UEi Test Instruments DM393 - Manual (Page 10)

G l o s s a ry of Te r m s
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 ten scaled upward (that is, calibrated) to read the RMS
value of a sine wave. in measuring pure sinusoidal waveform. This
technique is fast, accurate, and cost effective. However, in measuring
non-sinusoidal waveforms, 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 accurately
responds to the effective RMS value regardless of the waveform shapes
such as square, sawtooth, triangle, pulse trains, spikes, and transient
glitches as well as distorted waveforms with the presence of harmonics.
Non-sinusoidal waveforms may cause:
• Overheated transformers, generator and motors to burn out
faster than normal
• Circuit breakers to trip prematurely
• Fuses to blow
• Neutrals to be overheated due to the triplen harmonics present
on the neutral
• Bus bars and electrical panels to vibrate
Crest Factor: Is the ratio of the Crest (instantaneous peak) value to the
True RMS value, which is commonly used to define the dynamic range
of a True DMM. A pure sinusoidal waveform has a Crest Factor of 1.414.
A badly distorted sinusoidal waveform normally has a much higher
Crest Factor.
NMRR: (Normal Mode Rejection Ratio) is the DMM’s ability to
reject unwanted AC noise effect which can cause inaccurate DC
measurements. NMRR is typically specified in terms of db (decibel). The
meter has a NMRR specification of > 60db at 50Hz/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 existing on both the COM and Voltage input terminals of a
DMM, with respect to ground. CMRR is a DMM’s ability to reject
common mode voltage effect which can cause digit rattle or offset in
voltage measurements. The meter has a CMRR specification of > 60db
at DC to 60Hz in AC volts measurement function and > 120db at DC,
50Hz and 60Hz in DC volts measurement function.
Burden Voltage: Burden voltage is a voltage drop across the input
terminals of a current measuring device, caused by internal shunt
resistance. Burden voltage contributes measurement error, and should
be as low as practical.
Temperature Coefficient: Is a factor used to calculate the change in
indication or output of an instrument with changes in temperature.
Uncompensated changes in temperature contribute uncertainty by an
amount determined by the temperature coefficient to instrument.
DM391/393-MAN P. 9