On the Evic VT, or joyetech/Wismec mods in general, there's really no need to lock the resistance. The mod will maintain the initial resistance, and ask new coil/same coil if it detects a significant change in resistance. Locking the resistance is primarily useful if you grow tired of the new/same coil question. Otherwise, I'd actually recommend leaving the resistance unlocked (Evolv recommend the same for their chips). Yihi chips are different, with those you need to lock the resistance in order for tc to function properly.You need to lock the ohms for TC mode (Ni or Ti), and when you remove a atty & put in a different one, of course it will unlock, because it's a new atty.
When in TC mode, the atty needs to be locked, because when the power/joules/watts heats the coils, the resistance will rise if it is not locked.
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