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Micro Coils & Kanthal "Voodoo", or... why touching coils don't short out

Published by State O' Flux in the blog State's Stuff. Views: 3333

It is not voodoo, or related to internal combustion engines - it's the way the wire is designed - and this "feature" of Kanthal ('A' series, including A-1 and AE) has been well known since before you or I were born... and just one of the qualities that make it so common and useful. ;-)

When heated, Kanthal wire (iron-chromium-aluminum) builds up an aluminum oxide insulative coating on it's outside surface that protects the individual coils from shorting, one to another. This is called alumina (Al2O3).

This is most clearly noticed when you test fire a new compressed coil... the coil initially shorts and heats unevenly. The more you fire it (and shape it to final perfection) however, the more evenly it will heat - from the hot center out to the cooler ends.
This is the alumina layer developing and insulating the coils surface. Although the coils are "touching", the electrically conductive component of the coils are insulated by the alumina coating.

That is it. No magic... unless a self-generating, protective oxide layer is considered magic. Maybe it is magic - Kanthal wire is both thermally conductive and electrically resistive. Pretty convenient for us, and several billion other handy applications for heating coils.

Consider as well... for a given length and thickness of wire, the resistance does not change whether it's a 7 wrap conventional coil or a 7 wrap compressed coil.
Again... resistance for a given length does not change any appreciable amount.

What does change is the amount of heat the "compressed coil" is capable of generating. Reduced to a short, concentrated segment or element, the heat generated for a given resistance/area can be greater than the sum of parts.
Why you ask? The simplest of answers.

As stated, we have concentrated the heat generated by the individual coils into that smaller, more cohesive heat generating "element".
If you're still on the fence, consider this analogy. Ten matches, lit and separated by 1 inch per match, generate the heat of... 1 match or individual heat source per inch.
Now, ten lit matches, all within one square inch... 10 times the heat concentration / compressed down to one square inch.

Last, although "micro" is used to define a small diameter (1.5mm or 1/16" - or less), compressed coil heating element, you can compress a larger diameter vaporizer coil and have the same result of greater heat concentration for a given dimension of resistance/area occupied.

That's it. Share if you want, or keep the "magic" to yourself and continue to be entertained by the voodoo explanations. It is fun, ain't it?

You might be thinking, "boy SOF, you are one smart SOB".
Well, maybe... but I didn't invent Kanthal, and I'm not the first on ECF to comment about the wonders of the Kanthal wire design (for which there is a vast supply of interesting history and applications... if you like reading about such things that is).

After writing this, I was both generally curious - and because terms tend to be specific with this subject, didn't want to publish if there might be an impression that I was plagiarizing. I don't have much... but I do have a annoyingly huge sense of integrity.
So... I typed in "aluminum oxide layer electronic cigarette forum" (among other combinations) into Google (it parses down better than the ECF search engine) and the first to pop up (of perhaps more?) was a
post by LucentShadow.

So you see, not only not magic, but not a secret either.
Addendum for 12-17-2015: Something you will likely run into is that, when comparing your actual net resistance to the default resistance found for a given gauge of Kanthal wire... your actual doesn't match the default.
This is likely not any mistake you've made, but more likely to be a manufacturing variance in gauge thickness. Every single gauge of genuine, Temco sourced, Sandvik Kanthal A-1 wire I have on hand is not as thick as the default given in Steam Engine.
For example: Item number RW0249, 24 gauge Sandvik Kanthal A-1 has a listed "average" thickness of .0201/.511mm... which is what Steam Engine uses as a default.
The 4 rolls I have - which were obtained over the course of several months and not likely to be from the same production date - measure from a low of .018"/.465mm to a high of .019"/.494mm, but none match the average dimension.
So... always measure your wire with a caliper or mic, and if not the same or very close... substitute that value for the default. In Steam Engine, the gauge number box will change to "N/A", but with the right or "corrected" value, your measured resistance will be far closer to calculated.

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