# vane anemometer question



## Doctor Hattenhottie (Dec 31, 2012)

Hey guys, got a question about using the vane anemometer to measure CFM...Got myself an anemometer where, when measuring CFM, you key in the duct area in cubic feet so the machine can count CFMs accurately for you.
Simple enough, but here's where I ran into a snag. I am measuring a fresh air diffuser's output on a residential air-exchange system. The duct supplying the difuser is 5" diameter, BUT the diffuser is a one-size-fits-all type where the actual diameter is about 3" and tension clips hold it inside a larger duct. 
In this situation, do I key in the particulars for a 5" duct, or do I use the measurment of the narrowest part of this one-size-fits-all diffuser? Want to get an accurate reading of airflow. 
Hope you guys can help! I'm new to ventilation work and I have lots of questions. Thanks


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