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Re: Problems with "true" RMS Readings
--- Tesla list <tesla-at-pupman-dot-com> wrote:
> Original poster: Terry Fritz <twftesla-at-uswest-dot-net>
>
> Hi Harvey,
>
> Both my digital scope (60MHz) and my fancy HP
> 34401a (200kHz at 6 digits
> (I paid more than you did ;-)) do true RMS
> measurement to a bazaar level.
> It takes a while to get used to those readings
> compared to the old 'peak
> level times 0.707' meters. All these meters use one
> of the popular true
> RMS chips in them which are all very good. It does
> not sound like your
> meter is bad as long as it is measure something
> within an order of magnitude.
>
> ""True RMS meters read the equivalent DC voltage
> that would heat a
> resistor up as much as the odd waveform you are
> measuring.""
>
> The old meters will just read the peak value and
> multiply it by 0.707
> assuming that it is a sine wave, but they could be
> very far off while the
> true RMS device will be very accurate.
> Unfortunately, I must tell you that
> you should probably take all your old meters back
> and keep the new one ;-)
> True RMS meters are a wonder that modern electronics
> has brought us normal
> folks. My scope and HP meter cost about a $1000
> each, but 10 years ago
> they would have cost 10X that. For most things, the
> true RMS value is what
> we want. The old meters are just all they could do
> with a diode and a
> resistor... Since many things use sine waves that
> was usually enough.
> Doing the true real-time Root Mean Squared
> calculation requires some pretty
> heavy duty A/D converter and fast math power.
>
> I would be typing all night explaining all the
> ins-and-outs of RMS
> voltages and currents but rest assured that the
> meter is doing it's job
> well and you will find it a valuable tool.
> Oscilloscopes that have RMS
> readouts really make things obvious since you can
> see the messy signal and
> directly associated it with the RMS value.
I'll take your word for now, but I still dont see how
the discrepancies can be accounted for. In the large
amperage case we have what might be suspected: a
reading below the ordinary rms reading, actually
indicating less of the quantity of full sine wave
present. I have also used sensitive AC analogue meters
that indicate an amperage consumption of the 56/59
henry coils placed into the reverse condition made
when the circuit is connected between the series
resonant midpoints whereupon it then consumes q
squared less measured amperage. As I have indicated
that amperage is normally measured at .25 ma. No
mathematical argument can then turn what is supposed
to be a smaller resultant quantity, or realistic
percentage on true rms value to be smaller: which in
the smaller amperage case instead reveals itself
instead as a much larger comparative amperage shown
on the Quote "true" rms: instead several hundred
percent greater than the known quantities! So first
you have a smaller"true" value and then later on as a
different reading as a greater ratio "true" value/ how
can this be?
It is what it is: a piece of sheisse for making small
amperage mesurements, much better made by the former
pieces.
In Stubborn Attitude HDN
> If you are really worried I could test the meter
> for you but I think it is
> fine. Perhaps others could suggest a good book or
> web reference about RMS
> measurements that would make all this more clear?
>
> Cheers,
>
> Terry
>
>
> At 05:50 PM 11/1/2000 -0800, you wrote:
> >I recently purchased Radio Shack's Auto Ranging LCD
> >Digital Meter. This meter additionally has the
> quality
> >of reading the "true rms". All normal digital
> meters
> >have this rms reading also, and I have used Radio
> >Shack meters for years with no problems. But this
> one
> >is giving me a real headache Perhaps it is
> defective
> >but for now I can only ask for a replacement, but I
> >have a feeling that it will do the same thing.
> First
> >let me refer to the manuals explanation;
> >
> >WHAT DOES TRUE RMS MEAN
> >RMS stands for Root-Mean-Square. RMS is a standard
> >method for determining the effective value of a
> >varying or alternating voltage. The RMS value is
> the
> >DC voltage that would produce the same amount of
> heat
> >as the voltage you are measuring, if you were to
> apply
> >both voltages across identical resistors.
> >
> >The true RMS value is helpful when you need to
> measure
> >an AC voltage that is not a pure sine wave. Most
> >meters use average-responding AC converters. These
> >meters are calibrated with pure sine waves. If you
> >measure a sine wave that does not have harmonic
> >distortion, the meter gives an accurate reading.
> But
> >if the sine wave is not pure- there are distortions
> >within the waveform, or you measure triangle waves
> or
> >square waves- the average responding meters display
> an
> >incorrect value.
> >
> >For example a 2 volt peak to peak, 50% duty cycle
> >square wave measures correctly as 1.000 VAC on this
> >meter. An average responding meter displays 1.111
> VAC-
> >an error of more than 11 percent!
> >
> >The maximum frequency your meter can accurately
> >measure is 5000 Hz for AC voltage and 1000 hz for
> AC
> >current.
> >
> >Now these are all good things to know since I am
> >taking measurements from a three phase AC converted
> >car alternator at what the meter says is 188 hz.
> >Since these amperage figures were derived from
> >resonant circuits giving a lot of figures that
> didnt
> >jive I decided to compare the old meter "rms "
> >readings from the old 60 hz resonant amperage
> >conductions of the high voltage BRS using 56/59
> Henry
> >coils. I found great discrepancies not easily
> >explained by chop/chop analogies.
> >So I went back to measure the amperage input at
> short
> >in the high voltage wall driven BRS. This is been
> >measured numerous times by the old Radio Shack
> digital
> >meters at between .25 ma and .3 ma. However the new
> >"supposed true rms" reading from the new meter
> gives
> >1.15 ma! If Radio Shack is worried about a 11%
> error,
> >I am more surely worried about this >300% error and
> >will use more of those!!!!
> >
> >In short the meter seems very useless for small
> >amperage measurements. As I have complained to the
> >store where I purchased this the following;
> >
> >{I did the following tests by measuring the
> amperage
> >input at 4 times the former values by inputing 4
> times
> >more the voltage using a 440 transformer input to
> the
> >shorted high voltage
> >BRS}
> >
> >Taking a known circuit that consumes 1.12 ma by the
> >former meters -at- 440 volts the new meter only
> measures
> >.86 ma when the ampearge selection is at the 400 ma
> >range. Selecting the range down to the proper
> setting
> >of 4 ma then shows a reading of only .142 ma, a
> value
> >only 12.6 % the value the former Radio Shack
> "Normal
> >and not true" rms reading gives. In short the
> readings
> >the two ranges themselves give do not even resemble
> >each other!!! Getting more intrigued I decided to
> >measure the maximum wall amperage the high voltage
> BRS
> >gives at 10,000+ volts open condition using a 440
> >transformer, the normal meter reads .65 Amps but
> the
> >new quote "true" rms reads .528 A, so this is all
> BS,
> >those values cannot possibly be true. These are
> some
> >problems along the way and I certainly think Radio
> >Shack has some serious engineering problems with
> their
> >product, and unless someone could explain what is
> >wrong here I am sending them this letter.
> >Sincerely Unsatisfied; HDN
> >
> >
> >
> >
> >
> >=====
> >Binary Resonant
> Systemhttp://www.insidetheweb-dot-com/mbs.cgi/mb124201
> >
> >__________________________________________________
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> >
>
>
=== message truncated ===
=====
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