I'm a building contractor looking for a way to measure the airflow (in CFM) of the vent fans that are typical in homes. The usual suspect is in the range of 80-150 CFM, has a squirrelcage fan installed in a metal housing that's installed in the ceiling, and a 3" or 4" round outlet that ducted to the exterior (hopefully) with either rigid metal pipe or some sort of flexible pipe. The tool I have available at the moment is this item: http://www.extech.com/instruments/product.asp?catid=1&prodid=2 [Broken] My hunch is that the specific spot in which the vane is held will have a big effect on the accuracy of the reading. Some fans (when the ceiling grille is removed) have a nice round inlet, but more often than not it's a more complex shape. In addition, it's often not possible to hold the vane directly within the shape, but rather right in front of it. While googling around I've noticed a few manufacturers of flow hoods, but these seem to be both uncommon for HVAC diagnosis, and very expensive. I'd be grateful for any comments or advice from those with experience and understanding of this type of issue. My goal is to be able to get a reasonably accurate reading (+/- 10%) of actual airflow. The reason I don't just read the equipment nameplate is that the quality of the ductwork install has a major impact on actual flow. Thanks!