How do you calibrate a hygrometer?
How to Calibrate a Hygrometer
Most analog hygrometers are manufactured with a tolerance of +/- 4-5% (or more) of humidity within the range of 40-80%. Although humidity gauges are supposedly pre-set at the factory, it’s not unusual to find the calibration off once you get the unit home and placed in your humidor. If you want to see how accurate your hygrometer is, there is a simple, easy-to-do method you can use with items found right in your kitchen. All you need is some table salt, a sealed container (Tupperware™ type or ZipLock™ bag) and a plastic bottle cap.
Your hygrometer should now be properly calibrated, which will help you maintain a healthier overall environment for your cigars. If the gauge does not appear to be moving, try blowing warm, moist air into the back of it. One other thing you can do, which is also another way to test the unit, is by wrapping the hygrometer in a warm, damp paper towel for about 30 minutes or until it reads about 80-90% +/- a few points.