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Average sensing RMS calibrated
RMS (Root-Mean-Square) is the term used to describe the effective or equivalent DC value
of an AC signal. Most digital multimeters use average sensing RMS calibrated technique to
measure RMS values of AC signals. This technique is to obtain the average value by
rectifying and filtering the AC signal. The average value is then scaled upward (calibrated) to
read the RMS value of a sine wave. In measuring pure sinusoidal waveform, this technique
is fast, accurate and cost effective. In measuring non-sinusoidal waveforms, however,
significant errors can be introduced because of different scaling factors relating average to
RMS values.
True RMS
True RMS is a term which identifies a DMM that responds accurately to the effective RMS
value regardless of the waveforms such as: square, sawtooth, triangle, pulse trains, spikes,
as well as distorted waveforms with the presence of harmonics. Harmonics may cause :
1) Overheated transformers, generators and motors to burn out faster than normal
2) Circuit breakers to trip prematurely
3) Fuses to blow
4) Neutrals to overheat due to the triplen harmonics present on the neutral
5) Bus bars and electrical panels to vibrate
Crest Factor
Crest Factor is the ratio of the Crest (instantaneous peak) value to the True RMS value, and
is commonly used to define the dynamic range of a True RMS DMM. A pure sinusoidal
waveform has a Crest Factor of 1.4. A badly distorted sinusoidal waveform normally has a
much higher Crest Factor.
