A hygrometer is an instrument for measuring humidity. There are four basic types of hygrometer. The classic hygrometer, also called a psychrometer, uses two thermometers, one wet and one dry, to measure the rate of evaporation and therefore humidity. A hair hygrometer uses a piece of human or animal hair and measures changes in length due to moisture in the air. Electronic hygrometers measure the condensation on a chilled mirror to determine humidity. Lastly, gravimetric hygrometers actually weigh a sample of air and compare its weight to that of dry air.
The principle behind a psychrometer is fairly simple. The device contains two thermometers. One is dry, and the other is kept moist by wrapping the bulb in a wet piece of cloth. As the water around the wet bulb evaporates, the bulb cools down because the water absorbs heat from its surroundings as it changes phase. The rate of evaporation is dependent on humidity. Therefore, the bigger the difference between the wet bulb temperature and the dry bulb temperature, the lower the humidity. Different psychrometers have different ways of keeping air flowing across the bulbs. In a sling psychrometer, for example, the user swings the psychrometer by a piece of rope before taking a reading.
Hair Tension Hygrometers
In a hair tension hygrometer, a piece of human or animal hair is kept under tension. The length of the hair changes as it absorbs or loses water from the air around it. The length change is often magnified with a viewing lens so it can be interpreted.
Chilled Mirror Hygrometers
This type of hygrometer takes a very precise measurement of the temperature at which water condenses on a small mirror. This temperature is known as a the dewpoint, and it is directly related to humidity. The higher the humidity, the higher the dewpoint.
Gravimetric hygrometers work because of the very simple fact that moist air is less dense than dry air. The instrument very precisely measures the weight of an air sample and compares it to a sample of completely dry air of the same volume. This type of hygrometer is the most precise type, but it is also large, expensive, and impractical for field measurements. As a result, gravimetric hygrometers are generally only used to calibrate other types of hygrometers to ensure that they are working properly.