Here's an interesting app https://itunes.apple.com/us/app/rpm-turntable-speed-accuracy/id1030889902?mt=8
You set it on top of the platter, and it uses the phone's gyroscope to calculate speed. I had high hopes for it, but it told me my table was too fast, and then I realized that even on steady surfaces the gyro shows a 0.25 +-0.01 rpm rotation. I don't think I can compensate the reading by subtracting 0.25 from it, because I am not convinced the gyro's error is constant. See what you get...
Sample screen:
You set it on top of the platter, and it uses the phone's gyroscope to calculate speed. I had high hopes for it, but it told me my table was too fast, and then I realized that even on steady surfaces the gyro shows a 0.25 +-0.01 rpm rotation. I don't think I can compensate the reading by subtracting 0.25 from it, because I am not convinced the gyro's error is constant. See what you get...
Sample screen: