From: John De Armond
Subject: Re: AC panel voltmeter for RMS readings
Date: Thu, 07 Sep 2000 02:53:34 -0400
> In article <39B665CD.E3728F0D@bellsouth.net>,
> Neon John <firstname.lastname@example.org> wrote:
> > Absolutely incorrect. Moving vane meters respond to the RMS value
> > of the incoming waveform. You can verify that by either hitting the
> > books and learning how they work (I did, long ago) or you can
> > compare the instrument's reading to a true-RMS DVM (I have.)
> > John
> You can say it until you're blue in the face, John, and it will make no
> more sense. These meters have DC movements which do not respond to AC at
> all, and of course rms is a meaningless term without a waveform. The
> simple full-wave rectifier that converts the input to the DC current
> that these meters DO respond to does not produce the rms equivalent to
> any arbitrary waveform.
Iron vane meters do NOT have rectifiers, do not have permanent magnets
and respond equally accurately to AC and DC with one minor exception.
Because the iron vanes have a small amount of retentivity, on DC, the
current must be reversed, a second reading taken and the two averaged.
The amount of retentivity-induced error is small - a few minor divisions
at most - so it can be ignored for casual use.
> So what reference tome did you pull your misguided concepts from anyway?
Oh my, where to begin. This is like asking a civil engineer where
he learned of concrete. Let's see. At about the age of 10, the
Coyne Electrical Institute's Encyclopedia of Electricity was my
daily companion. Wonderful set of practical electrical books - you
really ought to try to find a set from a rare book dealer - you
obviously need a good course in electricity. Later in college, the
"Process Instruments & Controls Handbook", usually known as the
"Considine book" by those of us in the trade. Later I gained
intimate working knowledge of these instrument when I first worked
in and then headed the Sequoyah Nuclear Plant's metrology section.
Why don't I quote from Considine?
Sect 13.16 - "Moving Iron Instruments"
"An iron vane instrument provides a means for measuring AC or DC
current using an attraction-repulsion method. A field coil is wound
through which the current to be measured is passed. In the center
of this coil are placed two iron vanes. One vane is stationary, and
the other is made movable by connecting it to an axis, pointer, and
torque spring. The magnetic field setup by the current passing
through the coil induces like polarities in the iron vanes, causing
them to separate. The amount of separation is indicated by a
pointer. The principle is illustrated in Fig 16, and an early
magnet vane mechanism is shown in Fig 17. (the URL below has an
illustration close enough to Fig 16).
"The energy stored in the magnetic field can be written: E =
1/2*I^2*L Where L is the inductance of the system, and its
derivative, the torque T, is: T = dE/d<theta> =
"The system's response is proportional to I-squared.
(I'm going to type this very slowly, Steve, so you can be sure and
embrace every word)
>>>>> "The instrument indicates root mean square (RMS) current or voltage. <<<<<<
"By suitably shaping and arranging fixed and movable irons in the
field of the coil, the instrument scale can be made very nearly
linear over as much as 80% if its range. Alternatively, the upper
range of the scale can be greatly compressed, and a small portion of
the total range can be expanded to cover much of the scale. (Don't
miss this part either, steve) The latter arrangement is
particularly appropriate in a voltmeter used to monitor voltage
which is nearly constant most of the time, such as line voltage.
Over the years, various vane configurations have been developed, two
of which are shown in Fig. 18." (sorry, you gotta buy the book to
see that one.)
Oh, I might add, the above section was written by B. P. Gropp, R&D
Director, Weston Instrument Division, Sangamo/Weston, Newark, NJ.
Kinda like getting the 10 Commandments directly from Moses. If you
happen to disagree with Dr Gropp, perhaps you could take it up with
If you want an on-line reference, here's the very first hit from the
I'm sure there are better on-line references but this is enough to
demonstrate that you don't have the faintest idea of what you speak.
> I'm sure there are dozens of manufacturers of expensive rms voltmeters
> who will want to quash this publication so they can continue to foist
> their overpriced instruments on a naive society of engineers!
I can't imagine what sort of conspiracy theories you might cook up
in your little mind but I can speak to the power utility industry.
Every switchboard voltage or current indicator - AC or DC - made in
the last, oh, 100 years that I've ever seen is a moving iron
instrument. They are equally accurate on AC, DC, 25-500 hz, square
wave inverter waveform, distorted small generator output, distorted
saturable reactor output. In other words, just about anything you
want to toss at it. And there are no semiconductors to get damaged
by transients. Because a well-designed instrument responds equally
to DC and the RMS value of AC, the detailed 5 point calibration can
be done with simple DC instruments and then checked at a point or
two on AC as a cross-check.
