Analytical modeling of a coaxial cylindrical probe capacitive sensor based on MATLAB/Simulink for conductive liquids level measurements

Abstract

Measurement of liquid level is an important process variable in the operation of many process industries such as water treatment plants. There are different types of level sensors to measure this parameter, categorized into ultrasonic, capacitive, optical, microwave, magnetostrictive, resistive chain, magnetoresistive, hydrostatic pressure, air bubbler, gamma ray, etc. In this paper, a cylindrical probe capacitive sensor for measuring the level of conductive liquids has been modeled based on a step-by-step mathematical procedure, using MATLAB/Simulink. The steps take the relation between the different capacitances in the model with the height of the conductive liquid into account. The results show a linearity function between the sensor capacitance from 0.1 to 1.1 nF versus the height of liquid from 0 to 90 cm. In order to have a standardized instrumentation output, a signal conditioning circuit was designed to achieve an output current of 4-20 mA, representing the sensor capacitance, and the response of the sensor in terms of current to different heights of liquid was characterized.