Greenlee CMT-90, Instruction Manual, Page: 7 / 44

7
CMT-90
* The crest factor is the ratio of the peak value to the RMS value; it is
represented by the Greek letter
ξ
.
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 wave,
half wave or signal with harmonics. The ability to read true RMS provides
much more measurement versatility. The Greenlee CMT-90 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
Rectified Value 90 100 87 64
Crest Factor*
1.414 1 1.73 2
(
ξ
)