Manual Elma BM 821S (page 42 of 64) (English, Danish, Swedish, Norwegian)

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Elma BM 82Xs &

BM52Xs

Page 42

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.

NMRR (Normal Mode Rejection Ratio)

NMRR is the DMM's ability to reject unwanted AC noise effect that can cause inaccurate

DC measurements. NMRR is typically specified in terms of dB (decibel). This series has a

NMRR specification of >60dB at 50 and 60Hz, which means a good ability to reject the

effect of AC noise in DC measurements.

CMRR (Common Mode Rejection Ratio)

Common mode voltage is voltage present on both the COM and VOLTAGE input terminals

of a DMM, with respect to ground. CMRR is the DMM's ability to reject common mode

voltage effect that can cause digit rolling or offset in voltage measurements. This series

has a CMRR specifications of >60dB at DC to 60Hz in ACV function; and >120dB at DC,

50 and 60Hz in DCV function. If neither NMRR nor CMRR specification is specified, a

DMM's performance will be uncertain.

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Elma BM 821S Manual

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