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AC Measurement
AC measurements are usually displayed as RMS (root mean square) values. The RMS value is
equal to the value of a DC waveform, which would deliver the same power if it replaced the time-
varying waveform. Two AC measurement methods are average-responding RMS calibrated and
true RMS-reading.
The average-responding RMS calibrated method takes the average value of the input signal after
full wave rectification, multiplies it by 1.11, and displays the result. This method is accurate if the
input signal is a pure sine wave. The Greenlee CM-1500 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, square wave, triangle wave, half wave, or signal with harmonics. The ability to read true
RMS provides much more measurement versatility. The Greenlee CM-1550 is a true RMS meter.
The Waveforms and Crest Factors table shows some typical AC signals and their RMS values.
Waveforms and Crest Factors
Waveform
RMS Value
Average 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.
8
100
100
90
100
1.414
100
87
1
1.73
100
64
2
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