VORTEK VTD: Vortex Shedding Airflow Measurement System
The VorTek airflow measuring device is the only digital airflow sensing device on the HVAC market. By utilizing vortex shedding technology, the shedder mounted in the air stream creates pressure pulses, which are converted to an electronic frequency. This electronic frequency is related to airflow velocity in a linear manner. That is why the VorTek is able to maintain high accuracy over a very large range. 
VORTEK VTD: Vortex Shedding Airflow Measurement System Data Sheet