8
Waveform
RMS Value
100
100
100
100
Average Value
90
100
87
64
Crest Factor*
1.414
1
1.73
2
(
ξ
)
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-700 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-750 is a true RMS meter.
The Waveforms and Crest Factors table shows some typical AC signals and their RMS values.
Waveforms and Crest Factors
* The crest factor is the ratio of the peak value to the RMS value; it is represented by the
Greek letter
ξ
.
