Brymen BM857 User Manual page 6

5
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.
AC True RMS
AC True RMS, normally refers as True RMS, identifies a DMM function that is AC
coupled, and responds accurately only to the effective RMS AC component value
regardless of the waveforms. However, DC component plays an important role in the
distorted non-symmetrical waveforms, and will also be of interest sometimes. A full
wave rectified sine waveform is a good example, and the AC true RMS function will
only give the AC component reading which is at 43.6% of the total effective DC+AC
RMS reading.
DC+AC True RMS
DC+AC True RMS calculates both of the AC and DC components given by the
expression when making measurement, and can responds
accurately to the total effective RMS value regardless of the waveform. Distorted
waveforms with the presence of DC components and 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
AC Bandwidth
AC bandwidth of a DMM is the range of frequencies over which AC measurements can
be made within the specified accuracy. It is not the frequency measurement function,
and is the frequency response of the AC functions. A DMM cannot accurately measure
the AC value with frequency spectrums beyond the AC bandwidth of the DMM.
Therefore, wide AC bandwidth plays an important role in high performance DMM. In
reality, complex waveforms, noise and distorted waveforms contain much higher
frequency spectrum than its fundamental.