[vox-tech] [OT] Electrical Engineering Question

[Rod Roark]

On Friday 31 January 2003 06:20 pm, Peter Jay Salzman wrote:
> hi rod,
>
> well, the math looks good, but you most likely knew that anyhow.
>
> the relative error between 325 watts and 250 watts is about 23%.  pretty
> high, imho.

Well, either 250W is "23% too low" or 325W is "30% too
high".  :-)

> the implicit assumption is that the dial's rotational speed increases
> linearly with energy consumption.   i have no inner feeling for whether
> that's true or not.  however, i can tell you that the power delivered by
> an harmonic wave of any sort is proportional to the *square* of its
> amplitude.  that's kind of counter intuitive, and was just meant to
> illustrate that linear increase in angular speed may not result in
> linear increase in consumed power.

The meter seems all mechanical, just gears, and the dials
measure straight KWH, so I'm pretty sure the relationship is
linear.

> but i'm _very_ interested in hearing PG&E's response to this.  please
> send a followup to this when you have more info!

Sure.  I'll be doing some more careful testing first.
Checking that 100 revs/KWH assumption will be a pain.

Thanks for the feedback.

-- Rod

> pete
>
> ps- what kind of bulb consumes 250 watts?  a french fry lamp?   :)
>
>
> begin Rod Roark <rod@sunsetsystems.com>
>
> > I got my first electric bill at the new house; looks too
> > high.  So I decided to do an experiment.
> >
> > Outside the house is an electric meter.  It reads KWH
> > accumulated on 5 dials, and has a horizontal platter that
> > appears to spin about 100 revolutions per KWH (anyone know
> > if this is exactly true for a standard meter?).
> >
> > So I figure that means 10 watt-hours per rev, or 36,000
> > watt-seconds per rev.
> >
> > I timed one revolution with most things in the house turned
> > off.  45 seconds.  Then I turned on a 250W light bulb and
> > timed it again.  32 seconds.  So:
> >
> >   36,000 watt-secs / 45 secs =  800 watts
> >   36,000 watt-secs / 32 secs = 1125 watts
> >
> >   1125 - 800 = 325 watts -- for a 250W bulb.
> >
> > How come?  Should I complain to PG&E, or is there some
> > gotcha that I'm missing?
> >
> > Thanks,
> >
> > -- Rod