How Hygrometers Work
Hygrometers work in different ways depending on the application or purpose for testing moisture content in the air. The simplest type of hygrometer uses human hair, which causes strands to expand or contract to move a needle. The most common type of hygrometer, which is called a psychrometer, uses two thermometers: one with a wet bulb and one with a dry bulb. Temperatures drop as moisture evaporates from the wet bulb, and the relative humidity is determined by comparing the temperature differences between the two thermometers. Both hair and wet/dry hygrometers measure relative humidity reasonably accurately in a closed-room environment.