Siemens SITRANS LC500 Operating Instructions Manual page 18

R = Ratio between initial
capacitance and total
capacitance
Ca = Initial capacitance (air)
Cm = Capacitance Increase
(product)
C1 = Capacitance connection
point
C2 = Capacitance connection
cable
C3 = Capacitance Process
connection (includes active
part)
The measurement is further protected from interference by a buffer, which applies the
frequency signal from the measurement section to the active shield section. This effectively
eliminates any electrical potential difference between the shield and the measurement
section and prevents additional changes in capacitance occurring.
SITRANS
LC500
active shield
active
measurement
section
probe seal
(inactive)
The relative lengths of the measurement section and active shield section can be
specified to suit a particular application. If the measured range will be short relative to
the total length of the electrode, specify a short measurement section. This increases the
achievable resolution of the measurement, since any change in level will be greater
relative to the length of the measurement section.
Page 12 SITRANS LC500 – OPERATING INSTRUCTIONS
Conventional Capacitance
Measurement
R = (C1 + C2 + C3) + Ca
(C1 + C2 + C3) + Ca + Cm
Measuring-
Circuit
C1
C2
C3
Ca
Cm
empty full
tank tank
SITRANS LC500 with
Active Shield
R = Ca
Ca + Cm
Measuring-
Circuit
100%
buffer

frequency (f) K
capacitance (C)
0%
Ca
Cm
7ML19985GE04