You need to discover your preferred
vaping intensity (i.e., the power, measured in watts). Watts are not a property of either the atomizer ohms or the battery voltage, but the result of the two in combination:
Watts = Volts X Volts / Ohms
You can achieve your desired watts with an atomizer of virtually
any resistance on a variable voltage device like the Darwin (which, more precisely, is "regulated power", since it also takes into consideration the atty resistance). The atomizer resistance (ohms) is more critical with your
fixed voltage devices like the Riva and eGo. Unless you have the "Special Edition" Riva, that voltage is ~3.2V. A 2.5 ohm atty on it yields 3.2 X 3.2 / 2.5 = 4 watts ... which is too modest for most of us.
The "sweet spot" for many vapers is 6-8 watts. Thus the popularity of 1.5 ohm LR atomizers (yielding 6.8 watts on such batteries). Using your 2.5 ohm atty on your Darwin, dialing the voltage to between 3.9V and 4.5V will put you into the sweet spot zone. [YMMV, i.e.,
your sweet spot may be higher or lower than that.]
For more on this volts/ohms/watts stuff (and the amps that can fry atomizers), check out
this page of
The Wonderful World of Vaping Version 2 and
this article. And
this online utility makes the determination of volts/ohms/watts/amps quick and easy: enter any two values (e.g., ohms and volts) and it calculates the other two (e.g., watts and amps).
