VEGA Capacitive electrodes EK 4 … 20 mA - Compact - Manual (Page 4)

4 Capacitive electrodes EK 4 … 20 mA - Compact
22574-EN-020809
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 (admit-
tance processing). By this means, even diffi-
cult products such as adhesive, conductive
substances and solids with fluctuating mois-
ture content can be measured.
Through the use of screening tubes and
screening segments, inactive areas can be
created on the probe where pollution, conden-
sation 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 electrode surfaces (a)
- size of the electrode surfaces (b)
- type of dielectric between the electrodes (c)
Fig. 1.1 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 constant (DK
value) of the medium with respect to air, the
capacitance of the capacitor increases as the
level rises around the electrode.
Fig. 1.2 Capacitance change through varying
immersion level
a
b
c
The capacitance change is processed by the
oscillator and converted into a level-propor-
tional measured value. The measured value is
outputted as a standardised, floating
4 … 20 mA analogue signal, or optionally as a
digital signal with the HART
® communication
protocol. The sensor can be adjusted through
the integrated oscillator. As an option, adjust-
ment is also possible with a HART
® handheld
or PC.
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 E32 Ex or CAP E32 H Ex.
Continuous measurement requires a constant
dielectric value
ε
r
i.e. the medium should have
unchanging properties.