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fill a bottle cap with salt and a few drops of water and put it in an air tight box or bag overnight with the hygrometers, it should read 70% humidity, whatever it reads above or below that is how out they are, the super cheap ones you can't usually change manually but at least you'll know what to add/subtract to get the true reading. slightly more expensive ones or one's that work with an app can be changed easily.
Gewürzt
Some are rubbish. Either one or both of these.