VEGA Capacitive electrodes EL Profibus PA - Manual (Page 4)

4 Capacitive electrodes EL - Profibus PA
Product description
1 Product description
1.1 Function and configuration
Capacitive electrodes series EL can detect
the level of practically any kind of product,
regardless of whether it is a liquid, powder or
paste. This also applies to adhesive prod-
ucts.
The electrode simultaneously measures the
capacitance and the ohmic resistance (ad-
mittance processing). By this means, even
difficult products such as adhesive, conduc-
tive substances and solids with fluctuating
moisture content can be measured.
Through the use of screening tubes and
screening segments, inactive areas can be
created on the probe where pollution, con-
densation or permanent buildup cannot influ-
ence the measuring result.
Measuring principle
Electrode, medium and vessel wall form an
electrical capacitor.
The capacitance of the capacitor is generally
influenced by three factors:
- distance between electrode surfaces (a)
- size of the electrode surfaces (b)
- type of dielectric between the electrodes
(c)
Plate capacitor (schematic drawing)
The electrode and the vessel wall are the
capacitor surfaces. The medium is the dielec-
tric. Due to the higher dielectric (DK value) of
the medium with respect to air, the capaci-
tance of the capacitor increases as the level
rises around the electrode.
Capacitance change through varying immersion level
The capacitance change is processed by the
oscillator and converted into a level-propor-
tional measured value. The measured value
is outputted digitally as a Profibus PA signal.
The sensor can be adjusted through the
integrated oscillator. As an option, the adjust-
ment is also possible with a PC running the
VVO software.
In continuous level measurement, the level is
continuously detected and converted into a
level-proportional signal which is either di-
rectly displayed or further processed.
For this type of measurement, you need a
capacitive electrode with oscillator
CAP E34 PA EX.
Continuous measurement requires a constant
dielectric value
ε
r
i.e. the medium should have
constant properties.
a
b
c