"John Williamson" wrote in message
...

The most (theoretically) accurate way to measure RMS values is to use a
hot wire meter, which doesn't care what the waveform is, it just
measures the heating effect which is more or less frequency independent
& includes any DC offset automatically.

That's one way, subject to the problems with accurately measuring the
heating of a resistive element.

Another way to obtain a true RMS reading without complex electronics is to
use a certain kind of meter movement that mechanically integrates the
product of the current and the voltage. There are two sets of windings in
the meter, one for current and one for voltage. Their attraction or
repulsion that drives the pointer is based on the product of the current in
the windings. I have one that was made by RCA, and a very common tool during
the 50s, 60s, 70s, and 80s.

Yes, all agreed, but the shape of the mains waveform is immaterial,
unless you are talking a purely resistive load like a light bulb.

Simply not true. Even light bulbs have some degree of sensitivity to the
waveform, unless they have filaments with very long thermal time constants.

Historically rectifier-based power supplies have been very sensitive to wave
form shape, because their output voltage is strongly influenced by the peak
value of the power line wave.

In the old days some magnetic power line voltage regulators put out a fairly
pure square wave. This did a pretty fair job of heating tube filaments, but
did not provide full B+ voltage from the power supply. The problem was the
low peak voltage. If you jacked up the line voltage to get full B+, the tube
filaments ran hot and tube life suffered.

The point that I
have been trying to make all along is that when you are trying to measure
power, it's a function of both voltage and current draw, and in the case
of a SMPS, especially one that's in standby mode, the current draw from
the mains supply voltage, is very likely to be anything *but* sinusoidal.

Agreed. However there is a newer kid on the block, and that's the power
factor corrected SMPS. This technology has been reduced to an IC, and it
shows up in items as humble as compact flourescent light bulbs. If the
power factor is 1.00 or approaches it, then the current and voltage are
largely in-phase.