# Resistance Variation on RDA Coil



## BumbleBee (9/10/14)

Something weird happened yesterday, just before I left the shop I built a quick coil on my IGO, 26g Kanthal with a 3mm inner diameter, wicked it with cotton and it read 0.8 ohm on the SVD. I'm using it on a mech, just used the SVD to check the coil. I really wasn't impressed with it at all, took forever to warm up. So when I got home I took it apart to rebuild it but realised I had not brought my 26g with me. I took the coil out and just wrapped it around a 2mm. I used the same piece of wire and the same piece of cotton just thinned out a bit, didn't trim it at all... it came out to 0.6 ohm on the SVD. Does anyone have an idea why the resistance drops by going down to a smaller ID?


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## Riaz (9/10/14)

BumbleBee said:


> Something weird happened yesterday, just before I left the shop I built a quick coil on my IGO, 26g Kanthal with a 3mm inner diameter, wicked it with cotton and it read 0.8 ohm on the SVD. I'm using it on a mech, just used the SVD to check the coil. I really wasn't impressed with it at all, took forever to warm up. So when I got home I took it apart to rebuild it but realised I had not brought my 26g with me. I took the coil out and just wrapped it around a 2mm. I used the same piece of wire and the same piece of cotton just thinned out a bit, didn't trim it at all... it came out to 0.6 ohm on the SVD. Does anyone have an idea why the resistance drops by going down to a smaller ID?


because you using less kanthal maybe?


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## BumbleBee (9/10/14)

Riaz said:


> because you using less kanthal maybe?


nope, used the same piece if wire from end to end


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## johan (9/10/14)

BumbleBee said:


> nope, used the same piece if wire from end to end



better surface contact between the individual 2mm ID windings than the 3mm should give a lower resistance. It would be interesting to note what the resistance is when straighten.

Reactions: Thanks 1


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## BumbleBee (9/10/14)

johan said:


> better surface contact between the individual 2mm ID windings than the 3mm should give a lower resistance. It would be interesting to note what the resistance is when straighten.


OK, makes sense 

I'm going to pull this thing apart just now and straighten the wire out to see what reading the SVD gets.

Reactions: Thanks 1


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## BumbleBee (9/10/14)

1. Measured it now again, with and without wick it now reads 1.0 ohm, there is a bit of buildup on the coil surface.

2. Dry burn the crud off and it's still reading 1.0.

3. Stretched the wire out so now it's just one length, reading 1.1 ohm

There seems to be a very slight increase in resistance when the wire is stretched out, but doesn't explain the jump from yesterday's 0.6 to 1.0 today. I am suspecting the SVD of giving the incorrect readings.

Reactions: Agree 1


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## johan (9/10/14)

BumbleBee said:


> 1. Measured it now again, with and without wick it now reads 1.0 ohm, there is a bit of buildup on the coil surface.
> 
> 2. Dry burn the crud off and it's still reading 1.0.
> 
> ...



I agree the 0.4 ohm jump doesn't make sense, so the only culprit is the SVD.


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## BumbleBee (9/10/14)

johan said:


> I agree the 0.4 ohm jump doesn't make sense, so the only culprit is the SVD.


As I dig further down the rabbit hole I can see why a dedicated ohmmeter is a good idea, the SVD is great for checking resistance and for shorts and helps reduce the cost when starting on rebuildables, but the resolution isn't very good. If it reads 0.8 it could be anywhere from 0.81 to 0.89. I guess it is a good time to start looking out for a proper resistance checker 

Thanks for your input @johan


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## johan (9/10/14)

BumbleBee said:


> As I dig further down the rabbit hole I can see why a dedicated ohmmeter is a good idea, the SVD is great for checking resistance and for shorts and helps reduce the cost when starting on rebuildables, but the resolution isn't very good. If it reads 0.8 it could be anywhere from 0.81 to 0.89. I guess it is a good time to start looking out for a proper resistance checker
> 
> Thanks for your input @johan



No worries, was a riddle to me as well, as I assumed the ohm reading was precise


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