Works well but I have a complaint
PanoTuner is the most reliable free app that I have found. It will help you tune accurately, and I’ve been using it for years. However, they recently changed the layout in a way that does not make sense to me. Instead of a higher pitch being represented by the tuner going to the right side of the screen, the right sound now indicates lower pitches. This is contrary to how pianos are set up, to how any string instrument works, and how we are used to thinking of the musical alphabet as musicians (and how most other tuners on the market represent high and low). I do not understand this change and I wish it would go back to normal.