Heads up for anyone considering the PT-21 tuner: it's only meant to recognise guitar pitches EADGBE or 1 semi-tone below. Perfect if you play piccolo bass, but sadly no good for anything lower. Having said that, it's picking up the G,D and E strings on my electric mandolin, but not the A; this must mean it also picks up one octave above guitar pitch as well. It also works on the 12th (and 24th) fret of a 4 string bass... hooray.
Build quality is excellent though, and tunes fine; maybe the best value guitar pedal tuner out there if you only play standard, and tiny. The packaging and instructions are identical to Mooer, as is the pedal layout. Tracks as well as my Korg Pitchblack, and the display's as good.
All moot thouigh given this isn't guitarchat (unless there's some way to modify the tuning internally... more skilled hands than mine required I'm afraid.)