Made this out of a $9 three inch DC  muffin fan I got
at a surplus electronics place. I choose
the three inch fan rather than a smaller CPU
fan as it was much easier to work with. I am no watch 
maker. My fingers are too big.

I threw away everything except the fan, the ball bearings, the
ring permanent magnet, the shaft and the snap ring.
I machined a new bearing housing on my lathe out of some
Delrin I had lying around and hollowed out a little place
for the Hall sensor using a small burr on a Dremal tool.
Exact dimension will depend on the fan you get.
 
The spacing of this Hall sensor from the magnet must
be less than .7 mm. About .025 inches by my calcs. 
I used a bit of scotch tape to hold the tiny Hall sensor in 
place while a bit of RTV glued the 
wires in place. Sounds Mickey Mouse but it works well. 

More about this when I upload the skematic.

This should be used to check the flow through your
rad(s) to make sure your diffuser is the right shape.

Paul Lamar
 
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