Great info. Really helped me understand what's going on.
One question. Doesn't the resistance of the coil go down as it heats? Is it linear?
I don't have a clue how you could measure the resistance on a hot coil, but it would be interesting to know what the full draw circuit looks like versus the cold circuit.
Thats a very good question and one that I have thought a lot about recently. I have just been too freakin' lazy to test it. If I measure the voltage applied and the current which flows through the atomizer at the same time, I should be able to figure the actual resistance at that moment in time using the formula E/I = R. I will try and test this tonight.....
Edit #1: That was easier to test than I thought. I used
my test set to test this.
I used a recently charged Ultrafire 18650 battery which measured 4.11V static (not under any load). I used a KR-808D cartomizer as a dummy load. It measured 3.2 ohms. I put in the test set and measured voltage and amperage using two meters at the same time. After about 5 seconds, the voltage settled in at 3.85 volts and the amps was 1.22 amps. So, 3.85/1.22 = 3.155737... ohms.
Initially it would look as if the resistance dropped. However, the resolution of the ohms measurement on my meter only goes down to 10ths of ohms. Also, the accuracy specifications of the meter(s) are suspect also. I suspect that the I would need a much more precise test equipment to determine the answer to your question by direct measurement.
However, it is probably safe to say that the change in resistance is negligible for our purposes. Probably more important is how much the voltage dropped on the battery when it was placed under load.
Edit #2: It would appear from a table on
this page that the resistance would actually go UP between 2 and 3.5% if the wire goes from 68F to 400F. I would consider that to be fairly negligible.
Several unknowns here... How hot is the wire really getting? Which formulation of nichrome wire is the atty made with? etc. etc.