Page "Multimeter" Paragraph 66

Since the basic indicator system in either an analog or digital meter responds to DC only, a multimeter includes an AC to DC conversion circuit for making alternating current measurements.

Basic meters utilize a rectifier circuit to measure the average or peak absolute value of the voltage, but are calibrated to show the calculated root mean square ( RMS ) value for a sinusoidal waveform ; this will give correct readings for alternating current as used in power distribution.

User guides for some such meters give correction factors for some simple non-sinusoidal waveforms, to allow the correct root mean square ( RMS ) equivalent value to be calculated.

More expensive multimeters include an AC to DC converter that measures the true RMS value of the waveform within certain limits ; the user manual for the meter may indicate the limits of the crest factor and frequency for which the meter calibration is valid.

RMS sensing is necessary for measurements on non-sinusoidal periodic waveforms, such as found in audio signals and variable-frequency drives.