Hi,
Here is a paper on bags and other simple air measurement:
http://www.builditsolar.com/Referenc...lbnl-51550.pdf
and some more:
http://www.builditsolar.com/Referenc...lbnl-51551.pdf
I tried a really big bag to measure solar air heating collector flow rate. It worked fairly well:
Adventures in Measuring Airflow for Solar Collectors
Another way to do it is to get one of the little Kestrel wind meters and do a velocity survey across the duct, and average the results.
Gary