Coil Ohms

[happydave]

so has we have been talking i cut a 1 inch length of 28 gauge kanthal. it ohm'ed out at .5 ohms. i clamped in the ends of the wire in my pliers to protect the contact ends from oxidation and torched just the middle of the wire over and over and over allowing it to cool down between heat cycles... well i just checked it again and now im reading 1.0 ohms. so it just doubled the resistance value. now this could be caused by surface oxidation. but i have a strong gut feeling that its caused by the molecular structure becoming discombobulated. so now im going to use a new length of wire and use indirect flame

 

[happydave]

well it looks like its going to oxidize no matter what i do because of the atmosphere... DERP. i don't have a vacuum chamber so i'm out of ideas at this point. time for a vape.

 

[generic mutant]

Incandescent light bulbs are full of inert gas.

Measure the resistance of a bunch of new ones, and a bunch of oldish ones of the same design?

Again, not a perfect test, but it'd be interesting...

 

[generic mutant]

Even better, rig an incandescent bulb up to a circuit that turns it on and off every few seconds, measure resistance every few hours.

 

[happydave]

Improving Your Kiln's Element Life by Stephen J. Lewicki - Clay Times Magazine

found this in my search.... basically states what we already know

a kiln has a lot more wire in it than an e-cig so its easier to measure an increase in resistance... kilns also get much hotter and stay hot longer than an e-cig.... i throw my coils out after 4-7 days, long before i would notice any increase in resistance. but the OP was saying they will use coils for months.

Kanthal is what is used in most electric kilns.

 

[generic mutant]

Still leaves me wondering: Does dry burning lower the resistance again? If so, how?

My bet is it's temperature changing the structure.

 

[happydave]

Still leaves me wondering: Does dry burning lower the resistance again? If so, how?

My bet is it's temperature changing the structure.

i just don't see that occurring (lowering ohms). kanthal has a "Positive Temperature Coefficient of Resistance" meaning that an increase in heat will increase resistance.

the gunk could be electrically conductive to some degree. if it is, removing it from the coil could decrease resistance.

 

[Ryedan]

I have also noticed that coil resistance increases over time. You guys got me thinking and I had the time so I did a bit of testing with Kanthal. I cut about 3" of 27 gauge and made loops at each end for the meter probes. I torched the wire to the point of red hot and let air cool for 15 seconds (cool to the touch by then), multiple times and got these results:

Heat cool cycles #: Ohms:

1 1.3
6 1.3
11 1.3

Then I heated the center 1" by moving the wire rapidly back and forth so the whole inch got red and quenched the wire in water:

Quench cycle #: Ohms:
1 1.3
6 1.3
11 1.3

The quench cycles were done to the same wire I used in the air cool part. I did 2 meter measurements each time and for two sets I did three because I got a reading of 1.4 ohms twice. It's a cheap meter and only has one decimal point for ohms.

So, in 29 heat cycles I got no noticeable change in resistance. Going to check out your link now happydave.

 

[happydave]

when i ran my test i must have heated and cooled the wire more than 50 times. i found the answer. Kanthal FeCrAl alloy forms a protective layer of aluminum oxide when heated. this layer is not electrically conductive and does wear off. creating new area for aluminum oxide to form. this process repeated over and over and over makes the wire smaller and smaller increasing resistance.

Kanthal - Wikipedia, the free encyclopedia

 

[Ryedan]

You got it happydave. I just found the same data on the page about kilns you linked to:

"No matter what you do, elements will increase in resistance over time. This is because the part of the wire that actually carries the current gets thinner as more of the aluminum is transformed into alumina oxide."

I think this mystery is not a mystery anymore

 

[happydave]

even dealing with copper, heat will cause resistance to increase permanently i know this because i have worked on a TON of cars. the spark plug wires run right over the top of the engine and into the motor. they are often the cause of a poor running engine. brand new, resistance is pretty low but over the years the resistance increases.. most manufactures list a range of resistance (ohms) for the spark plug wires.
knowing something is true and finding a way to prove it on the internet are two very different things.

 

[generic mutant]

Thinking about it, pretty obvious that's why an incandescent bulb is full of inert gas.

They wouldn't do it for the giggles, would they?

