[Integza] built a Tesla turbine and wanted to know how fast it was spinning. However, he didn’t have a tachometer, and didn’t want to buy one. After a false start of trying to analyze the audio to measure the speed, he decided to use a tried-and-true method. Let the wheel break an infrared (IR) optointerruptor and count the spokes of the wheel as they go by. If you know the spacing between the spokes, you can compute the speed. There was only one problem: it didn’t work.
Turns out, PLA is at least somewhat transparent to IR. Knowing that it was a simple matter to fix some tape to the wheel that would block IR and that made things work much better. If you missed the video where he built the turbine, you might want to watch it first.
The results were good. Thanks to the Internet, [Integza] was able to appropriate some code from a nine-year-old Ukranian and use it as a base. With breath power he got a pretty good spin going, but with an air compressor, he handily broke 10,000 RPM.
We were surprised the audio method didn’t work out. We know big companies spend money with other companies to listen to compressors run and decide if they are about to fail. We suspect some audio processing through GNU Radio or some other DSP software could have recovered the actual signal, but we don’t know that.