Due to the fact that they are mechanical devices, analog (dial-type) hygrometers can often be inaccurate and need to be calibrated. Before doing so, you need to determine whether the hygrometer can be calibrated. Look at the back of the hygrometer to see if there is a small screw in the center. If so, the hygrometer can be adjusted, and you can proceed with this method. If not, you may want to replace the unit.

The first thing you should do is to make sure that your hygrometer is reading accurately. Bear in mind that mechanical hygrometers may need to be checked frequently for precision. We suggest checking your device at least once every ninety days. There are three easy ways to check the hygrometer for accuracy -

1. Wrap the hygrometer with a dampened paper towel. After a period of a half-hour or so, look at the dial to see if it reads 100%. If it reads less, turn the calibration screw until it is at 100%. The problem with this method is that it is useless if the hygrometer is reading too high. Of the three methods, this one is the least precise.
2. Get a clean, dry, empty mayonnaise jar, a cap from a Snapple bottle, some table salt, and water. Fill the Snapple cap with salt and drip water on it until it is saturated, but not fluid. Lower the cap into the jar and put it off to one side and place the hygrometer in the jar on the opposite side and put the lid on. After 3 to 4 hours, check the hygrometer. It should read around 70%. Once again, if not, recalibrate the unit by turning the screw.
3. The simplest and most accurate method is to use a Boveda Calibration Kit - Single Kit. You place the hygrometer in the provided, pre-conditioned envelope and follow the directions. Once adjusted, this method should give you the most reliable readings.