In article ,
"Arny Krueger" wrote:

"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.

And a third practical way, which I suspect devices like the kill-a-watt
meter and "true RMS" digital multimeters use, is to sample the
voltage/current waveform and then use a microcontroller to perform the
appropriate integration. Microcontrollers are amazingly cheap and
powerful these days. As long as the waveforms you're measuring are
relatively slow (compared to the sampling frequency), it should be quite
accurate. For mains work, this isn't a hard thing to achieve--a few
kilohertz sampling rate is probably overkill.

--
Andrew Erickson

"He is no fool who gives what he cannot keep to gain that which he cannot
lose." -- Jim Elliot