A psychrometer is a device that can be used to measure humidity. It is a type of hygrometer, a family of devices that are designed to measure relative humidity. Psychrometers are among the simplest hygrometers and they can even be made at home with a few supplies from a hardware store. It is also possible to buy them through scientific supply catalogs, as well as stores that stock supplies for measuring climate conditions like temperature and barometric pressure.
To measure relative humidity in an environment, the psychrometer relies on the fact that humidity will change evaporation speed. The device consists of two thermometers. One is a dry bulb thermometer that is exposed to the ambient air to measure the temperature. The second is a wet bulb thermometer, with the bulb dipped in a wet material.
The process of evaporation will cause the wet bulb thermometer to cool down, generating a lower reading than the dry bulb thermometer. The drier it is, the more quickly the water will evaporate and the more radical the cooling effect will be. People can compare the two temperatures and determine the humidity on the basis of the difference. When the two thermometers have the same reading, the relative humidity is 100%. Rarely, the wet bulb thermometer will be warmer, because it is so cold that the water forms ice and the ice insulates the bulb.
A psychrometer chart is often printed on the back of the device or provided with the packaging so that people can quickly look up the relative humidity after they have taken a reading. Charts are also available online and in the manuals that come with some types of scientific instruments. People making a psychrometer at home can take advantage of published charts to read their devices. A simple home made psychrometer can be made with a pair of thermometers, with a muslin wrap for one of the bulbs to wick water up to the bulb for evaporation.
In a variation known as the sling psychrometer, the device is designed to be swung through the air to speed up the evaporation process. Other devices known as ventilated or aspirated psychrometers are designed to work with fans ventilating the wet bulb thermometer. This also speeds evaporation and is designed to allow the water to evaporate more evenly so that the reading will be more accurate. Care must be taken to avoiding heating the thermometer or placing it in a location that might skew the reading.