VEGA Capacitive electrodes EK … with signal conditioning instrument - Manual (Page 11)

Capacitive electrodes EK with signal conditioning instrument 11
Product description
+
– 0 %
5 % 10 %
15 % vol.
fully insulated
electrode
partly insu-
lated elec-
trode
Solid
humidity (%)
Level
measured value
Fig. 1.3 Humidity change
Oscillator E18
The oscillator E18 with the patented process-
ing (phase selective admittance processing)
extends the application range of capacitive
level measurement technology.
In conjunction with the fully insulated rod
electrode EK 24 the oscillator E18 compen-
sates even very conductive build-up.
Mounted in an individual rod or cable elec-
trode type EK, E18 ensures also the exact
measurement in solids with varying humidity
contents.
The oscillator E18 processes the measuring
currents according to their phase position.
Measuring current with a defined phase
shifting as they occur with build-up or humid-
ity changes are filtered out.
Humidity change
A humidity change in solids causes a change
of the dielectric constant figure (
e
r
). In parallel
the ohmic value of the medium changes. Due
to the change also a phase shifting of the
measuring currents is caused.
With a capacitive measurement already low-
est humidity changes cause measuring er-
rors. Typical products are e.g. sand,
aggregate in the cement industry, hop or
plastic granules (after drying).
When using the oscillator E18 humidity
changes of 15 % vol. do not influence the
accuracy of the measurement. Even layering
of product with different humidity does not
play a role for the measuring accuracy.
When the humidity contents exceeds 15 %
vol., fully and partly insulated electrodes
react differently (see also "Fig. 1.3 Humidity
change“). Whereby the measured value on
fully insulated electrodes raises with steady
level, the measured value on partly insulated
electrodes drops.
1) Actual value level
1)