I have a viscous fan in my vehicle and I like to measure via Arduino fan speed. What's the best way to detect fan speed? Max engine rpm is about 3800 and the fan has 7 blades. The viscous fan fully engages at 90 C. The detector will get quite hot, in excess of 120 C.
Will something like this work?
TCRT5000 infrared reflection sensor
Possibly put white paint on one of the fan blades so that only one blade will reflect the signal? They say max operational distance is 25 mm.