Structure design and performance analysis of a coaxial cylindrical capacitive sensor for liquid-level measurement

This paper presents the design and fabrication of a liquid-level measurement system based on a grounded cylindrical capacitive sensor for conductive liquids. Fabrication of the cylindrical capacitive sensor is performed with a novel approach of shielded cable. The inner conductor of shielded cable is connected to ground and the outer conductor is used to measure the liquid level. The relationship between the capacitance value of the cylindrical capacitive sensor and the liquid level is deduced by the analysis and calculation of physical parameters. Moreover, a backward difference calculation is adopted in order to offset the effects of the parasitic capacitance. The performance of the system has been experimentally tested in both grounded metallic and plastic containers. The experimental results show that the nonlinear error is smaller than 5 mm over a range of 90 cm. Over a wide measuring range of 200 cm, the liquid-level measurement system has a nonlinear error smaller than 1.35 cm.