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3.2 Glossary of Terms for Digital Multimeters
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 (that is,
calibrated) to read the RMS value of a sine wave. In measuring pure
sinusoidal waveform, this technique is fast, accurate, and cost effective.
However, in measuring non-sinusoidal waveforms, 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 accurately responds
to the effective RMS value regardless of the waveform shapes such as
square, sawtooth, triangle, pulse trains, spikes, and transient glitches
as well as distorted waveforms with the presence of harmonics.
Non-sinusoidal waveforms may cause:
– Overheated tansformers, generator and motors to burn out faster
than normal
– Circuit breakers to trip prematurely
– Fuses to blow
– Neutrals to be overheated due to the triplen harmonics present
on the neutral
– 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, which is commonly used to define the dynamic range
of a True RMS DMM.
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