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I'm trying to measure the rotational speed of a turbocharger, but the laser tachometer I have can only measure up to 99,999 RPM and it isn't enough (display starts to output letters) and devices that can measure higher than that are quite expensive. I have seen Arduino based tachometers that use a photodiode coupled with a laser but it seems to require the beam to be broken, something that can't be done with the turbocharger. Is there any other option to measure its rotational speed, even it is less accurate?

It's for a small project so buying a $200 tachometer isn't justifiable.

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  • $\begingroup$ Can you down gear? Then the RPM you are reading will be lower. $\endgroup$ Aug 3 '17 at 10:46
  • $\begingroup$ That might work on a gas turbine... $\endgroup$
    – Solar Mike
    Aug 3 '17 at 11:30
  • $\begingroup$ take a deeper look at the Arduino based tachometers. You have pretty much the same thing with the beam "broken" by alternately being reflected or not. You probably just need a more powerful source laser. Make sure your laser is only reflected at one point in the rotation of course. $\endgroup$
    – agentp
    Aug 3 '17 at 12:32
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The people I worked with replaced the nut on the inlet side (as it is cooler than the exhaust side...) with a magnetised one and picked up the changing field with a Hall effect sensor (if memory serves...) to get the rpm.

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  • $\begingroup$ temperature may be an issue for the magnets. Most magnets don't manage correctly over 80-200 Cº $\endgroup$ Aug 4 '17 at 10:57
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    $\begingroup$ Which is why I clearly suggested that it is fitted on the inlet side where the incoming temperature is basically ambient when you are dealing with a car turbocharger application...... As having the coolest air coming in helps reduce the exit temperature which is also reduced prior to entering the engine by using an intercooler.... And do I understand you are thinking of adding magnets? I made it clear that a magnetised nut is used.... $\endgroup$
    – Solar Mike
    Aug 4 '17 at 11:13
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100,000 RPM is only about 1.6KHz which isn't a challenging frequency to measure. All you need is a transducer that will produce a "once per revolution" signal. An LED, a photodiode, and a stripe of reflecting paint on the rotor (or black paint if the rotor itself is reflecting) would probably work fine. Any cheap PC sound card and free audio software will sample the signal at 44.1 KHz or even higher, which is more than enough.

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    $\begingroup$ Some turbos go to 280,000 rpm plus any dust plus any oil mist and will the paint survive ? As I pointed out a magnetic nut avoids these problems and is / was an accepted solution by those who were measuring the turbo performance. $\endgroup$
    – Solar Mike
    Aug 3 '17 at 19:24

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