Liquid Level Measurement System using Capacitive Sensor
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Abstract
The accuracy of liquid level measurements using capacitive sensors can be compromised in remote sensing applications due to parasitic capacitance from signal cables and interface circuits, leading to reduced precision in detecting subtle changes in capacitance. This paper introduces a capacitive method for measuring liquid levels in water and alcohol solutions, where the liquid height is assessed based on variations in capacitance between the sensing electrode and the ground indicate the liquid level in a tank. The system uses Programmable Phase-Locked techniques along with an All-Pass Filter circuit to generate a 90º phase shift and convert the locked frequency into capacitance values, unaffected by parasitic capacitance from coaxial cables and op-amp pins, which arise from circuit design and connection. The system incorporates an Improved Active Shield circuit, which has been enhanced to eliminate parasitic capacitance from both the coaxial cable and the op-amp pins, enabling capacitance measurement in the range of 1–100 pF with a normalized level error (NLE) of less than 2%. This system operates in real-time on a microcontroller, providing immediate display results. It is cost-effective, compact, and suitable for use in hazardous environments.
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