10
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 wave-
form. 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 DM-200A and DM-210A are average-responding meters.
The true RMS-reading method uses internal circuitry to read the true RMS value. This method is accu-
rate, 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 DM-510A 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 crest factor is the ratio of the peak value to the RMS value; it is represented by the Greek
letter x.
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