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.

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?

I guess the answer might be the same as:
Why do I have Tennis Shoes if I don't play tennis?
Why do they call the pants you wear when you are dressing up, not slacking
at all, "Slacks"?

--Lawrence


----- Original Message -----
From: "michael brown" <n5qmg@AMSAT.ORG>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, February 27, 2002 9:58 AM
Subject: Re: [pic] Math Question - Measuring AC Voltage


> Doug says:
>
> > There are many ways to measure AC voltages.  The most common is RMS
> > which is the usual 220VAC (110VAC USA).  RSM voltage is litterally "Root
> > Mean Square", so you square your readings, average them over one cycle,
> > and take the square root.  It would help if you could sync your readings
> > to the zero crossings of the power line.
>
> Why would syncing to the line make any difference.  As long as he samples
> one complete cycle why would it matter where he started and where he left
> off?
>
> michael
>
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