Design and Fabrication of Fringing Field Capacitive Sensor for Non-Contact Liquid Level Measurement

The capacitive sensors are widely used for liquid level measurement in different applications, but the contact type capacitive sensors suffer from frequent maintenance and drift of the sensor output. The present paper proposes a simple noncontact interdigital (IDT) fringing field capacitance sensor for liquid level measurement. The sensor can be made using flexible /non-flexible substrate, and it can be used to measure the liquid level of the vessel for both planar and non-planar surfaces. Theory of level measurement using the sensor has been discussed. Four different IDT sensor (1-n-1) configurations such as 1-1-1, 1-3-1, 1-5-1 and 1-11-1 have been designed, analysed, simulated (finite element method), and fabricated to measure water and insulating transformer oil levels from 0 to 40 mm. The analytical expressions are derived for 1-n-1 configuration to estimate the value of capacitance and the maximum field penetration depth of the sensors. The capacitance value of all four sensors varies almost linearly with the variation of liquid level. Experimental results show that the sensitivity of the 1-5-1 IDT sensor is 40 fF/mm. Performances of the sensors were compared with other capacitive sensors reported in the literature. The sensor is capable to detect the water level with a resolution of 0.1 mm.