22-08-2020, 05:49 PM
Craig Wrote:Oddly enough there is no traceable definition of RMS voltage. The only way that NPL/PTB/NIST etc do it is using a thermal transfer standard. Because DC voltage is defined in terms of the Josephson effect to umpteen places of decimals, you compare the heating effect of the AC voltage being calibrated with the heating effect of a DC voltage derived from the Josephson standard.
I'd disagree here on two counts.
Firstly, the volt isn't a basic unit defined in terms of Josephson effect to umpteen places of decimals (it may numerically equal this, but that's not its definition).
A volt is the potential difference which exists if one joule of energy is produced or absorbed, by moving one coulomb of charge.
One joule is a derived unit, from metre, second, and kilogram (actually 1 kg m² / sec²) and one coulomb is a derived unit, from ampere and second (actually 1 A sec).
Metres, seconds, kilograms, amps, are all fundamental units (the amp was actually redefined last year in terms of the coulomb! Up to then, it was defined in terms of the force between two current-carrying wires).
So the volt is a derived unit.
Secondly, once a volt is defined, then RMS volts are easy to define. For a voltage constant with time, then RMS volts = actual volts. For a voltage which varies periodically with time, the RMS volts = square root of the average over one cycle, of the square of the voltage.
So to measure it, just take loads of samples over a cycle, square each sample, find the average of this, and take the square root. The error is relatively straightforward to calculate from the mathematical exact figure, by knowing how much the voltage varies between samples.
Coming back to the OP's question, yes, a voltage is assumed RMS by convention, unless otherwise stated.