To summarize, Steve, you don't have the faintest idea what you talk
about. But as usual, you speak your mistake authoritatively and you
caustically ridicule those who DO know and dispute what you say.
Pretty typical. The only thing that makes this instance remarkable
is that you're so starkly wrong about a basic, fundamental
electrical principle that even a freshman EE student knows that it
only took a minute to completely refute what you said and to post
authoritative references. Other than myself, of course. Never has
that old saying, "better to remain silent and be thought a fool than
to open your mouth and remove all doubt." been more apropos.
From: John De Armond
Subject: Re: AC panel voltmeter for RMS readings
Date: Thu, 07 Sep 2000 19:29:32 -0400
> In article <39B73B6E.46F2FCE5@bellsouth.net>,
> Neon John <johngdNOSPAM@bellsouth.net> wrote:
> > Sect 13.16 - "Moving Iron Instruments"
> > "An iron vane instrument provides a means for measuring AC or DC
> > current using an attraction-repulsion method. A field coil is wound
> > through which the current to be measured is passed. In the center
> > of this coil are placed two iron vanes. One vane is stationary, and
> > the other is made movable by connecting it to an axis, pointer, and
> > torque spring. The magnetic field setup by the current passing
> > through the coil induces like polarities in the iron vanes, causing
> > them to separate. The amount of separation is indicated by a
> > pointer. The principle is illustrated in Fig 16, and an early
> > magnet vane mechanism is shown in Fig 17. (the URL below has an
> > illustration close enough to Fig 16).
> > "The energy stored in the magnetic field can be written: E =
> > 1/2*I^2*L Where L is the inductance of the system, and its
> > derivative, the torque T, is: T = dE/d<theta> =
> > 1/2*I^2*(dL/d<theta>)
> > "The system's response is proportional to I-squared.
> > (I'm going to type this very slowly, Steve, so you can be sure and
> > embrace every word)
> > >>>>> "The instrument indicates root mean square (RMS) current or
> voltage. <<<<<<
> Ok, I admit, I'm confused. Explain to me please what any of this has to
> do with the Simpson taut-band (rectified) panel meters you provided a
> link to? Silly me, I assumed this was your recommendation.
If you go to the first URL in my original article, which points to
Simpson's web page, and go to the Wide-Vue meter's data sheet, here:
You'll see that the Wide-Vue format is available in both rectifier
and iron vane types. The instrument I specified, #10220 is an iron
vane, 0-150 volt AC voltmeter. If you look at the top of that page
under "Specifications", you'll note that both types of movements are
jewel-pivot and not taut band. I certainly prefer the taut
band-style movement but I thought it more important to specify an
instrument that can be easily purchased at Graingers from stock.
Graingers web page appears to be down as I write this note so I
can't double-check the URLs I provided to the catalog pages. Since
I double-checked them when I wrote the article, I have confidence in
> Note too the contradiction in adjacent sentences of what you claim to
> have found which states that the response is proportional to the power,
> or I^2, and then to rms - which is the square root of that. Can't have
> both. Get your money back.
I^2*R = power. R is the coil resistance. Power is what does work
to move the pointer. He was simply stating that the pointer
movement is proportional to (real) power. The scale is linearized
via the shape of the iron vanes. Since the RMS value of a complex
waveform IS the real power, the two are equivalent. Moot discussion
unless you plan on making your own movement, since the manufacturers
provide you a black box with two terminals to hook the voltage or
current to and a fairly linear scale. Think I'll keep the book. I
suggest you drop the $150 for a copy too.
I might add that in a rectifier-type meter, the RECTIFIER and not
the meter movement is what causes the instrument to not read the RMS
value of the input. If the polarity of the field magnet could
change in sync with the input AC wave, that type meter would also
read RMS value. And indeed the field magnet can change - in an
electrodynamic (ED) meter movement. The ED movement looks quite
similar to the conventional movement except that the field is a
wound coil that is connected to the input terminals. It responds
equally to DC or the RMS value of AC. Indeed, before the
development of the fancy computing standards, the ED meter was the
usual transfer standard between AC and DC. The standard meter was
calibrated against a DC standard and then used to calibrate AC
instruments. The movement is too large and fragile for use in an
RV, of course, but it is of interest nonetheless. At least to me.