VEGA Capacitive electrodes EL … - Manual (Page 11)

Capacitive electrodes EL … 11
Product description
+
– 0 %
5 % 10 %
15 % vol.
fully insulated
electrode
partly
insulated
electrode
Solid
humidity (%)
Level
Meas. value
Fig. 1.3 Humidity change
Oscillator E18
Oscillator E18 with patented processing (phase
selective admittance processing) extends the
application area of the capacitive level
measurement technology.
In conjunction with the fully insulated rod
electrode EL 24 the oscillator E18
compensates also strong conductive build-up.
Mounted into an individual rod or cable
electrode type EL, E18 ensures also the exact
measurement of solids with varying humidity
contents.
The oscillator E18 processes the meas.
currents acc. to their phase position. Meas.
currents with a defined phase shifting as they
can occur with build-up or humidity changes,
are hence filtered out.
Humidity change
A humidity change in solids causes also a
change of the dielectric constant figure (
ε
r
).
Furthermore the resistance of the product
changes. Due to the change also a phase
shifting of the meas. currents will be caused.
With a capacitive measurement, even smallest
humidity changes cause meas. errors. Typical
products are e.g. sand, aggregate in the
cement industry, hop or plastic granules (after
drying).
When using oscillator E18 modifications up to a
humidity contents of 15 % vol. do not influence
the accuracy of the measurement. Even
layerings of different humid mediums do not
play a role for the meas. accuracy.
When the humidity contents is higher than
15 % vol. the fully and partly insulated
electrodes react differenty (see also "Fig. 1.3
Humidity change“). Whereas the measured
value with fully insulated electrodes increases
with steady level, the measured value reduces
with partly insulated electrodes.
1) Real value level
1)