# measuring power usage

Robert G. Brown rgb at phy.duke.edu
Tue Jun 3 21:09:23 EDT 2003

On Tue, 3 Jun 2003, 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.

Yeah, it's hard to say whether it does or doesn't even from the brochure
that comes with it.  My recollection is that it "implies" that it does,
since they go out of their way to indicate that appropriate usage is to
measure draw of e.g. refrigerators and other loads that typically have
PF<1.

However, the (non-PFC) switching power supplies used in PC's only draw
current for roughly the middle third of each half-cycle and have a
significant harmonic load (and harmonic distortion) at 180 Hz; it isn't
just a matter of evaluating <V_0 sine(wt) I_0 sine(wt + delta)>_avg for
w = 2\pi 60 and some small/measured delta.  So it is anybody's guess if
it does it right.

Still, even with a 10% or so fudge on the power factor, I find the
device to be very useful.  Without it, it is fairly difficult to have
ANY idea of the power draw of a given PC.  Even if it is basically
returning just (1/2)V_0 I_0 that's useful information.

And all for less than \$50 delivered...

rgb

--
Robert G. Brown	                       http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at phy.duke.edu

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