. If you do a basic calculation taking into account heat generated, rate of heat loss, and increasing resistance with temperature for conductors of different diameters, then you get a nice distribution curve that shows the maximum temperature of each of the conductors.

Although this doesn't take into account how long it takes to reach incandesence, it does look quite nice on screen.

The best thing that could be done is to find the wire that reaches the ignition temperature of the mix fastest, and use that one.
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Unread postAuthor: Blackett » Sun Sep 02, 2007 11:43 am

For me, I would just stick with one of Latke's chamber shorts or spark gaps. This method for potential ignition would not be worth the hassle. (my opinion)

For the extra few dollars you can go get a stungun.

Sure this is neat but is actually a similar concept to steel wool, just a larger gauge that doesn't burn up immediately.

The most efficient thing that i could think of for graphite replacement would be using a fuse clip. however unless you managed to make an excessively long piece reach incadesence, I don't see why you wouldn't just go with multiple sparks.

Besides the amount of heat generated would scare me, even if only used for a few seconds unless it was in th exact center and not near the chamber wall.
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Unread postAuthor: PVC Arsenal 17 » Sun Sep 02, 2007 12:08 pm

I tried this light using mechanical pencil lead refills (thin stuff) and a 9V. It worked but the battery got hot.
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Unread postAuthor: Blackett » Sun Sep 02, 2007 11:27 pm

actually I tried this with .7mm graphite-aka mech. pencil lead.


I got it to reach incandesence through the use of two nine volts, however I had an issue with temperature in the time it took to reach incandesence. I burned myself when the plastic around my alligator clip melted.



But I suppose if you could get it to incandesce a significantly longer piece it would be a plausable ignition source, considering it should work just as well as a spark strip through the length of the chamber.

mounting such a piece might be a slight inconvenience however, which makes me a stungun guy through and through.
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