7ML19985GE04
SITRANS LC500 – OPERATING INSTRUCTIONS
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Oper
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Transmitter
Operating Principles
Capacitance
1
measurement operates by forming a variable capacitor resulting from the
installation of a vertical measurement electrode in a vessel or silo. The tank wall usually
forms the reference electrode of the capacitor. Whatever material is sandwiched
between the two electrodes forms the dielectric. This will be composed of the vessel
contents (air, vapor, liquid, solid, or a combination) and, if the measurement electrode is
insulated, the insulating layer (PFA, for example). The dielectric gives a capacitance value
that is proportional to level.
Capacitance is affected by the surface area of the electrodes, the separation distance
between the electrodes, and the dielectric constant of the vessel contents. The dielectric
constant is the measure of a material’s ability to store energy. The relative dielectric
constant of air (vacuum) is 1: all other materials have a higher value.
The SITRANS LC500 variable frequency oscillator
The SITRANS LC500 probe is equipped with a variable frequency oscillator which
responds to the capacitance: a change in capacitance is registered as a change in
frequency. The relationship between capacitance and frequency is inverse, resulting in
high resolution and accuracy. The variable frequency maintains a constant relationship to
the reading.
Capacitance measurement in a cylindrical metal tank
In a cylindrical tank, it is possible to determine the initial capacitance in air by factoring in
the length of the probe, diameter of the probe, diameter of the tank, and the relative
dielectric constant of air.
1.
For definitions relating to capacitance, see the glossary, page 107.
Note:
To preserve linearity of the measurement, both electrodes must be parallel.
(When the vessel contents are conductive, the measurement electrode is insulated
and the interface between the insulating layer and the contents acts as a parallel
reference electrode independent of the tank wall.)