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Capacitance is formed between the level sensing probe and metallic wall of the vessel which act as two electrodes, and the application media acts as a dielectric medium when the sensor is introduced into the vessel. These electrodes are separated by an insulator by suitable media like solid and liquid known as a dielectric medium. The dielectric is the main factor in the capacitance principle as its value will be influenced by temperature and moisture content of operating material. These factors should be taken into account while designing the probe.
When the level of process material rises, the value of dielectric changes which results in change in the value of capacitance between the level sensing probe and the vessel wall. This change can be detected and measured by the Radio Frequency technique. These RF signals are interpreted by the evaluation unit and it converts these signals into a relay output signals. RF current flowing in the capacitor helps the electronics detect the presence of application media.
The design of the probe comprises a sensing rod and grounding part, typically made of stainless steel to resist stains and corrosion while providing robust support. The probe design can vary based on tank type and application media:
Metallic Tank: Acts as a ground, requiring only the sensor rod without additional grounding.
Non-metallic Tank: Requires a reference probe for proper grounding, achievable through various arrangements:
Probe Design Capacitance Principle: Utilizes a capacitance-based level sensor.
Twin Probe: Provides grounding for materials with low permittivity, utilizing both reference and sense rods.
Still Well: Ensures grounding for low-dielectric media and creates turbulence over the sense rod.
Based on electronics, capacitance-based level sensors are categorized into two parts:
Level Switch: Point level switches detect the presence of material at a predefined level, triggering an alarm condition to prevent overflow when that level is reached.
Level Transmitter: Designed for continuous level measurement, it monitors the material level throughout the process at any given time.
Build-up on the capacitance probe can cause the capacitance sensor to malfunction. The capacitance level sensor starts giving a false alarm when material sticks on the probe.