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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 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, half wave or signal with harmonics. The ability to read true
RMS provides much more measurement versatility. The Greenlee CMI-200 is a true RMS meter.
The Waveforms and Crest Factors table shows some typical AC signals and their RMS values.
Waveforms and Crest Factors
W
v a
f e
r o
m
R
M
S
V
l a
e u
1
0 0
R
e
t c
i f i
d e
V
l a
e u
0 9
C
e r
t s
F
c a
o t
* r
1
4 .
4 1
(ξ)
* The crest factor is the ratio of the peak value to the RMS value; it is represented by the
Greek letter ξ.
8
1
0 0
1
0 0
1
0 0
7 8
1
. 1
3 7
1
0 0
4 6
2
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