Measuring air flow in an HVAC duct can be a tricky business. Paddle wheel and turbine flow meters introduce not only resistance but maintenance issue due to accumulated dust and debris.

[Nubmian] set the transducers up in a PVC pipe and blew air into it with a fan ... ground did the trick. For more HVAC projects check out this pinwheel flow meter and [Adam]’s piece on HVAC ...