Measuring TT Speed

the RPM app for cell phones is accurate and a lot easier to use than a strobe. Check it out !

In my phone LG v30+ RPM metter shows 32.83 instead of 33.33
Both my turntables has strobe's and one of the even has custom built. And if I do measure friends machines I am getting the same error always so for me it's usable but it's not measurement tool. It's phone with gyroscope which is not calibrated.

its a bit extreme but the proper way to measure is with a calibrated test tone disk and a frequency counter. You can also send the output to a scope and visually watch the deviations.

jsman,
yes, it's the illuminated number icons that count. If there's drift, and you have a way to adjust platter speed, you can usually get the illuminated numbers to hold steady, plus or minus. I always make speed adjustments with the tonearm lowered and playing the first song of a LP sitting under the strobe disc. It's surprising how much stylus drag there is.
If you don't have a way to adjust speed you can get a close estimate of actual speed and error using this technique posted at the KAB website:

TO EVALUATE DRIFTING SPEED, [COUNT] THE NUMBER OF ICONS THAT MOVE IN AND OUT OF THE SPOTLIGHT IN 60 SECONDS. 

ACTUAL SPEED = 1 + (#ICONS DRIFT PER MIN / 3600) X RPM 

DRIFT IN PERCENT = (#ICONS DRIFT PER MIN / 3600) X 100

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.