General Description; Measuring Average Values Andtrms Values; Definition Of True Root Mean Square Value And Crest Factor - HT HT64 Manuel D'utilisation

2. GENERAL DESCRIPTION

The instrument carries out the following measurements:
 DC voltage
 AC, AC+DC TRMS Voltage
 DC current
 4-20mA% display
 AC, AC+DC TRMS current
 Resistance and Continuity test
 Diode test
 Capacity
 Frequency
 Duty Cycle
 Temperature with K-type probe
 Data Logger function and display of graphs of measured data
Each of these functions can be selected by means of the appropriate switch. The
instrument is also equipped with function keys (see § 4.2), analogue bargraph and LCD
TFT high-contrast colour display. The instrument is also equipped with an Auto Power OFF
function which automatically switches off the instrument after a certain (programmable)
idling time.
2.1.

MEASURING AVERAGE VALUES ANDTRMS VALUES

Measuring instruments of alternating quantities are divided into two big families:
 AVERAGE-VALUE meters: instruments measuring the value of the sole wave at
fundamental frequency (50 or 60 Hz).
 TRMS (True Root Mean Square) VALUE meters: instruments measuring the TRMS
value of the quantity being tested.
With a perfectly sinusoidal wave, the two families of instruments provide identical results.
With distorted waves, instead, the readings shall differ. Average-value meters provide the
RMS value of the sole fundamental wave; TRSM meters, instead, provide the RMS value
of the whole wave, including harmonics (within the instruments bandwidth). Therefore, by
measuring the same quantity with instruments from both families, the values obtained are
identical only if the wave is perfectly sinusoidal. In case it is distorted, TRMS meters shall
provide higher values than the values read by average-value meters.
2.2.

DEFINITION OF TRUE ROOT MEAN SQUARE VALUE AND CREST FACTOR

The root mean square value of current is defined as follows:
an alternating current with a root mean square value of 1A intensity, circulating on a
resistor, dissipates the same energy that, during the same time, would be dissipated by a
direct current with an intensity of 1A". This definition results in the numeric expression:
T
t
0
1

2
G= g ) ( t dt The root mean square value is indicated with the acronym RMS.
T
t
0
The Crest Factor is defined as the relationship between the Peak Value of a signal and its
RMS value: CF (G)=
sinusoidal wave it is
as wave distortion increases.
G
p
This value changes with the signal waveform, for a purely
G
RMS
2 =1.41. In case of distortion, the Crest Factor takes higher values
EN - 4
In a time equal to a period,
"
HT64