Level instruments

Point level measurement - Capacitance switches

Capacitance 5/6

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Overview

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Mode of operation Introduction Common Terms Inverse frequency shift capacitance point level switches and continuous level transmitters are designed to withstand the harsh environments of high pressure and high temperature ap-plications. Capacitance The property of a system of conductors and dielectrics that per-mits the storage of electricity when a potential difference exists between the conductors. Its value is expressed as the ratio of a quantity of electricity to a potential difference and the unit is a Farad.Capacitor manufacturer Inverse Frequency Technology Siemens inverse frequency shift capacitance devices incorpo- rate a unique frequency-based approach to level measurement. The capacitance units monitor the effect of capacitance based on frequency change. The relationship between capacitance and frequency is inverse. Because small level changes result in a large frequency change, the result is excellent resolution and accuracy. A device in a circuit that has the potential to store an electric charge. Typically a capacitor has 2 conductors or electrodes separated by a layer of a non-conducting material called a di-electric. With the conductors on opposite sites of the dielectric layer oppositely charged by a source of voltage, the electrical energy of the charged system is stored in the polarized dielec-tric.Dielectric constant Principle of Operation Inverse frequency shift capacitance devices require two compo- nents: a reference electrode of a variable capacitor and the measurement electrode. In capacitive level measurement, the environment (typically the vessel wall) acts as the reference electrode, while the probe supplies the measurement electrode. The dielectric is composed of the vessel contents and, if the measurement electrode is insulated, the insulating layer. The ability of a dielectric to store electrical potential energy un-der the influence of an electric field. This is measured by a ratio which compares the capacitance of a professional condenser with the mate-rial as dielectric to its capacitance with a vacuum/dry air as di- electric: the dielectric constant of air is 1.Active shield

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The portion of the probe isolated from the active measurement section. The sensor signal is connected to the active shield por- tion of the probe, eliminating the electrical potential difference between the shield and the measurement section. So, the shield portion of the probe near the process connection is not affected by changes in vapour concentration, material buildup, dust or condensation.
Inverse frequency shift capacitance operation Capacitance is affected by the surface area of the electrodes, the separation distance between the electrodes and the dielec- tric constant of the vessel contents. The dielectric constant is the measure of a material’s ability to store energy. The relative di-electric constant of air (vacuum) is 1; all other materials have a higher value.
Siemens FI 01 · 2009