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Problems with "true" RMS Readings
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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