When you bring calculus into the picture it becomes possible to calculate the RMS value for any arbitrary waveform (the hard part is to integrate the square of the voltage over one cycle). But is it possible to measure the RMS value of any waveform? It should be possible for a digital processing oscilloscope to calculate the RMS value of a waveform. Otherwise the only voltmeters I am familiar with all assume that the input waveform is a sine wave which is rectified and displayed as a DC voltage on an RMS scale. For instance, the RMS value of a DC voltage should be the DC voltage value. When I use a VOM to measure the voltage of a 9V battery, the DC voltage is 9.4V When I measure the battery as AC, the RMS value is 12V. The only sense I can make of that is, Don't try to measure the RMS value of anything except an AC sine wave. Indeed, VAC implies the RMS value of an alternating current (AC) sine wave.