Design method of a bidirectional flow meter for a metered dose inhaler

This paper presents the design of a bidirectional flow meter for metered-dose inhalers, addressing the challenge of accurately measuring airflow over vastly different ranges: 0-150 L/min for inhalation and up to 700 L/min for exhalation to capture peak expiratory flow. The design process utilized numerical simulations to understand the complex flow physics and a series of physical measurements to determine the precise influence of two critical geometric parameters: the width and height of the proposed flow element. Results demonstrate a predictable quadratic relationship between pressure drop and flow rate in both directions. Notably, the exhalation measurement range was found to be highly sensitive to the flow element's height, while the inhalation measurement remained robust against minor geometric variations. This process yields a validated flow meter that adds minimal resistance to the inhaler, enabling the accurate quantification of patient-specific respiratory waveforms for improved drug deposition studies and efficacy assessment.