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Check the frequency setting used on the tuners (as it is generally adjustable). Set a=440 herz, unless otherwise directed by instructor. Strangely, all of your tuners and app could be reading correctly if they were set to different “a” frequencies.
Research is what I’m doing when I don’t know what I’m doing. —Werner von Braun
Without deviation from the norm, progress is not possible. —Frank Zappa
Experience is a difficult teacher, it gives the test first and the lesson after.
Great suggestion Irv. Fiddlerman has some nice drones in all keys on his site, linked to YouTube.
I made some bagpipe/highland pipes drones awhile back in all keys. Lots of pleasant harmonics in those. Here's a link to them for a free download. Once downloaded you won't need wifi to play them. Most computer audio players have a repeat feature that will let you play them indefinitely. Better yet, if you have computer music recording software such as Cakewalk( It's free you know).......you can load the audio, set loop points and have a more gapless experience.
Here's the link to those if interested-