Operation & Application

SITRANS LC 500: Transmitter

Operating Principles

Capacitance1 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.

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.)

The SITRANS LC 500 variable frequency oscillator

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.

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 109.

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SITRANS LC 500 – INSTRUCTION MANUAL

7ML19985GE01

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Siemens instruction manual Sitrans LC 500 Transmitter, Operating Principles, Operation & Application