Measuring TT Speed

Mijo, You are certainly OK to prefer and use the RPM app on your cell phone, but your little experiment doesn't prove anything about its accuracy relative to an un-named strobe to which you compared it.  I am only speaking for the KAB strobe, not strobes in general. I got the idea not to trust cell phone apps based on numerous posts to this forum and the Vinyl Asylum website where users complain about their lack of accuracy, or they had a problem with tt speed that ultimately was proven to be related to their reliance on a cell phone app.  Like I said, I don't recall whether anyone specifically named the RPM app.

@johnss:  Correct.  And that is what you get with AnalogMagik.  But still there are potential variables, including proper center hole on test LP, warp, etc.

@ lewm.  Some RPM apps are better than others.  The preferred ones have a way to be calibrated.

@edwyun .... re. “Some RPM apps are better than others. The preferred ones have a way to be calibrated“

Please name some of the apps that have a way to be calibrated.

+1 on the Timeline

Lewm, You have to place the phone directly over the spindle for it to measure accurately. Most just lean it against the spindle so the phone is anything but horizontal. I use a record weight with a flat top so that the phone is right over the spindle and horizontal. If it is not dead center the wow and flutter readings will be off. Readings are repeatable and ease of use relative to a strobe is superior. How long do you want to stare at a strobe to see if it is drifting. You also do not get wow and flutter readings off a strobe. I use an iPhone by the way in spite of hating Apple. I have used several strobes. Why don't you give this a spin and see for yourself. If you can't get it to read accurately and repeatedly stick with your strobe. No big loss.  
The phone does not have to be calibrated. It already has an accurate built in clock and this is a measurement against time, Revolutions Per Minute. Counting against the clock is an extremely simple function.