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A C m e a s u r e m e n t s
6.6
Crest factor
The crest factor is derived by dividing the peak value by the rms
value of a signal. It is very important for the correct measure-
ment of pulse signals and a vital specification of a measuring
instrument.
û
C = — — = —————— —— ——
V
rms
For sinusoidal signals the crest factor is
√2 = 1.414
If the maximum permissible crest factor of a mea-
suring instrument is exceeded, the result will be
inaccurate because the measuring instrument will
be overdriven.
The accuracy of the rms calculation depends on the crest factor,
it deteriorates with increasing crest factor. The specification of
the maximum crest factor relates to the full scale value (see
specifications). If a range is not used up to full scale, the crest
factor may be higher (e.g. 230V measured in the 500V range.)
See figure form factors.
6.7
DC and AC currents
Current measurements are performed in the HM8112-3 by
using precision shunts. The voltage drop across the shunt is
measured. Due to the resistance of conductors and cables R
a total load voltage V
accrues which may lead to false measu-
B
rement results.
R
s
V
s
Fig.: Principle of the current measurement using shunts
V
= Source voltage
S
V
= Burden voltage
B
R
= Resistance of conductors and cables
L
The measurement error in % follows from:
100 x V
B
Error (%) =
— —— ——–
V
S
48
Subject to change without notice
peak value
rms value
DMM
R
L
R
V
R
= resistance of the source
S
R
= Shunt inside the multimeter
Form factors
7 Temperature measurement
L
In the international SI system of units the Kelvin (K) was defi-
ned as the basic unit for temperature measurements. Degree
Centigrade (°C) is a lawful unit, derived from the SI units, and
internationally accepted. In the USA, temperatures are still
mostly given in degrees Fahrenheit (°F).
Absolute temperatures are mostly in degrees Cen-
tigrade (°C). Relative temperatures or temperature
differences are given in Kelvin (K).
Kelvin (K)
Centigrade (°C)
0 K
-273,15 °C
255,38 K
273,15 K
373,15 K
Conversion table:
°C to K:
T
= T
[K]
[°C]
°K to °C:
T
= T
[°C]
[K]
°C to °F:
T
= 9/5 x (T
[°F]
°F to °C:
T
= 5/9 x (T
[°C]
Abbreviations and symbols:
T
temperature given in [K]
[K]
T
temperature given in degrees Centigrade [°C]
[°C]
T
temperature given in degrees Fahrenheit [°F]
[°F]
7.1
Temperature sensors
The temperature sensors used most are the NiCr – Ni thermo-
couple (K-type) and the platinum sensor PT100. The charac-
teristics of the temperature sensors are defined in the norms
only for a limited range. Outside this range there are no reliable
values. If the measuring range of the temperature sensors is
exceeded, the HM8112-3 hence indicates „Overrange".
Crest
factor
C
2
2
2
3
Fahrenheit (°F)
459,67 °F
-17,77 °C
0 °C
100 °C
212 °F
+273,15 K
–273,15 K
+32 °F
[°C]
–32 °F)
[°F]
Form
factor
F
π
= 1,11
2
2
π
= 1,11
2
2
π
= 1,57
2
2
= 1,15
3
0 °F
32 °F
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