I was talking with another tech this morning about checking RPM. this brought up the subject of strobes. As we were discussing strobes it occurred to me someone had probably created an app with RPM already. Shazaaam!!! Here's a app that will allow you to test RPM on fans. You can also test motor rpm, if, you place a mark on the shaft. https://itunes.apple.com/us/app/strobe-light-tachometer-to/id708094321?mt=8 To check fan speed just dial it in till the fan appears to be not moving at all. That'