A Single Carbon Nanotube-paper Composite Electrode Sensor Based Brake Oil Degradation Detection

Brake oil is essential to the performance and safety of hydraulic braking systems, but its degradation—primarily caused by water absorption—can lead to reduced boiling points, corrosion, and brake failure. This study presents a non-invasive method for real-time monitoring of brake oil degradation by detecting changes in water content using a single-electrode capacitive sensor based on a carbon nanotube paper composite (CPC). The sensor operates on the principle of fringing electric fields, enhanced by highaspect-ratio carbon nanotube fibers that increase local field intensity and dielectric sensitivity. Unlike conventional twoelectrode designs, this configuration offers structural simplicity and is well-suited for embedded automotive platforms. Experimental testing was conducted using two types of brake fluids, with incremental additions of deionized water (0.5% by volume) to simulate moistureinduced degradation. The sensor exhibited a strong linear response to increasing water concentration, with consistent slopes across fluid types, enabling a generalized calibration model for real-time water content estimation. The CPC sensor demonstrated high sensitivity, fast response, and excellent repeatability, making it an effective solution for insitu brake fluid monitoring. This work supports the development of predictive maintenance systems aimed at improving vehicle safety and operational reliability.