Being able to measure ducted airflow cheaply and non-intrusively, like with this ultrasonic flow ... to turn into a functional sensor. [ItMightBeWorse] is using readily available HC-SR04 sensor ...

Enclosed are the ultrasonic ... sensor and an LED indicating the power status. On the other side of the CAT5 cable can be found an Arduino compatible board with an XBee shield and a 9V battery ...