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We bought a digital anemometer to check the CFM of various fans we have on hand. My husband designed and 3D Printed this duct that securely mounts fans on to the anemometer for accurate readings, which seems to work great.

The unit we have takes an input of FT² and automatically converts that to CFM, but we don't know how to determine the surface area in FT² of this duct design, given the cone shape, fins and solid center area.

We tried using Pi x R ^ 2 with Radius from the small fan side, and again with Radius from the large anemometer side, but this isn't quite correct. We're using a high end 6 CFM fan as a baseline, and readings are roughly 2.5 CFM off depending on which end we measure the radius of.

Any advice on how to properly find the surface area of this would be great. We tried loading the STL file in CAD software like Fusion 360, but it measure the design, not the channels for airflow.

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    Which side is the fan mounted to and where is the anemometer taking readings from? I suspect that the manufacturer specs are taken from a calculation involving fan speed and blade design and never actually measured. – JPhi1618 Feb 20 at 20:40
  • @JPhi1618 Here is a rendered image showing the duct in use. i.imgur.com/Z44E2HG.jpg This particular anemometer doesn't have an arrow showing airflow direction, but the instructions say it should blow from the back side, so I mounted the duct on the rear and the fan blowing towards it (rather than pulling air through) – Aidan Knight Feb 20 at 20:44
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    That looks very cool, but I wonder if you can expect accuracy with such a large size difference. At the very least, there is a lot of curving the air has to do to change shape from the small fan to the large anemometer and it flows over a large surface area. That seems like a significant amount of drag, that would lower you numbers. I'm also not sure about the large center shaft. – JPhi1618 Feb 20 at 21:11
  • In the first design the center shaft was hollow, I think he just closed it in because airflow wasn't really going through it (it covers the round center part on the small fan and slightly larger center part on the anemometer fan). I do have my concerns about the duct shape though, where as you mentioned, it goes from relatively small to quite large. – Aidan Knight Feb 20 at 21:26
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When I worked in a clean room we had to certify the flow hoods and the flow in the pipes. With pipes we had to measure multiple locations and average the readings close to the outside ends up being slower flow than 1/3 way to the center. On a square plenums we had to take 9-12 reading depending on size. Prior to doing this many times I would have thought the outside was the fastest but the drag of the sides slows the air down, in the center close to the fan there is very little flow unless squirrel cage blower. So try taking multiple measurements and average the results it may be more accurate.

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