At 10:45 AM 2/27/02 -0600, Lawrence Lile wrote:
>If you could guarantee you are really measuring for a full cycle, then yes
>it won't make any difference where you start.  However, starting at a zero
>crossing and ending two zero crossings later (both pos and negative cycle)
>guarantees you have measured a full cycle and it's easy to do.

Seems to me that you might get more jitter from the discrete number of
measurements if the beginning and end took place close to the peak of the
cycle.

>Now, a dumb question.  If RMS is calculated by squaring, averaging, then
>taking the root, then why didn't the (possibly dislexic) genius that
>invented it call it SQUARE MEAN ROOT since that is really the order you do
>these things in?

Sounds like you have an anti-reverse-Polish-notation prejudice. ;-)

To the OP: Do you really need RMS? What are you trying to measure? It's
appropriate if you are looking at heating (such as an electric heater or a
light bulb). OTOH, average may be preferable or good enough for your
application. If the waveform is sinusoidal, then you can relate the
average to the RMS and just do one calculation. You can also get the
average with a low pass filter and take fewer readings.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
9/11 United we Stand

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