Prototyping a Flowmeter for a Continuous Positive Airway Pressure Device

Continuous positive airway pressure (CPAP) is an effective therapy for obstructive sleep apnea. Although the controlled variable in CPAP devices is pressure, flow measurement can supplement monitoring of therapy effectiveness. Here, we describe desired sensor characteristics, application constraints and a theoretical framework for designing a differential pressure flowmeter for a CPAP device. Our aim is to validate this framework through experimental verification of the permanent pressure loss and flow-pressure curve of a 3D-printed prototype. Pressure at patient interface was less than 5% lower than that measured before the restrictive element, despite pressure drop in such element being always above that threshold. The predicted quadratic flow-pressure relationship was achieved with a 97% approximation. These findings indicate that the type of restrictive element chosen for the prototype, a Venturi tube, is suitable for the application, and the used framework is adequate for sensor design. However, refinements to the current geometry are needed to adjust flow sensibility and the fitting to other CPAP device’s components.