On 8/14/2007, Gene E. Bloch posted this:
On 8/14/2007, Arfa Daily posted this:
"William Sommerwerck" wrote in message
...
So are you saying that a meter that measures *true* RMS
anything -- power, current, volts -- will do so correctly even
if the waveform is asymmetric about the zero point? I always
believed that an even distribution about the zero point was a
requirement for an accurate representation of an RMS value.

The RMS value of a waveform has nothing to do with the shape or symmetry
of
the waveform.

Assuming the meter is correctly designed, yes. RMS has a clear, specific
definition, and if the measurement is correctly implemented, the reading
will be correct.

Two qualifications... If the waveform is non-periodic, the measured RMS
value will vary according to the sample period. Also, if the waveform
includes a DC component, and the meter blocks it with a capacitor, then
the
RMS reading will not include the DC component.


Ah, OK. Perhaps it's non-periodicity that I'm getting confused with. As I
say, college seems a long-haired good music time ago now ...

Arfa

I agree with William Sommerwerck's more thorough analysis; the DC problem
(which I hadn't thought of) is pretty interesting too :-).

I agree about the confusion you mention - it makes sense; BTW, the way around
it is to use long sampling times, as I said in my post, although I didn't go
that one step further: to think about non-periodicity - & understand it :-)

BTW, some true RMS meters use an entirely different approach[1]: they measure
the temperature of a resistor which is heated by the current flowing through
it when it is shunted across the line. This is (1) potentially pretty
accurate, and (2) probably very very slow to respond to changes - i.e, an
implied long sampling interval :-)

I guess we're OT here, but I admit that I'm having fun...

[1] Well, I'm not sure why I said 'different' - we haven't discussed the
method used, have we?

I spoke just a bit too soon - the calorimetric method was mentioned in
some recent posts in this thread that I just saw.

--
Gene E. Bloch (Gino)
letters617blochg3251
(replace the numbers by "at" and "dotcom")