Hygrometer
From Stogiepedia
Hygrometers are instruments used for measuring relative humidity. A simple form of a hygrometer is specifically known as a "psychrometer" and consists of two thermometers, one of which includes a dry bulb and the other of which includes a bulb that is kept wet to measure wet-bulb temperature. Evaporation from the wet bulb lowers the temperature, so that the wet-bulb thermometer usually shows a lower temperature than that of the dry-bulb thermometer, which measures dry-bulb temperature. When the air temperature is below freezing, however, the wet bulb is covered with a thin coating of ice and yet may be warmer than the dry bulb. Relative humidity is computed from the ambient temperature as shown by the dry-bulb thermometer and the difference in temperatures as shown by the wet-bulb and dry-bulb thermometers. Relative humidity can also be determined by locating the intersection of the wet- and dry-bulb temperatures on a psychrometric chart. Hygrometers come in both analog and digital forms. Digital hygrometers generally display the temperature as well.
[edit] Calibration
The simplest way of calibrating a humidor is the "salt method." This requires a cup, table salt, a large plastic sealable bag, and distilled water. Begin by filling the cup with 1/2 cup of salt. Then, mix in distilled water until the mixture looks like wet sand. Place the cup containing the mixture and the hygrometer in the bag and seal the bag. Place the bag in an area out of direct sunlight for approximately 8-12 hours. After waiting 8-12 hours, check the hygrometer (without removing it from the bag). If it is perfectly calibrated, the reading should be exactly 75% relative humidity. If the hygrometer read something besides 75%, make a note of how far off it is from the proper number and add or subtract that amount (eg. if the hygrometer reads 70% you will add 5%, if it reads 77% you will subtract 2%). Place the hygrometer back in the bag and let it sit for at least two hours. After that, it should stabilize at 75% and be fully calibrated.
