Amprobe AM-120 TRMS User Manual page 5

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Analog bar-graph
The analog bar graph provides a visual indication of measurement like a traditional
analog meter needle. It is excellent in detecting faulty contacts, identifying
potentiometer clicks, and indicating signal spikes during adjustments.
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.