Newbie questions on hygrometers

Sorry for the newbie questions, but I'm not familiar at all with
relative humidity, dew points, and temperature, or they they
correlate. I've tried wiki-ing it, but have not found the answer.

I'm trying to find very accurate hygrometers (perhaps a chilled-mirror
hygrometer) but the RH accuracy listed for these high end hygrometers
are listed like "0.5C", which I believe is the dew point. How can I
use this number to find the RH accuracy in RH% units? I'm guessing the
temp factors into it, but I need the most accurate hygrometer for an
indoor room that could be anywhere from 0-120 F, but in real life the
temp will be between 60 and 80 F.

Also, can anyone recommend a very accurate hygrometer?

Thanks!
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