[SI-LIST] Re: How they measure true RMS
- From: Paul Levin <levinpa@xxxxxxxxxxxxx>
- To: a.ingraham@xxxxxxxx
- Date: Wed, 17 Dec 2003 07:19:30 -0800
Dear Andrew,
Perhaps you are thinking of the watt meter style. I still own a Weston
meter that has the voltage applied to the field coil and a sample of
the current (i.e., voltage across a shunt resistor) going through the
moving coil. In this way, the deflection is proportional to the
average value of volts x amps. If you were to apply the voltage to
both coils (through appropriate scaling factors, of course,) then the
reading would be the average value of volts x volts, or RMS voltage.
Regards,
Paul
--
Paul Levin
Senior Principal Engineer
Xyratex Storage Systems
______________________
Ingraham, Andrew wrote:
>>I was in Sears today and saw a very nice digital multimeter with * TRUE
>>RMS
>>* AC capability for $69.00 That got me thinking about HOW they measure
>>true RMS value. In the old days they passed the waveform through a
>>resistor
>>and measured the heat.
>
>
> In the old days of analog mechanical meters, there also was a special meter
> movement (not the usual d'Arsonval type) that would respond moderately well
> to the RMS value. I think it had more than one coil and responded to RMS by
> way of special (nonlinear, perhaps?) magnetic characteristics. Wish I could
> remember more, sorry.
>
>
>>Can a $69 multimeter have enough processing power to
>>be truly sampling and integrating the waveform to get the true RMS value?
>
>
> With a $5 IC, you can do almost anything. Provided, of course, that it
> doesn't need to do it very fast.
>
> These things use a mini processor to calculate RMS. They are probably
> accurate only for audio frequencies, maybe up to 100 KHz or a MHz or so,
> but not beyond. They've been around for some time.
>
> It can be done faster, even with nonlinear analog circuits (limited only by
> the analog circuit's speed), but I doubt many multimeters do that. Anyway
> most engineers hopefully wouldn't use multimeters (with clipleads) to
> measure MHz signals. (That's a silly assumption on my part, I know.)
>
> Regards,
> Andy
>
>
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