That's a nice unit, then. I have a fairly cheesy power meter
from Lacrosse, similar in style to the Kill-a-Watt, and it
clearly does not consider power factor, just current as measured
across a shunt resistor.
More recently I got a TED 1000 (www.theenergydetective.com). It has clamp-on
sensors for both main feed conductors and measures true RMS. It also has a PC
interface for logging, etc. I'm pretty happy with it, but it is not meant to
measure power usage of individual appliances like a Kill-a-Watt.
Doug
--- On Sun, 3/1/09, Randall <tr3driver@ca.rr.com> wrote:
> From: Randall <tr3driver@ca.rr.com>
> Subject: Re: [Shop-talk] tube fluorescent wattage
> To: shop-talk@autox.team.net
> Date: Sunday, March 1, 2009, 5:08 PM
> > And measuring a complete bulb+ballast fixture with a
> > Kill-a-Watt style meter may not be very accurate
> because of
> > the power factor of a magnetic ballast, and non
> linearity of
> > an electronic one.
>
> While I have not tried one myself, it's my
> understanding that the
> Kill-a-Watt meter measures both apparent and true RMS
> power. It has
> displays for watts, volt-amps, power factor, etc.
>
> Fairly easy to do, all it takes is a small microprocessor
> and a couple of
> A/D converters (one measuring current, the other voltage).
> We're only
> dealing with a 60 Hz waveform, so a cheap 50kHz A/D will
> give us almost 1000
> samples per cycle. That's plenty to give us a very
> good approximation of
> true power. I'm sure there are better techniques
> available, but it wouldn't
> take much processing power at all by today's standards
> to capture a few
> cycles worth of data points; multiply each voltage,current
> pair together;
> and do a brute-force RMS calculation on them. (Multiply
> each value by
> itself, add up the squares, divide by the number of
> samples, and take the
> square root of that.)
>
> Compared to what my digital camera does every time I push
> the button, that's
> nothing.
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