**For an Analog** <Works with digital as well but can't adjust it after and could damage the electronics inside.>Take the hygrometer and put it in a ziploc bag. Milk carton lid will salt, add a few drops of water, not enough to dissolve it. Let it sit 6 hrs. It should read 75%. Adjust it accordingly as quickly as possible when you remove it from the bag.
I suck at directions so you may want/need to google "Testing and calibrating hygrometers".
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