when a coil begins to wear out does the resistance go up or down?
lets say for example a fresh 1.8ohm coil gets used for 6 months straight. what's the resistance after 6 months?
Assuming Kanthal A1, in an RBA where wick is replaced and the coil dry burned regularly... the wire will "consume" it's aluminum content over time, in the form of it's electrically resistive, thermally conductive aluminum oxide outer layer.
This
"alumina layer" (Al2O3) is the key feature for vapists who wrap compressed coils... although most aren't even aware of it's existence or function. What is left is the bare iron-chromium content, which will corrode and degrade from use and exposure.
Although there are
several variables to consider, as most tend to notice this condition occurring withing 2-4 months (perhaps less with your starting resistance value)... either visually, by performance or by taste, chances are you'd change the coil, before your term of 6 months.
As you might imagine, as all this occurs, resistance increases. If the coil starts out as a compressed coil, is repeatedly dry burned and occasionally "re-compressed" (some people just can't leave stuff alone)... at some point, the coil may "short out", wrap to wrap, because the alumina layer that insulates the iron-chromium is gone.
Using a 10X loupe after a dry burn... when I see a black, rough textured coil, rather than a nice smooth, dull silver, or I "taste" metal... it's time (actually, well past time) for a new coil. ;-)
