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

Capacitive electrodes EK - Profibus PA 9
+
- 0 %
5 % 10 %
15 % vol.
Product description
fully insulated
electrode
partly
insulated
electrode
solid
humidity(%)
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 the capacitive level measurement
technology. This function can be switched on,
see "4 Setup".
In conjunction with a fully insulated rod
electrode, the oscillator also compensates
heavy conductive buildup.
Mounted with any rod or cable electrode type
EK, this oscillator enables exact meas-
urement of solids with varying moisture
content.
The oscillator processes the meas. currents
acc. to their phase position. Meas. currents
with a defined phase shift, commonly
occurring when buildup forms or moisture
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 meas.
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
measurement. Even layers containing diffe-
rent amounts of moisture do not affect the
accuracy.
1) real value level
1)
If the moisture content goes above 15 % vol.,
the fully and partly insulated electrodes react
differently (see also "1.3 Varying moisture
content"). Whereas the measured value from
fully insulated electrodes increases with
unchanging product level, the measured
value from partly insulated electrodes falls.