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When I first started playing my ukulele, I was happy to use a free smartphone app. Now that I've restrung my uke with a low G, the app I was using doesn't recognise the note, so I got a proper tuner that can detect what note is being played (rather than how close the frequency is to the note that it thinks the string should be). So many players recommend this tuner, and I am very happy with it, too.
Seems to work perfectly. Very clear and easy to read. You do need some advanced knowledge of what notes your strings should be tuned to but I guess this makes it more flexible as you could custom tune your instrument.