Capacitive fill level sensors are discussed in DE 195 11 556 C1 and DE 197 28 280 A1. The sensors have two cylindrical metal electrodes situated one inside the other. The liquid may flow into an interspace between the two cylinders and fill up the interspace, depending on the fill level. The liquid, which has a higher dielectric constant, displaces air, which has a comparatively low dielectric constant. The electrical capacitance between the two electrodes is obtained according to the proportion of liquid between the two electrodes. An evaluation circuit determines the capacitance and estimates the fill level on that basis.
When using fill level sensors, it is important to be sure that their electrodes are resistant to the liquids. In the case of highly reactive liquids, e.g., uric acid, extremely high-grade electrodes of noble metals or corrosion-resistant steels must therefore be used.