Sensors can be used to balance the airflow in complex ducting systems which provides cost-saving energy efficiencies. See also environmental monitoring.
The VaV box is controlling a particular airflow into a room or duct. The air needs to be distributed throughout the building through these VaV boxes. Sensors enable monitoring valves that controls the air volume. The sensors can test air pressure throughout the system to know and monitor the airflow through the duct.
You can accomplish this by recording the pressure drop through a diversion to your pressure sensor while measuring at low pressure and using a high-sensitivity pressure sensor.
Application note, PDF