suggestion: howto measure Fan output CFM...?

[silvervarg]

A few people have tried this technique and a few very similar, like taping a small weight on top of the bag and see how high it will life up. Do a few seaches and you will find a few threads.
The major problem is that you will get lots of pressure from the bag, so the speed will decrease as the bag is filling up. Fans that can handle pressure better will do better in this test, so the results may differ a lot from how the fans will work in a PC.
It could be close to how a PC with lots of intake fans and no exhaust fan will work, but it would be far from how a typical negative pressure case PC will work.

By letting different fans blow air through a short tunnel and put a wind meter on the end of the tunnel (the kind that you can buy for slightly more advanced outdoor temperature measurements called "weather stations" should work fine).
The wind speed should be directly proportional to fan CFM, and the pressure in the short and open tube will be rather low.

[wim]

good idea.

some issues..

-might be very hard to decide precisely when the bag is 'filled' just by looking at it
-mass of bag itself tend to cause pressure differential between outside and inside (i.e. the fan has more work to do than just move air, also has to push edges of bag out)...maybe raises some complications because fan pressure (or force) and amount of airflow are not the same thing?

to reduce gravity effects from the bag, i'd suggest
-making your own bag and making it as light as possible (eg. from cling-wrap)
-hanging the bag below the fan (so the fan blows air downwards/fill the bag from above)


edit: d'oh, the swede beat me to it