General Description; Measuring Average Values Andtrms Values; Definition Of True Root Mean Square Value And Crest Factor - HT Instruments HT62 Bedienungsanleitung

2. GENERAL DESCRIPTION

The instrument carries out the following measurements:
 DC Voltage
 AC TRMS voltage
 DC Current
 AC TRMS Current
 Resistance and Continuity test
 Diode test
 Capacitance (HT62)
 Current and voltage frequency
 Duty Cycle
 Temperature with K probe (HT62)
Each of these functions can be selected by means of the appropriate switch. The
instrument is also equipped with function keys (see § 4.2), an analogue bargraph and
backlight. The instrument is also equipped with an Auto Power OFF function (which can be
disabled), which automatically switches off the instrument 15 minutes after the last time a
function key was pressed or the rotary switch was turned. To switch on the instrument
again, turn the rotary switch.
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; TRMS 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
1 0

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
HT61-HT62
In a time equal to a period,
"