KYORITSU 2009R - Manual (Page 8)

̶ 6 ̶
*Effective Value (RMS)
Most alternating currents and voltages are expressed in effective
values, which are also referred to as RMS (Root-Mean-Square)
values.
The effective value is the square root of the average of square of
alternating current or voltage values.
Many clamp meters using a conventional rectifying circuit have
"RMS" scales for AC measurement. The scales are, however, actually
calibrated in terms of the effective value of a sine wave though the
clamp meter is responding to the average value. The calibration
is done with a conversion factor of 1.111 for sine wave, which is
found by dividing the effective value by the average value. These
instruments are therefore in error if the input voltage or current has
some other shape than sine wave.
*CF (Crest Factor) is found by dividing the peak value by the
effective value.
Examples:
DC: CF =1
Sine wave: CF=1.414
Square wave with a 1: 10 duty ratio: CF=3
Waveform
Crest factor
CF
Average value
Vavg
Effective value
Vrms
Conversion
factor
Vrms/ Vavg
Reading errors for
average sensing
instrument
≒0.637 ≒1.111
≒1.414
≒1.155
≒1.732
＝-3.8％
×100％
＝11.1％
≒0.707
2
1
2 2
π
A
3
D
1
3
2
D
1
3
A
0.5A×1.111-
3
A
A
A A
A
1
0.5A
1
0％
2
A π
＝
AD
f
A
＝A・D
T
2
A
A×1,111-A Ч100
Ч100
D A
D
1
＝ A
D A
（1.111 -1）
D
A
0
A
0
A
0
A
0
T
D=f/T
f
3