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

4 Capacitive electrodes EK - Profibus PA
Product description
1 Product description
1.1 Function and configuration
Capacitive electrodes series EK 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
products.
The electrode simultaneously measures the
capacitance and the ohmic resistance of the
product (admittance processing). By this
means, even difficult products such as
adhesive, conductive 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,
condensation or permanent buildup cannot
influence 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 the electrode surfaces
(a)
- size of the electrode surface (b)
- type of dielectic between the electrodes (c)
Plate capacitor (schematic drawing)
The electrode and the vessel wall are the
capacitor surfaces. The medium is the
dielectric. Due to the higher dielectric (DK
value) of the medium with respect to air, the
capacitance 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
adjustment 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
directly displayed or further processed.
For this type of measurement, you need a
capacitive electrode series EK 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