A flow sensor for a medical ventilator system is disclosed herein. The flow sensor includes a valve assembly having a valve seat, and a valve flap attached to the valve seat. The valve flap is composed of an elastomeric material configured to generate a pre-load that biases the valve flap into engagement with the valve seat such that the valve assembly remains closed in the absence of an externally applied force. The flow sensor also includes a pressure transducer configure to measure a first pressure level at an upstream position relative to the valve assembly, and a second pressure level at a downstream position relative the valve assembly. The flow sensor also includes a processor configured to estimate the flow rate of a fluid passing through a medical ventilator system based on the first and second pressure levels.