Nonlinearity Issues in Respiratory Flow Measurement in Constant Temperature Mode

Accurate respiratory flow rate measurement is critical for optimizing mechanical ventilation, a life-support intervention widely used in Intensive Care Units (ICUs). A precise monitoring is required to deliver the right amount of gas to patients, however, conventional sensor designs that patients often face cost and complexity barriers. For this purpose, Hot Wire Anemometers (HWAs) are particularly well-suited as they offer high sensitivity, and fast response.This paper presents the development of a cost-effective thermal flow meter leveraging a Negative Temperature Coefficient (NTC) thermistor as a sensing element, operating in Constant Temperature Mode (CTM). A closed-loop control system dynamically adjusts heating current to compensate for airflow-induced thermal losses, enhancing sensitivity and stability under low-flow conditions. A custom calibration setup was implemented to validate the system’s performance under controlled flow scenarios.While the inherent nonlinearity of the thermal feedback system poses challenges, the primary contribution of this work lies in investigating the causes of these nonlinear behaviors and exploring methods to mitigate their impact on flow accuracy. This system emphasizes cost reduction and adaptability for ICU use without compromising fast response or precision. By addressing these nonlinearity issues, the proposed system demonstrates its potential as an effective, low-cost solution for real-time respiratory monitoring in mechanical ventilators.