Elma BM 82X & BM52X
Page 64
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.
