HT instruments HT8100 - Manual (Page 5)

HT8100
EN - 4
2. GENERAL DESCRIPTION
HT8100 carries out the following measurements:
 DC and AC+DC TRMS voltage
 DC and AC+DC TRMS current
 Resistance and Continuity test
 AC voltage and current frequency
 Diode
test
 Current generation with amplitude up to 24mA DC with display in mA and %
 Generation with selectable ramp outputs
 Output current measurement from transducers (Loop), HART 250 resistor included
 Simulation of an external transducer
Each of these functions can be selected using the 8-position rotary switch, including an
OFF position. The instrument is also equipped with function keys (see § 4.2) and an
analogue graphic bar. The selected quantity appears on the LCD display with the
indication of the measuring unit and of the enabled functions.
The instrument is also equipped with an automatic display backlighting function
(Autobacklight) and with an Auto Power OFF function which automatically switches off the
instrument approx. 20 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 AND TRMS 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 instrument’s 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: “In a time equal to a period,
an alternating current with a root mean square value with an intensity of 1A, circulating on
a resistor, dissipates the same energy that, during the same time, would have been
dissipated by a direct current with an intensity of 1A”. This definition results in the numeric
expression:
G=

T
t
t
dt t g
T
0
0
) (
1
2 The root mean square value is indicated with the acronym RMS.
The Crest Factor is defined as the relationship between the Peak Value of a signal and its
RMS value: CF (G)=
RMS
p
G
G
This value changes with the signal waveform, for a purely
sinusoidal wave it is 2 =1.41. In case of distortion, the Crest Factor takes higher values
as wave distortion increases.