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Different capacitive and resistive elastomer sensors for the measurement of the filling level of water in a vertical plastic tube are introduced in the paper. The filling level sensors consist of elastomer membranes carrying electrically conductive layers with special geometrical designs. Three different basic sensor principles in various versions were investigated. First, capacitive strain sensors with different electrode designs detect the pressure of the water column above the elastomer membrane by measuring the change of electrical capacitance due to the stretch deformation. The shape of the electrode layers on these sensors affects their measuring sensitivity. Second, resistive strain sensors are also stretched by the water pressure acting on the membrane, but here the stretch changes the electrical resistance of the sensor. The resistor on the membrane is a silicone layer with conductive particles, which consist of carbon black or alternatively of silver-coated copper flakes. Depending on the kind and the concentration of the particles, the resistance of the sensor and its variation upon stretch can be tuned. Third, capacitive proximity sensors detect the height of the water level by measuring the capacitance between two neighbored electrode layers on the membrane. In this configuration, the sensor is a long elastomer film along the length of the plastic tube. The advantages and disadvantages of the different sensor principles are discussed. This discussion includes the sensor characteristics in terms of the change of capacitance or resistance vs. water level in the tube and also the creep behavior of the different filling level sensor types at constant water filling level.
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