 

[Ryedan]

Thinking about it, pretty obvious that's why an incandescent bulb is full of inert gas.

They wouldn't do it for the giggles, would they?

Things like this are mostly obvious, once the light bulb flicks on

 

[Nataani]

Awesome to wake up to such a thorough discussion!

The build up of aluminium oxide is a reasonable explanation for an increase in resistance, my question now becomes... How? Usually an aluminum oxide coating protects the underlying metal from oxidation. Take the example of a soda can, if you scratch it you will find that the surface layer of aluminum is very thin and underneath is pristine metal. I can see that expansion may crack this layer, giving access to layers beneath, but over cycles this effect would diminish until there is enough flex in the coating to permenantly protect the metal beneath. Could it be that the juices react with the coating to remove it?

 

[happydave]

The Alumina oxide IS oxidation and it's very brittle. Juice could wash it off it. The expanding and contacting wire could "rub" the alumina oxide into the wick. More than anything it will gradually flake off over time. It's important to note that ceramic kilns run at much higher temperature for much longer periods of time than an e-cig. But the material (kanthal) is the same.

 

[CommaHolly]

exactly your heat cycling a coil thousands of times, and that's going to increase resistance. you can try cranking up the voltage a bit, to compensate for the increase in resistance.

that's what I've done

 

[CommaHolly]

ummmmmmmm y'all are way beyond me now

I do want to say, that I vape a lot,,like 14ml a day,,,,,and these heads are about 2 months old,,and I dry burn almost daily,,,so that's TONS AND TONS of heating,,,,,

 

[AttyPops]

electrons MOSTLY flow across the surface of the wire. lets say i have a 2 gauge wire that is made of one solid length of copper and for comparison i have 2 gauge wire that is made up of 10 strands of wire. the multi stranded wire will have a lower resistance than the solid wire because of its increased surface area. so i could see how covering the wire with a substance COULD impede the flow of electrons. i have no idea if "e-liquid gunk" would cause this. but i have a pretty good way of finding out...

I used to think that too. Then, I was told that's mostly for AC or signals. For DC, it's the cross sectional area. Or so I was corrected. IDK for sure.



I think the wire micro-pits a bit too. Fissures and pits will reduce the conductivity a bit. So it's the heating =oxidizing and/or cracking = less conductivity = higher ohms thing. Nothing to do with gunk. Although gunk adversely effects performance of the coil vaporizing juice.

And, yeah, heat raises resistance while it's hot too. So a hotter wire has higher resistance while it's hot than the same wire cooled. It's some physics thing. If it were not so, the universe would probably go "poof". But IDK the specifics, and that's a bit off topic. Other than IDK if the gunk/heat of the wire is a factor when measuring resistance....AKA....X.X ohms @ Z degrees C. My guess is that slightly warm vs room-temp has a negligible effect. And we don't measure coil ohms while the wire is glowing.

 

[crxess]

Oxidation and dissimilar metals.

Coil resistance greatest increase is caused by a loss of clean contact between the coil lead ends and the device.
As heat is induced, especially on longer draws, the outer shell starts to oxidize.
The wire ends touching the SS housing(or other metal) start to lose contact. Carbon starts to form and contact continues to be reduced until the coil finally reads open, even if not broken.
Even at this point the coil may be good, but will not function do to contact loss. Cleaning the leads of oxidents would bring it back to normal function.
Problem is working on tiny coils in many applications and getting them reattached properly. Most are not worth the effort compared to quick replacement.

Items like Kanger bases need to be inspected and cleaned when rebuilding or coils tend to read higher than actual build.

 

[AttyPops]

Contacts, huh?

Too bad we can't solder Kanthal. Anyone have a coil with silver-wire leads that wants to do a test? Solder the silver wire leads into something so you can avoid oxidation. Prove it's the lead contacts and not the wire....

(Think. Test ohms several times and average it out. And don't wreck whatever you're using or make sure it's trashable.)

Besides, if it was mostly the lead contacts, that would mean that silver lead wire doesn't drop in ohms like pure Kanthal wire....or the silver must oxidize the same (doubt it).

I'm betting on the wire itself. Although your point about contacts is well made and a factor too.