Operating Principle of Capacitance Level Sensors ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

Operating Principle of Capacitance Level Sensors

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Capacitance level instruments operate on the basic principle of the variation of the electrical capacity or capacitance of a capacitor formed by the sensor, vessel wall and dielectric material. A capacitor is made up of two conductive plates which are separated from each other by a dielectric. The storage capability of a capacitor defined by the capacitance, C, is directly dependent on the plate areas (A), their distance apart (d) and the dielectric constant of the material between the plates:

C = ЄA/d

Capacitive probes are used in liquids that are non-conductive and have a high dielectric constant (Є)and can be used for continuous level monitoring. A capacitive probe shown below, consist of an inner rod with an outer shell immersed in the liquid that we intend to measure the level.

The capacitance of the probe is measured between the inner rod and the outer shell with the aid of a capacitance bridge. In the portion out of the liquid, air serves as the dielectric between the rod and outer shell. In the immersed section, the dielectric is that of the liquid that causes a large capacitive change as the level of liquid changes. If the tank is made of metal it can serve as the outer shell. The capacitance change is directly proportional to the level of the liquid. The dielectric constant of the liquid must be known for this type of measurement.

The dielectric constant of the process material is the most important aspect of this level measurement application. It should be noted that temperature, moisture content, humidity and density can change the dielectric constant of the process material. These factors are often taken into account and corrected for in the design of capacitive probes for level measurement. The higher the dielectric constant of the process material, the easier and most reliable would be the level measurement. Materials with low dielectric constants are not suitable for capacitance level measurement application.

Areas of Application
Capacitive level measuring instruments are used for level detection in silos, tanks and bunkers, for both limit detection and continuous measurement. These instruments are typically used in all areas of industry and are capable of measuring liquids as well as solid materials.

  • Highly suitable for liquids and bulk solids
  • They have no moving parts
  • Suitable for highly corrosive media
Limited in its application for products of changing electrical properties (especially moisture content)