SEFRAM 7314 Notice D'utilisation page 47

Average sensing RMS calibrated technical :
Most digital multimeters use an average sensing RMS calibrated
technique of an electrical average circuit to measure RMS value
of AC signals. This technique is scaling the output average
value of the average sensing circuit by the ratio of RMS value
to average value, for sine wave the ratio is 1.11. Unfortunately,
this ratio varies widely as a function of the wave form, it
will give grossly incorrect results in many cases. The table
shows a few representative examples comparing RMS to
average.
Waveform Crest Fac- True rms Average
Type 1 Volt tor (V / Value Responding
PEAK
Peak Ampli- V rms) Circuit Cali-
tude brated rms
Value of Sine
Waves Will
Read
V
Peak 1.414 0.707V 0.707V
V
Peak
1.00 1.00V 1.11V
V
Peak
1.73 0.577V 0.555V
Rectangular 2 0.5V 0.278V
Pulse Train 10 0.1V 0.011V
* % of Reading Error =
Average Responding Value - True rms Value
True rms Value
M731400M00
% of Read-
ing Error*
Using
Average
Respond-
ing Circuit
0%
+11.0%
-3.8%
-44%
-89%
x 100%
28