4
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.
4) OPERATION
CAUTION
Before and after hazardous voltage measurements, test the voltage function on a known
source such as line voltage to determine proper meter functioning.
AutoCheck
TM
mode (
TBM525 & TBM829 only
)
This innovative
AutoCheck
TM
feature automatically selects measurement function of DCV,
ACV or Resistance (
) based on the input via the test leads.
●With no input, the meter displays “Auto” when it is ready.
●With no voltage signal but a resistance below 60M
is present, the meter displays the
resistance value. When below “Audible Threshold“ is present, the meter further gives a
continuity beep tone.
●When a signal above the threshold of 1.5V DC or 3V AC up to the rated 1000V is present,
the meter displays the voltage value in appropriate DC or AC, whichever larger in peak
