I think I am going with the fiber optic turbo RPM sensor.
No semi conductors in the sensor itself to be affected by the heat
and ten times the frequency response. That 5K hertz proximity sensor 
could be failing to respond at RPM's over 130,000 or so.

http://www.home.agilent.com/USeng/nav/-536893493.0/pc.html

This is an 8 mm bolt modified to hold two 1 mm fiber optic cables
pointed at the fins. It will be potted with epoxy to hold the
cables securely. 8 mm is the same size as the proximity sensor.

Paul Lamar
 
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