VEGA Capacitive electrodes EL … - Manual (Page 4)

4 Capacitive electrodes EL …
Product description
1 Product description
1.1 Function and configuration
Capacitive electrodes series EL detect levels of
virtually all kind of products, whether liquids,
granules or pastes. This is also valid for
adhesive products.
The electrode measures the capacitance and
the product resistance (admittance
processing). Hence also problematic products
such as adhesive, conductive products and
solids with fluctuating humidity contents can be
measured.
When using screening tubes and screening
segments, inactive parts can be created on the
probe where pollution, condensation or perma-
nent build-up do not influence the meas. result.
Meas. principle
Electrode, medium and vessel wall form an
electrical capacitor.
The capacitance of the capacitor is generally
influenced by three factors:
- distance of the electrode surfaces (a)
- size of the electrode surfaces (b)
- kind of dielectricum between the electrodes
(c)
Fig. 1.1 Plate capacitor (schematic drawing)
The electrode and the vessel wall are the
capacitor plates. The medium is the
dielectricum. Due to the higher dielectric
constant value (DK-figure) of the medium
against air, the capacitance of the capacitor
increases with the degree of the electrode
covering.
Fig. 1.2 Capacitance change with covering
a
b
c
The capacitance change is converted by the
oscillator into a level proportional, floating
current in the range of 4 … 20 mA or into a
switching command.
Continuous level measurement
With the continuous level measurement, the
appropriate level is continuously detected and
converted into a level proportional signal which
is either directly displayed or further
processed.
A capacitive electrode series EL with oscillator
and a signal conditioning instrument VEGAMET
are required which converts the proportional
current of the oscillator into standardized
current and voltage signals.
The continuous measurement requires a
constant dielectric constant figure
ε
r
i.e. the
medium should have constant features.
The floating meas. signal of the electrode
electronics is in the range of 4 … 20 mA and
connection to other processing systems such
as e.g. VEGALOG is hence possible without
additional potential separation.