An Inexpensive Air Flow Meter
A DC motor will operate as a DC voltage generator when its shaft is turned fast enough.
I made an inexpensive air flow meter by doing the following:
I bought an inexpensive hand-held, battery-operated personal fan from the 99 cent store.
I removed the batteries.
I removed the rubber fan (propeller) blade (because it would not spin the motor-generator fast enough.
I attached an old, larger-blade, fan propeller blade I had left over from a bathroom fan to the shaft, so the air flow would spin(turn) the DC motor-generator in the hand-held fan.
I connected a DC Voltmeter to the wires where the battery terminals had been placed.
I attached this combination to a pole and held it near the air duct outlets where the air flow seemed to be the greatest.
I measured the flow rate out of each register of interest (by measuring and writing down the DC voltage).
This is a relative measurement, not an accurate qualitative measurement. However it tells generally what the relative flow rate is coming out of each register.
(I did try to calibrate this device knowing that 60 MPH is equivalent to 88 feet per second, but I am too embarassed to tell you what happened with that experiment. Suffice it to say, the experiment was quickly terminated).
Good Luck