Siemens LC 500 Manual De Usuario

Page 8

SITRANS LC 500 – INSTRUCTION MANUAL  

7ML19985GE01

 

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 LC 500 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.

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 109.

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.)