Capacitive level measurement is based on the capacitor principle. A capacitor is formed by 2 electrodes, electrically insulated from each other. The electrodes must be conductive and are typically made of metal. Capacitors have the ability to store energy in an electric field between its electrodes, caused when a voltage or potential is applied to a circuit. The property of capacitance relates to the amount of energy stored in its field to the applied potential. By placing liquid between the electrodes the ability for the capacitor to store energy increases and so the capacitance increases.
Features of elobau’s capacitive level sensors
•Conductive and non-conductive liquid measurement
•Analog output signals: 1-5V, 4-20ma, 0-10V, 0-5V, 0-20ma
•Stainless steel electrodes
•IP65 protection level
•-20° to +80°C
•Up to 2500mm total length
•Reverse connection & short circuit proof
These level sensors function without a movable float which can be affected by the liquid. The actual operation is as follows:
The liquid is known as the dielectric. The key property of dielectric materials is the dielectric constant or relative permittivity. This property is the amount of charge the liquid can absorb.
As the liquid is introduced between the electrodes of the capacitor the capacitance changes progressively and liquid level can be determined. To measure variations in capacitance electric energy flowing into and out of the electrodes is measured as the circuit potential is varied. A regular flow of energy is established by connecting the electrodes to an alternating current measurement circuit. The more energy flow to the electrodes, the greater the capacitance, meaning more liquid between the electrodes.
For sensor calibration reference measurements at empty and full must be taken. Generally we need to know the dielectric constant of the liquid being measured to calibrate the sensor at its full condition. With the empty and full outputs are established, liquid level sensing comes to relating sensor output to these values.
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