Bypass ultrasonic flow sensor for respiratory monitoring: flow-division-based structural adaptation and optimization

Accurate airflow measurement remains challenging in respiratory monitoring because the required operating range is wide and the aerodynamic conditions are complex. This study proposes a bypass ultrasonic airflow sensor and investigates the effects of the throttling structure and the bypass inlet diameter on flow-division characteristics. Computational fluid dynamics (CFD) simulations and bench experiments were conducted for orifice-plate and vane-type restrictors with bypass inlet diameters of 2–6 mm. Based on the numerical and experimental results, three optimized configurations were selected for representative respiratory-monitoring scenarios, and piecewise-polynomial calibration models were established. After calibration, the three selected configurations achieved accuracies of ± 1.25%, ±1.86%, ±2.17% F.S., with the best repeatability of ± 0.13% F.S. and a response time below 20 ms. Robustness assessments demonstrate the sensor's resistance to moisture interference and clarify the changes in split ratio and system applicability caused by inlet contamination. This study provides a practical framework for the structural design, selection, calibration, and evaluation of clinical-scenario adaptability of bypass ultrasonic airflow sensors for respiratory monitoring.