Calibrating an Analog Hygrometer
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 that the hygrometer can actually 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.
There are three ways to check the hygrometer for accuracy:
- 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.
- 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 right around 70%. Once again, if not, recalibrate the unit by turning the screw.
- The simplest method is to use a Boveda calibration set. You just place the hygrometer in the provided, pre-conditioned envelope and follow the directions.