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where to buy a R.P.M. gauge to read the total R.P.M. of my turbojet engine or how to make something this is a turbocharg Answered

 i need a device to read the R.P.M.s of my home made turbojet engine? what can i use (turbocharger) weel speed 

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NachoMahma (author)2010-02-15

.  Since turbojets spin at up to 100K+ RPM, you're going to need a fast tach (especially if you are triggering on every blade.
.  I don't think you'll be able to build an optical tach that fast, but steveastrouk's noise freq sounds like a good idea. With a little bit of calibration, it should be very accurate.

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steveastrouk (author)NachoMahma2010-02-15

You think it'll be too fast ? Hadn't thought of that, but lets make some assumptions, and say that the fans have 24 blades,  2.4 E6 blades/minutes, but that's only 40kHz, which is a PRF of 25usec. I'd put money on a fairly straightforward circuit working, if you can kick it with a nice bright light.

You CAN buy turbojet tachos for the modelling community.

I'd like to try my noise idea, mind.

Steve

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NachoMahma (author)steveastrouk2010-02-15

.  I shoulda done the math. ;)
.
.  A piezo (or similar) detector could be mounted almost anywhere on the engine. Maybe even on the craft, instead of the engine.

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steveastrouk (author)NachoMahma2010-02-15

Better idea still ! Yes, a piezo would be a great transducer.

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steveastrouk (author)2010-02-15

The best method is a non-contacting optical tachometer. You can buy them for about 30 USD, or you can make one with a small microcontroller like the Arduino.

I do wonder though, if you know how many blades are on the turbocharger, if you can relate the noise of the thing to speed, since the fundamental frequency will be shaft speed x blade number.

Then you can measure speed with a soundcard scope.

Steve

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