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is there any way to measure rpms cheaply?

we are making wind turbines in one of my classes and my team is making a tesla turbine as ours. we want a way to measure how fast the disks are spinning. please help.☺

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Hello people,

Answers were appreciable. I have a question similar to this. Can anyone tell me how can i measure rpm of an object if it is rotating underwater? Approximate rpm will b around 250 but i need to be more precise. Is there any device which can help? Optical tachometer is not working for me.

Please help. thanx in advance

orksecurity6 years ago
Attach a gear to the shaft. Hold a card against the teeth of the gear and listen for the musical note produced. Find that note on a piano. Look up or calculate its frequency to get teeth per second. Divide that by number teeth on the gear to get rotations per second. Multiply by 60 to get rotations per minute.

Not hugely accurate, but you're looking for cheap rather than precise.
Creative!
rickharris6 years ago
put a white dot/bar on the disc, look at it with a photo transistor. the output will be proportional to speed.

IF you can attach something to the spinning shaft then a DC motor will give an output proportional to speed - all you need is to calibrate it.
Photodetector does have the advantage of minimum added drag on the system.
+1
If you are willing to spend a little money, you could get a device like this one, which sells for $30. 

http://www.harborfreight.com/digital-contact-tachometer-66400.html

I have an older version of this device, and it has proven handy on occassion.  You basically hold the shaft up to the shaft you want to measure.  The device comes with various tips to help couple to the shaft.

I don't know much about Tesla turbines, but I believe that they typically operate at high rpms.  The device at the link above claims to work to 20000rpm.  I have only used my meter for speeds of a few thousand RPM.

If your measurement also has to be used as feedback to some other device, then you would be best using an approach like those suggested in the other answers.