A hygrometer is a device that measures the relative humidity in the air. Its most common use is in an enclosed container that needs to be maintained at a specific humidity, such as a cigar humidor. An analog hygrometer typically uses a needle with a numbered scale to provide the relative humidity, whereas a digital display provides a specific number for the humidity. The steps in calibrating a hygrometer generally involve exposing the hygrometer to an atmosphere with a known humidity and adjusting the hygrometer to display that humidity value. Calibration is typically necessary only with analog hygrometers.
Calibrating a hygrometer requires some specific equipment in addition to the hygrometer itself. This includes the cap from a plastic soda bottle and an airtight plastic food container large enough to hold the hygrometer. Additional materials needed for calibrating a hygrometer include a small amount of table salt, water and a flat head screwdriver that fits the calibration screw for the hygrometer.
The first step is to pour a teaspoon of salt into a clean, dry soda bottle cap. Just enough water should be added to make the salt wet without actually dissolving the salt. The bottle cap and the hygrometer should then be placed in the plastic container as close together as possible. The lid on the container should be sealed without pressing down to ensure the air in the container is at normal pressure.
The container can be opened after about eight hours and the hygrometer removed. After reading the humidity level, the hygrometer can be adjusted if it does not show a reading of 75 percent. This procedure for calibrating a hygrometer is based on the fact that the damp salt produces a relative humidity of 75 percent, when the test is properly performed.
The hydrometer is adjusted by turning the calibration screw. This generally involves turning the calibration screw clockwise to increase the hygrometer’s reading and counter-clockwise to decrease the hygrometer’s reading. If the hygrometer does not have a calibration screw, any deviation from 75 percent should be noted and applied to future measurements. For example, if the hygrometer reads 65 percent after the test, then ten percent should be added to that hygrometer’s humidity measurements.
Analog hygrometers should be calibrated about every six months, as they tend to fall out of adjustment over time. Digital hygrometers are generally calibrated at the factory and are not adjustable by the user. A faster but less accurate method of calibrating the hygrometer involves wrapping the entire hygrometer with a damp cloth, and adjusting the hygrometer to read 96 percent.