Turbo tachs are well established.  A magnet is inserted under the compressor nut
and a simple coil outside the (aluminum) compressor housing outputs a once per
rev pulse.
Jeff Spitzer

How plebian.  You'd think they would use a Hall effect sensor at least.

Russell Kent

What is the rise time of the Hall effect sensor?
Lets see one pulse per rev 150,000 revs per minute
2500 revs per second.. 400 usec. OK I guess that will work.

Don't hall effect sensors have to be right next to the nut?

Just a narrow angle lens photo transistor looking at
the nut across the air passage with a painted white flat with 
a light source might work too. Course the temp there is 200 F
and that may be why they use a mag sensor.

Paul Lamar
 
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