thanks guys... Davep - what is RMS?
It's the measure of the actual voltage of a waveform based on it's Root Mean Square value. Since alternating or pulsing voltage varies constantly during the duty cycle, peaks positive, and may or may not cross zero and go negative, some method is needed to determine the actual value of its ability to perform work as compared to a direct current voltage.
True voltage of a waveform is the measurement of the voltage at all points in the wave and averaging them to arrive at a meaningful representation of the what it would be if it were a DC voltage. A sine wave will be different than a square wave or a triangle waveform. In ecig mods, the wave is usually a square wave pulse. Every so many milliseconds the value reaches the same level (amplitude), remains there for a time (duration) and then declines suddenly to zero and repeats. That's repeated over and over. The duty cycle (and the equivalent RMS voltage) can be changed by varying the pulse width (pulse width modulation), the frequency of the pulses (frequency modulation), or the amplitude of the pulses (amplitude modulation).
The idea is to aproximate the value that is equal to an equivalent DC voltage to compare the waveform to DC in terms of the amount of work it could do.
For a sine wave, it's .707 times the peak value of the wave. For a square wave, it's the peak value unless it's a pulse with time at zero between the pulses and no negative component, then you have to calculate the duty cycle (on/off/negative time). For a triangle waveform, it's .577 times peak.
It's much better understood with pictures and diagrams and formulas.
Explaining rms voltage and current | Nuffield Foundation