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.
- Place a teaspoon of salt in the bottle cap and add a few drops of tap or distilled water to moisten it. Don’t overdo it. You don’t want to dissolve the salt. Add only enough water to dampen the salt. When water is added to common table salt, it will maintain an exact 75% humidity in a perfectly sealed environment.
- Carefully place the salt and your hygrometer into the airtight container. (Try not to get any moistened salt on the gauge.) Check the unit to assure its current reading is somewhere in the 40% to 80% range.
- Seal the container tightly but don’t try to remove any remaining air trapped inside. Now, wait for at least several hours until the environment has stabilized (this could take up to 6 hrs.). Do not open the container. Read the gauge’s humidity % level. It should be exactly 75%. If it is not, note the deviation as being the amount your hygrometer is out of calibration. If for example, it reads 65%, the gauge is 10% low. If it reads 80%, the gauge is 5% high.
- Carefully remove the unit from the container/bag. Assuming your hygrometer has a calibrating screw on the back (most better ones do) take a very small flathead screwdriver and turn it slowly while watching the dial on the front. If your gauge was low by 10%, turn the screwdriver so the dial is set 10 percentage points higher than it was previously. Conversely, if your gauge was high by 5%, turn the screw in the opposite direction to set the dial 5 percentage points lower.
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.