Manual Ranging Multimeter  (220-0181A)    True RMS      Faxback Doc. # 7173

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 - 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 the meter can accurately measure is 1000 Hz for AC
voltage and current.  The allowable crest factor is 1:1 to 3:1.  The crest
factor is the ratio of the peak signal value divided by the RMS value.  A
10% duty cycle square wave with a peak voltage of 3 volts has an RMS value
of about .95 volts.  This gives a crest factor of 3.16 (3V/ .95V), and is
outside this meter's range of accuracy.

(br/all-7/18/94)