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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.
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@jellybaby
Some are rubbish. Either one or both of these.
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