PlatterSpeed is an easy way to calibrate your turntable's speed to proper 331/3 RPM or 45 RPM respectively by using a 3150 Hz testtone.
Using any testrecord with a 3150 Hz track you can record and adjust with your iPhone (or iPad, or iPod Touch) from the loudspeaker in an endless loop.
Measuring the speed this way is absolutely exact in contrast to using a traditional strobe disk.
Please note that you absolutely need a record with the test tone of 3150 Hz for PlatterSpeed to work. The Adjust+ 7 inch testrecord is a perfect fit - others might work as well but there's no warranty about the accuracy.