Transmitter; Operating Principles; The Sitrans Lc500 Variable Frequency Oscillator - Siemens SITRANS LC500 Operating Instructions Manual

Capacitance transmitters

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Transmitter

Capacitance

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 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.

For definitions relating to capacitance, see the glossary, page 107.

7ML19985GE04

SITRANS LC500 – OPERATING INSTRUCTIONS

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