Greenlee DM-300 Manuel D'instructions page 10

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AC Measurement
AC measurements are usually displayed as RMS (root mean squared)
values. Two methods of AC measurement are average-responding RMS
calibrated and true RMS-reading.
The average-responding RMS calibrated method takes the average value of
the input signal, multiplies it by 1.11, and displays the result. This method
is accurate if the input signal is a pure sine wave. The Greenlee DM-310 is
an average-responding meter.
The true RMS-reading method uses internal circuitry to read the true RMS
value. This method is accurate, within the specified crest factor limitations,
whether the input signal is a pure sine wave, a square wave, sawtooth
wave, half wave or signal with harmonics. The ability to read true RMS
provides much more measurement versatility. The Greenlee DM-300,
DM-330, and DM-350 are true RMS meters.
The Waveforms and Crest Factors table shows some typical AC signals and
their RMS values.
Waveforms and Crest Factors
Waveform
RMS Value
Rectified Value
Crest Factor*
(ξ)
* The crest factor is the ratio of the peak value to the RMS value; it is
represented by the Greek letter ξ.
10
100 100
90 100
1.414 1
100 100
87 64
1.73 2