Dampen a towel so it's soaked but not dripping. Wrap it completely around your hygrometer.

Wait 30 to 40 minutes, unwrap, and quickly check your hygrometer reading. It should read 98% or higher.

If it does, you're done and your hygrometer is correctly calibrated.

If it doesn't, then turn the screw in the back of the hygrometer until the reading is 98% or greater. You've successfully calibrated your hygrometer and you can return it to the humidor. You should calibrate it again in about a year.

If you don't want to fiddle with the screw on the back of your hygrometer, then simply make a mental note about how far off your hygrometer was. Say after the towel, your device read 80%. You know it's about 20% off.

Tips:

Calibrate your hygrometer at least once a year. We recommend twice a year.

Never use tap water in your humidifier. The mineral deposits can damage your hygrometer over time. They can also damage your humidifier and leave unpleasant tastes and odors on your cigars.