An Analytical Model for Monitoring Liquid Volume in Absorbent Media Using Capacitive Sensors

Sweat rate monitoring is essential for the accurate interpretation of biomarker concentrations in wearable sweat sensors. In addition, sweat rate measurement is particularly relevant for the early detection of diseases associated with hyperhidrosis and hypohidrosis, conditions that affect thermoregulation, hydration balance, and metabolic function. This work presents an absorbent-based capacitive sensor for sweat rate monitoring applications. The sensor consists of a parallel-plate architecture, where an absorbent textile acts as the dielectric. A novel analytical model based on mixing rules enables real-time estimation of absorbed liquid volume from capacitance measurements. To account for slow response and evaporation effects, an empirical dynamic model is introduced, improving accuracy under varying sweat rates. Experimental validation across four infusion rates demonstrates a volumetric estimation error below 10% and average sweat rate error below 3%. These results position the system as a viable candidate for autonomous sweat monitoring. Moreover, the sensing architecture is well-suited for industrial applications involving fluid absorption in porous media, such as textile processing or hygiene product testing.