Manual Elma BM 869 (page 38 of 60) (English, Danish, Swedish, Norwegian)

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Elma BM869 side 38

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 triplen 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.

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Elma BM 869 Manual

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