VEGA Capacitive electrodes EL Profibus PA - Manual (Page 9)

Capacitive electrodes EL - Profibus PA 9
+
- 0 %
5 % 10 %
15 % vol.
Product description
fully insulated
electrode
partly insulated
electrode
Solid
moisture (%)
Level
Meas. value
Varying moisture content
Oscillator
The oscillator CAP E34 PA Ex with patented
processing (phase-selective admittance
processing) extends the area of application
of capacitive level measurement technology.
This function can be switched on, see "4
Setup".
In conjunction with a fully insulated rod elec-
trode, the oscillator also compensates heavy
conductive buildup.
Mounted with any rod or cable electrode type
EL, this oscillator also enables exact meas-
urement of solids with varying moisture con-
tent.
The oscillator processes the measuring cur-
rents acc. to their phase position. Measuring
currents with a defined phase shift, com-
monly occurring when buildup forms or mois-
ture content changes, are hence filtered out.
Varying moisture content
A change of moisture content in solids
causes a change of the dielectric constant
(
ε
r
). A simultaneous change of the ohmic
resistance of the product also occurs. In
addition, the change causes a phase shift of
the measuring currents.
With capacitive measurement, even minor
changes of moisture content cause measur-
ing errors. Typical products are e.g. sand,
aggregate in the cement industry, hops or
plastic granules (after drying).
When using oscillator CAP E34 PA Ex,
changes of moisture content up to 15 % vol.
do not influence the accuracy of the meas-
urement. Even layers containing different
amounts of moisture do not affect the accu-
racy.
1) Real value level
1)
If the moisture content goes above 15 % vol.,
the fully and partly insulated electrodes react
differently (see illustration "Varying moisture
content"). Whereas the measured value from
fully insulated electrodes increases with un-
changing product level, the measured value
from partly insulated electrodes falls.