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4.7 True RMS measurements / Crest factor
(only Clamp Meter CHB 35)
True RMS measuremets
Commonly available clamp meters give an average value, and are
therefore only suitable for measuring DC, and pure sine-wave AC
currents and voltages.
If the AC current or voltage is not a true sine-wave, for example
due to distortion from a phase controller, then true RMS measure-
ments must be made – with the UNITEST Clamp Meter CHB 35,
they can be.
Table 1, below, shows a comparison of measurements made with
the UNITEST Clamp Meter CHB 35, and those made with conven-
tional clamp meters that have average and peak value functions:
Crest factor:
The crest factor is the ratio of the peak value to the RMS value of
a signal.
This is important for RMS measurements (UNITEST Clamp Meter
CHB 35). Although the measured value may lie within the chosen
range, if the peak value of the signal is too high, the UNITEST
Clamp Meter CHB 35 may be overmodulated and give false rea-
dings.
4.8 MIN/MAX and peak values
The MIN/MAX button can be used to find either the largest or the
smallest value of a series of measurements. Pressing the MIN/MAX
button activates first the MIN mode, so that the smallest value is
selected. Pressing it a second time changes to MAX mode, for the
largest value. Pressing the button a third time returns the instrument
to normal operation.
Wave form
RMS value
Rectified value
Display UNITEST
Clamp Meter
CHB 15
Display UNITEST
Clamp Meter
CHB 35
Deviation
Deviation
PTDB93468-05.qxd 14.12.2005 9:41 Uhr Seite 26
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