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Re: [Shop-talk] tube fluorescent wattage

To: Randall <>
Subject: Re: [Shop-talk] tube fluorescent wattage
From: Doug Braun <>
Date: Sun, 1 Mar 2009 16:43:37 -0800 (PST)
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 ( 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.


--- On Sun, 3/1/09, Randall <> wrote:

> From: Randall <>
> Subject: Re: [Shop-talk] tube fluorescent wattage
> To:
> 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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