Contact and spaced coils with SS

[Powerman]

I asked this on another forum. I C&P here. Hopefully it's not that much out of context... Honestly, I'm still having a hard time wrapping my head around this. In any circuit you can get a short. So a wire 10 cm long between +/- terminals has X resistance. Coil it, loop it and touch only the tops together now you have a short. Not the distance between the two +/- terminals is 2 cm. Who cares... Except now 2 cm of wire only has 1/5 the resistance of the straight wire. So this is the part I don't get. I can only assume. I going to use numbers straight out of my ass to illustrate the point. I assume measuring the effect of temp on resistance is on a material at a equal temp. So 2 cm of wire at the same temp for the whole length of the wire. 2 cm wire with 1 ohm at 100 degrees. Heat it up to 200 degrees it has 2 ohms. But if 1 cm is 300 degrees, and 1 cm is 100 degrees, the average is 200 degrees. 2 ohms. If a coil is contacting only at the top, and current is shorted, I could see the short heating up much hotter than the rest of the coil. I could see TC limiting power because it sees the rise. The top is hotter than the bottom and the average is the rise. But if there is a short, then that 10 cm wire is now only 2 cm. And the natural resistance of the wire would show that. 10 cm with 1 ohm is now 2 cm with 0.2 ohm resistance. The part I don't get is why does my mod see the full resistance of the full length of the wire, but TC seems to only see the rise in one small part of the wire. Path of least resistance should be measured. A 10 cm length of wire shows 1 ohm on mod, not 0.2 of a shorted contact coil? I hope that makes sense.

[Powerman]

So the point of this was I got some 430 SS and put the 138 TCR in from steam engine. But the vape was very anemic. Dual 24g 9.5 wraps at 2.5 mm coming in at 0.16 ohm. I had to bump the TCR all the way up to 170 to get it to run right. That was a contact coil. I built 3 spaced coil decks and all work right with the 138 TCR. So fine I have to do spaced. I just want to know why.

[RoofMonkey911]

Just taking a stab in the dark here, but spaced coils not only give more surface area for juice, but also for airflow. Wire touching itself will get 'hotter' / retain heat due to a lack of airflow or juice between the coil wraps. ?? Maybe????

Powerman said:

So the point of this was I got some 430 SS and put the 138 TCR in from steam engine. But the vape was very anemic. Dual 24g 9.5 wraps at 2.5 mm coming in at 0.16 ohm. I had to bump the TCR all the way up to 170 to get it to run right. That was a contact coil. I built 3 spaced coil decks and all work right with the 138 TCR. So fine I have to do spaced. I just want to know why.

[Powerman]

That's reasonable, but the point of contact coils is even heating and less hot spots. So if it's only seeing a part of the coil, it should still even out better than spaced.

[Jaquith]

If you want 'contact coils' then Annealing, but just know Oxides will form. Info "For sub-critical annealing, heating should be performed to 760-815°C (1400-1499°F), followed by air-cooling or water quenching. This stainless steel cannot be hardened by thermal treatment." Further, most SS wire is from China and SS430 has at least 3 SAE alloys nevermind China's GB conversion. Besides not knowing 'exactly' what you have, for Temperature Control increase (hotter) or decrease (cooler) the TCR until the desired temperature is obtained. In other words IGNORE posted TCR settings .. assuming proper setup and configuration. Tip doing this blind, disable Preheat and set the Power way, way too high relying on TC regulation NOT by some erroneous TCR setting or limited by a Power setting.

[lewisss]

Powerman said:

So the point of this was I got some 430 SS and put the 138 TCR in from steam engine. But the vape was very anemic. Dual 24g 9.5 wraps at 2.5 mm coming in at 0.16 ohm. I had to bump the TCR all the way up to 170 to get it to run right. That was a contact coil. I built 3 spaced coil decks and all work right with the 138 TCR. So fine I have to do spaced. I just want to know why.

It could be this:-

kanthal when heated up forms an oxide layer outside the wire, this oxide has an extremely high resistance and is extremely strong. so when current passes through the coil it cannot 'jump ' loops and the current has to follow the full route of the wire. as the oxide layer is forming an effective isulation for the wire.

nickel also forms an oxide layer, but it has poor insulation properties. so if the loops of the coil touch they effectively short, hence why nickel should be used only as spaced coils.

Titanium and SS fall in between these two. 

I have had very little luck with contact coils in SS 
titanium contact coils are much better as long as you heat them to a dull red first (at your own risk!) 

at this temp they change colour to all colours of the rainbow, I find a metallic blue works best.

If you heat them any further they go white (this is the formation of titanium Dioxide) 
at this point throw them away .......... you have gone too far!