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How does capacitive level sensing work?

30/04/2014

How does capacitive level sensing work?

To explain capacitive level sensing, we must first understand the concept of a capacitor.

A capacitor is formed by two electrodes, electrically insulated from each other. The electrodes themselves must be conductive and are typically made from metal. They can be any shape, although two parallel plates are easiest to visualise.

Capacitors have the ability to store energy in an electric field between these electrodes when a voltage or ‘potential’ is applied to the circuit. The property of capacitance relates the amount of energy stored in this field to the applied voltage or potential.

By placing non-conductive material between the electrodes, the ability for the capacitor to store energy increases and so the capacitance increases. This material between the electrodes is referred to as the ‘dielectric’.

The key property of dielectric materials is the amount of charge that can be stored.

As a dielectric liquid is introduced between the electrodes of the capacitor, the capacitance changes proportionately and liquid level can be determined.

To measure variatioMain 300x225ns in the capacitance, electric energy flowing into and out of the electrodes is measured as the voltage or potential is varied. This flow of energy is created by connecting the electrodes to an alternating current measurement circuit. The greater the energy flow to the electrodes, the greater the capacitance, meaning more dielectric between the electrodes.

For level sensor calibration, reference measurements at empty and full tank levels must be taken. Generally, the dielectric constant value of the liquid being measured is required, to enable calibration of the sensor at its ‘full’ level. With the empty and full outputs set, liquid level sensing comes down to relating the sensor output to these values.

Capacitor plates can also be designed as a concentric tube and rod, with the advantage of reduced interference as well as improved mechanical stiffness and robustness, as engineered in the Gill liquid level sensors.

All signal processing is managed on-board with the fully integrated electronics. Dielectric fluctuation caused by temperature change is managed using offset values stored within the sensor memory, which are typically programmed during factory calibration at Gill Sensors.

Gill Sensors capacitive liquid level sensors are often constructed from aluminium, stainless steel or carbon fibre allowing Gill to cater for harsh applications from fuel tanks in Formula 1 race cars, to oil transmission systems in mining and industrial machinery.

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