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Reliable detection of limits
Limit detection (empty or full indicator) of media in containers or pipes can be readily accomplished with NEGELE sensors in a reliable and cost-effective manner. We offer a wide range of measurement procedures and solutions for virtually every application - whether in aqueous, pasty or foaming media.
Capacitive measurement principle
This method is suitable for limit detection and dry running protection in fluids or pasty and adhesive media, and does not require a minimum conductivity of the medium.
The capacity of a capacitor is influenced by three factors:
- Distance between the electrodes
- Electrode surface area
- Type of dielectric between the electrodes
The type of dielectric plays an important role in capacitive limit switches.
The measurement electrode and the vessel wall act as the capacitor plates, while the medium in the vessel serves as the dielectric. Because the medium has a higher dielectric constant than air, the capacity increases when the electrode is submerged. This change in capacity is evaluated by the electronics and converted into a corresponding switch command.
Conductive measuring principle
This method is used for limit detection or dry running protection in aqueous media with a minimum conductivity of 1 µS/cm.
The conductive limit switches are used together with an evaluation device for detecting limits. The evaluation device generates an alternating current that is applied to the limit switch. When contact is made between the sensor rod and the medium, the electric circuit is closed and the resulting current is evaluated by the limit device.
The sensitivity depends on the resistance of the medium and must be adjusted on the limit device: The higher the resistance, the higher the sensitivity setting.
Potentiometric measuring principle
This method is ideal for limit detection of extremely adhesive and viscous media with a minimum conductivity of 1 µS/cm, e.g., in the preparation of cream cheese, or the manufacture of ketchup and mayonnaise.
The sensor rod acts as a potentiometer via which a measuring bridge is formed to the vessel wall. The measured value changes in proportion to the filling level. When the sensor is installed from above and from the side, the user obtains a "full" or "empty" indication. When installed from below, a 4-20 mA signal is output.
