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From: Paul E. Schoen on 11 Apr 2010 20:50

"Phil Allison" <phil_a(a)tpg.com.au> wrote in message
news:82cqm0FjbgU1(a)mid.individual.net...
>
> "The Phantom"
>
>
>> You can also get moving iron voltmeters which will read true RMS.
>
> ** Moving iron meters always read "true RMS" - within the limits of their
> frequency responses.
>
> The force on the pointer is proportional to the average of the *square* of
> the applied current ( hence it automatically rectifies AC) - and the
> scale on the face is somewhat non linear as it indicates actual current.

I have used and calibrated iron vane meters, and I don't remember them as
true RMS for all waveforms. Particularly, they have a slightly different
response to a DC component, which is part of the true-RMS value (although
some meters have an option of RMS and AC+DC RMS). Here is some information
about older RMS metering devices:
http://ca.fluke.com/Fluke/usen/Community/Fluke+Plus/ArticleCategories/DMMs/True-rms+Facts.htm

The other types mentioned, electrodynamic and thermocouple, AFAIK are true
RMS with any waveform and DC component. The thermocouple type is especially
good for high frequency into the RF range.

Here is more information that may be helpful for the OP:
http://www.allaboutcircuits.com/vol_2/chpt_12/1.html
http://www.allaboutcircuits.com/vol_2/chpt_1/3.html

It also mentions the electrostatic voltmeter, which also responds to AC as
well as DC. But they are very specialized, expensive, and usually for very
high voltages.

I found other references that state true-RMS performance of iron-vane
instruments, and perhaps they do respond accurately, although they are
definitely limited in frequency response to power-line and low audio
signals. Without actually performing tests I will accept that iron vane
types are at least better than rectifier types for RMS readings. Here is a
discussion with much detail that seems to prove your assertion:
http://yarchive.net/electr/analog_voltmeters.html

For the OP's purposes of voltage measurement, a silicon or germanium diode
bridge will work well, and can be used with most commonly available meters.
AC current is best measured with a CT, and there are some small inexpensive
ones that will handle currents of 5 to 30 amps with a ratio of about 500:1
such as this:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=237-1103-ND

The output can be rectified and provides a current of 10 mA for 5 A with
enough voltage to produce a linear output through a silicon diode bridge.

Paul

From: phantom on 11 Apr 2010 23:33
On Sun, 11 Apr 2010 10:52:33 -0700, Fred Abse
<excretatauris(a)invalid.invalid> wrote:

>On Sun, 11 Apr 2010 05:05:01 -0500, phantom wrote:
>
>> On Sun, 11 Apr 2010 12:32:23 +1000, "Phil Allison" <phil_a(a)tpg.com.au>
>> wrote:
>>
>>>
>>>"The Phantom"
>>>
>>>
>>>> You can also get moving iron voltmeters which will read true RMS.
>>>
>>>** Moving iron meters always read "true RMS" - within the limits of their
>>>frequency responses.
>>
>> Why are you repeating what I already told the OP? I already explained
>> to him that moving iron voltmeters read true RMS.
>
>No, you said "You can also get moving iron voltmeters which will read true

So I did, and it is quite true. In addition to moving iron ammeters,
you can ALSO get moving iron voltmeters, and they do indeed read true
RMS. I never said anything to the effect that only some moving iron
voltmeters read true RMS.

I first described moving iron ammeters which read true RMS, explained
about scale problems to watch out for, and then I told him that (as
well as ammeters), you can ALSO get moving iron voltmeters (the)
which read true RMS.

If you and Phil think that I was saying that only some moving iron
voltmeters read true RMS, that is your mistake, not mine.

>RMS."
>
>"also"
>
>>
>>
>>>The force on the pointer is proportional to the average of the *square*
>>>of the applied current ( hence it automatically rectifies AC) - and
>>>the scale on the face is somewhat non linear as it indicates actual
>>>current.
>
>
>Phil is correct.
From: Phil Allison on 11 Apr 2010 23:55

<phantom(a)aol.com>


> I first described moving iron ammeters which read true RMS,


** You never said a thing about MI ammeters being " true RMS " and the
matter was not raised before.

The wording YOU used was misleading and wrong !!

That you now try to re-word the whole thing proves the point.

My post stated the case clearly and gave an explanation too.


Your whining about it is PATHETIC !!



..... Phil







From: phantom on 12 Apr 2010 00:02
On Sun, 11 Apr 2010 22:18:47 +1000, "Phil Allison" <phil_a(a)tpg.com.au>
wrote:

>
><phantom(a)aol.com>
>>
>>>
>>>> You can also get moving iron voltmeters which will read true RMS.
>>>
>>
>>>** Moving iron meters always read "true RMS" - within the limits of their
>>>frequency responses.
>>
>> Why are you repeating what I already told the OP? I already explained
>> to him that moving iron voltmeters read true RMS.
>
>
>** Fraid you told the world that only * SOME* of them did.
>
I said "You can also get moving iron voltmeters which will read true
RMS."

I could have said "You can also get moving iron voltmeters (some of)
which will read true RMS."

Or, I could have said "You can also get moving iron voltmeters (a few
of) which will read true RMS."

Or, I could have said "You can also get moving iron voltmeters (most
of) which will read true RMS."

Or, I could have said "You can also get moving iron voltmeters (all
of) which will read true RMS."

I didn't bother to use the last locution because the convention in
English is to take a missing quantifier to be the universal quantifier
"all". If you want to take it to be something else, you certainly
may, but don't attribute an error to me because of your choice.
From: Mike Cook on 12 Apr 2010 03:03
> For the OP's purposes of voltage measurement, a silicon or germanium diode
> bridge will work well, and can be used with most commonly available meters.
> AC current is best measured with a CT, and there are some small inexpensive
> ones that will handle currents of 5 to 30 amps with a ratio of about 500:1
> such as this:
> http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=237-1103-ND
>
> The output can be rectified and provides a current of 10 mA for 5 A with
> enough voltage to produce a linear output through a silicon diode bridge.
>
> Paul

Excellent. Thank you Paul for the CT explanation. That looks like my solution
for ampere measurement.

Cheers.

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