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From: rsd@sei.cmu.edu (Richard S D'Ippolito)
Newsgroups: sci.electronics
Subject: Meters and RMS, was: Powerline voltage too high...
Message-ID: <9839@as0c.sei.cmu.edu>
Date: 2 Jan 91 19:20:25 GMT
References: <5402@rsiatl.Dixie.Com>
Reply-To: rsd@sei.cmu.edu (Richard S D'Ippolito)
Organization: Software Engineering Institute, Pittsburgh, PA
Lines: 58

In article <5402@rsiatl.Dixie.Com> larry@rsiatl.UUCP (Larry Kahhan) writes:

>>rsd@sei.cmu.edu (Richard S D'Ippolito) writes:
>>
>>>... Again, there is NO SUCH THING as "true RMS"; it's just RMS! ...
>>
>
>Not so! Early AC voltmeters used a technique to measure the RMS values
>of sinusoidal waveforms by a simple calibration technique. 

Uh, Larry, how old are you??!  

At best, what you meant to say is mis-stated by confusing measuring
technique with calibration technique, but I catch your drift.  "Early" AC
voltmeters of the iron-vane repulsion or thermopile type co-existed with and
in many cases preceeded some of those average-responding D'Arsonval
movements so common until the advent of digital display meters.  These were
in common use in various industries, including the electric utilities and
standards labs.

(For those who may not know, the "simple calibration technique" is insuring
that the calibration source is a pure sinusoid and adjusting the output
scaling so that the rms-equivalent is displayed.)

The lower cost of the "newer" meters made them available to a wider, but not
necessarily more educated clientele, who weren't aware that the analog (and
now digital) meters would not read the correct RMS of a non-sinusoid, now
more important as the resolution increased (mostly far in excess of even the
short-term accuracy) by using digital displays.


>When AC
>voltmeters came along which could determine the RMS value of an 
>arbitrary waveform (using any one of a number of techniques) these
>meters were called "true RMS" to distinguish them from ealier RMS
>meters. 

Nope, the goofy phrase was coined by a combination of marketers and at-best
apathetic engineers who didn't insist on "RMS-responding" (doesn't sound as
catchy, does it?), and now we're stuck with a re-education task, always
harder the original educating would have been.

Your KWHr meter has always been RMS-responding.

>However, there is only one real definition of RMS, as
>Richard points out.
>
>Larry Kahhan - NRA, NRA-ILA, CSG, GSSA , & GOA 


Other goofy marketing anomalies (or should I just be honest and call them
the mistake they are?) are the 3-1/2 (or whatever) -digit tag on a meter
whose full scale reading is 1999 counts (anybody know logarithms?), and the
meter with "0.1%" accuracy, enthusiastically promising a reading no more
than a factor of 1000 off!


Rich