VEGA Capacitive electrodes EL 4 … 20 mA - Compact - Manual (Page 9)

Capacitive electrodes EL 4 … 20 mA - Compact 9
22652-EN-020809
Product description
+
– 0 %
5 % 10 %
15 % vol.
fully insulated
electrode
partly insu-
lated electrode
Solid
moisture (%)
Level
Meas.
value
Fig. 1.3 Varying moisture content
Oscillator
The oscillator CAP E32 (H) 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 measure-
ment of solids with varying moisture content.
The oscillator processes the measuring cur-
rents acc. to their phase position. Measuring
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 (e
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 measure-
ment errors. Typical products are e.g. sand,
aggregate in the cement industry, hops or
plastic granules (after drying).
When using oscillator CAP E32 (H) Ex,
changes of moisture content up to 15 % vol.
do not influence the accuracy of the measure-
ment. 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 also „Fig. 1.3 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.