8
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 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-450 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
100
100
100
100
Average
Value
90
100
87
64
Crest Factor*
(
X
)
1.414
1
1.73
2
*
The c
rest factor is the ratio of the peak value to the RMS value; it is
represented by the Greek letter
X
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