Measurement Techniques And Considerations; True Rms Vs Average-Responding Meters; Measuring Voltage, Ac/Dc - Fluke 27/FM Operator's Manual

True RMS and Measuring Voltage
MEASUREMENT TECHNIQUES AND CONSIDERATIONS
The following measurement techniques and considerations will assist you in
using your meter safely and effectively.
True RMS vs Average-Responding Meters
One of the most useful features of the Fluke 27/FM is the direct measurement
of true rms or effective ac voltages and ac currents. Mathematically, rms is
defined as the square root of the sum of the squares of the ac and dc
components. In physical terms, rms is equivalent to the dc value that
dissipates the same amount of heat in a resistor as the original waveform.
The reason that rms is valuable is that it greatly simplifies the analysis
of complex ac signals. Since rms is the dc equivalent to the original waveform,
it can be used in the relationships derived from Ohm's law (E = I x R), and
it provides a reliable basis for comparing dissimilar waveforms.
Most meters today have average-responding ac converters rather than true rms
ac converters. Usually the gain in average-responding meters is adjusted so
that the reading gives the rms value, provided the input signal is a
harmonic-free sinusoid. However, if the signal is not sinusoidal, the
average-responding meter does not give correct rms readings.
The Fluke 27/FM actually calculates the rms value through analog computation.
This means that readings are accurate rms values not only for harmonic-free
sinusoids, but also for mixed frequencies, modulated signals, square waves,
sawtooths, 10%-duty-cycle rectangular pulses, etc.
Figure
3 shows some common waveforms and a comparison of the readings
displayed by your true rms meter and average-responding meters. Figure
3
also illustrates the relationship between ac and dc measurements for
ac-coupled meters. For example, consider the first waveform, a 1.414V (0-pk)
sine wave. Both true rms and the rms-calibrated average-respondings meters
display the correct rms reading of 1.000V (the dc component equals 0).
However, consider the 1.414V (0-pk) rectified square wave. Both types of
meters correctly measure the de component (0.707V). Only the true rms meter
correctly measures the ac component (0.707V). The average-responding meter
measures 0.785V, which amounts to a 5.6% error in the total rms measurement
calculated from the ac and dc components. Since average-responding meters
have been in use for so long, you may have accumulated test or reference
data based on them. The conversion factors in Figure
3
should help you
convert between the two measurement methods.
Measuring Voltage, AC/DC
The multimeter features five ac voltage ranges and five dc voltage ranges.
All ranges present an input impedance of approximately 10 megohms in
parallel with less than 100 pF. Measurement errors, due to circuit loading,
can result when making either ac or dc voltage measurements on circuits with
high source resistance. However, in most cases the error is negligible (0.1%
or less) if the measurement circuit source resistance is 10 kilohms or less.
If circuit loading does present a problem, the percentage of error can be
calculated using the appropriate formula from Figure 4.