measuring power usage

Dave Lane dlane at ap.stmarys.ca
Tue Jun 3 20:00:21 EDT 2003


At 04:35 PM 6/3/2003 -0700, Trent Piepho wrote:

> > There is nothing in the flier that indicates that it measures rms amps and
> > hence rms watts. Computers are not resistive loads and they draw 
> current in
>
>It measures power factor and frequency, so it should be doing what is
>necessary to find true rms.

Measuring true rms current depends on measuring in some way the area under 
the curve from the shape of the waveform. Non-rms meters usually rectify 
the voltage to all positive voltage and low pass filter it to convert to a 
DC value, then a fudge factor is used which assumes the waveform is a sine 
wave (which it isn't).

... Dave

ps. what you really need for an accurate measurement of RMS power and power 
factor is something like: 
http://ca.fluke.com/caen/products/features.htm?cs_id=5179(FlukeProducts)&category=PHASE1(FlukeProducts) 
which unfortunely is about $1900US - ouch! There are likely other 
alternatives that are cheaper.

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