So with so much expertise on line, how does one determine the accuracy of a particular tuner given the existence of tempered scales.

I'm happy with the couple of ancient Boss tuners I have (the lack of auto power save is a pain when others leave it on) and I know that I'll have to tweak guitar in particular to best suit the key I'm in. I don't know what they're called but that brand that 'sticks' to hlat surfaces and has the row of LEDs, never seems to give the accuracy of my Boss - I can easily hear that the fancy one is wrong. I don't even know if this type is still on the market.

But I'm not worried about which brands are better, rather the conundrum that any tuner can only ever be perfectly accurate in one key. Any clues please?

Regards, John