- LI Capacitive sensor
- Technology
LI capacitive sensor: Technology
The LI sensor uses capacitive technology which measures the ability of a media to hold an electric charge (permittivity). This allows the sensor to reliably determine if media is present and to suppress the detection of foam and residue. The adjustable sensitivity of the LI fine tunes the capacitive measurement to a specific media, ensuring accurate detection and eliminating false triggering.
The basic principle of the technology uses a constant voltage supply to charge a capacitor plate which creates an electrical field. The single plate capacitor then measures capacitance between the sensor element and its surroundings, which uses earth ground as the reference. Any media that comes in between the sensor probe and the tank wall changes the measured capacitance, which is detected by the sensor.
The LI capacitive technology is suitable for media with low (i.e. oils) and high (i.e. water) dielectric constants, but a minimum dielectric constant of 1.8 is required for reliable detection. Most liquids are acceptable, and the below table provides reference values for common conductive and non-conductive medias.
| Relative permittivity (dielectric constant) of common media | Value (> 1.8 required) |
|---|---|
| Mineral oil | ≈ 2 |
| Coolant emulsion | ≈ 25...75 |
| Glycol | ≈ 37 |
| Water | ≈ 80 |
Level Measurement Zone
The unit operates with radial detection characteristics. Therefore, media below the active zone (A = 28 mm) are not detected.
Temperature Measurement
There is an RTD element at the lower end of the probe used to measure the media temperature.
Residue Supression
When the capacitive measurement unit is adjusted to the presence of a specific type of media, it can also suppress or ignore the presence of build-up or foam. The capacitive measurement will be high when media is present and lower when build-up or foam is around the sensing tip. The difference in how each influences the capacitive measurement allows the sensor to distinguish between them and ignore capacitive signals below that of the actual media being detected.