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BM869s 36
_v61010-2-033:2019
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.
AC True RMS
AC True RMS, normally refers as True RMS, identifies a DMM function that is AC coupled,
and responds accurately only to the effective RMS AC component value regardless of the
waveforms. However, DC component plays an important role in the distorted non-
symmetrical waveforms, and will also be of interest sometimes.
DC+AC True RMS
DC+AC True RMS calculates both of the AC and DC components given by the expression
when making measurements, and can responds accurately to the
total effective RMS value regardless of the waveform. Distorted waveforms with the
presence of DC components and 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 triple harmonics present on the neutral
5) Bus bars and electrical panels to vibrate
AC-Bandwidth
AC-bandwidth of a DMM is the range of frequencies over which AC measurements can be
made within the specified accuracy. It is not the frequency measurement function but is the
frequency response of the AC functions. A DMM cannot accurately measure the AC value
with frequency spectrums fall beyond the AC-bandwidth of the DMM. Therefore, wide AC-
bandwidth plays an important role in high performance DMMs. In reality, complex
waveforms, noise and distorted waveforms contain much higher frequency spectrum than
its fundamental.
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.