Vega Capacitive electrodes EK 4 … 20 mA - Compact Bedienungsanleitung PDF herunterladen (Seite 4)

4 Capacitive electrodes EK 4 … 20 mA - Compact

22574-EN-020809

Product description

1 Product description

1.1 Function and configuration

Capacitive electrodes series EK can detect

the level of practically any kind of product,

regardless of whether it is a liquid, powder or

paste. This also applies to adhesive products.

The electrode simultaneously measures the

capacitance and the ohmic resistance (admit-

tance processing). By this means, even diffi-

cult products such as adhesive, conductive

substances and solids with fluctuating mois-

ture content can be measured.

Through the use of screening tubes and

screening segments, inactive areas can be

created on the probe where pollution, conden-

sation or permanent buildup cannot influence

the measuring result.

Measuring principle

Electrode, medium and vessel wall form an

electrical capacitor.

The capacitance of the capacitor is generally

influenced by three factors:

- distance between electrode surfaces (a)

- size of the electrode surfaces (b)

- type of dielectric between the electrodes (c)

Fig. 1.1 Plate capacitor (schematic drawing)

The electrode and the vessel wall are the

capacitor surfaces. The medium is the dielec-

tric. Due to the higher dielectric constant (DK

value) of the medium with respect to air, the

capacitance of the capacitor increases as the

level rises around the electrode.

Fig. 1.2 Capacitance change through varying

immersion level

a

b

c

The capacitance change is processed by the

oscillator and converted into a level-propor-

tional measured value. The measured value is

outputted as a standardised, floating

4 … 20 mA analogue signal, or optionally as a

digital signal with the HART

®

communication

protocol. The sensor can be adjusted through

the integrated oscillator. As an option, adjust-

ment is also possible with a HART

®

handheld

or PC.

In continuous level measurement, the level is

continuously detected and converted into a

level-proportional signal which is either directly

displayed or further processed.

For this type of measurement, you need a

capacitive electrode series EK with oscillator

CAP E32 Ex or CAP E32 H Ex.

Continuous measurement requires a constant

dielectric value ε

r

i.e. the medium should have

unchanging properties.

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