you can also test your hygrometer with a boveda pack. Each pack is marked with its humidity (65%,69% and 70%) . However humidity level you want to go..put both hygrometer and say, a 65% boveda pack in a tightly sealed jar for 24 hours.

the next day....read the humidity level in the hygrometer
the reading overnight in the hygrometer minus the 65(%) in the boveda will give you the difference your hygrometer is OFF the standard.

I just put a small scotch tape over either digital or analaog hygrometer to remind me, how much this hygrometer is OFF the standard.

but thats just me and how I do it...

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