To test a hygrometer's calibration, take a bottle cap or other small shallow container, fill it most of the way with table salt. Add enough water to make a stirrable slurry, no standing water. Mix it up and seal the cap with salt and your hygrometer in a Ziploc bag. The hygrometer reading should eventually stabilize at exactly 75%. Any variation from this is how far off your hygrometer is.
Originally Posted by ryknoll3
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