A psychrometer measures humidity and is a type of hygrometer. It consists of two thermometers, a dry bulb and a wet bulb, and works by measuring the difference in temperature caused by evaporation. A psychrometer chart is often provided for quick reference. Homemade versions can be made with a pair of thermometers and a muslin wrap. Variations include sling and vented psychrometers, but care must be taken to avoid distorting the reading.
A psychrometer is a device that can be used to measure humidity. It is a type of hygrometer, a family of devices designed to measure relative humidity. Psychrometers are among the simplest hygrometers and can even be made at home with just a few supplies from a hardware store. You can also purchase them through scientific supply catalogs, as well as stores that stock supplies for measuring weather conditions such as temperature and barometric pressure.
To measure relative humidity in an environment, the psychrometer relies on the fact that humidity will change the rate of evaporation. The device consists of two thermometers. One is a dry bulb thermometer which is exposed to ambient air to measure temperature. The second is a wet bulb thermometer, with the bulb immersed in a moist material.
The evaporation process will cause the wet bulb thermometer to cool, resulting in a lower reading than the dry bulb thermometer. The drier it is, the faster the water will evaporate and the more radical the cooling effect. People can compare the two temperatures and determine the humidity based on the difference. When the two thermometers have the same reading, the relative humidity is 100%. Rarely, the wet bulb thermometer will be hotter, 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 in the packaging so people can quickly look up the relative humidity after taking a reading. Graphs are also available online and in manuals that come with some types of scientific instruments. People who make a psychrometer at home can take advantage of the published graphs to read their own devices. A simple homemade psychrometer can be made with a pair of thermometers, with a muslin wrap for one of the bulbs to absorb water up to the bulb for evaporation.
In a variation known as a sling psychrometer, the device is designed to be swung in the air to speed up the evaporation process. Other devices known as vented or aspirated psychrometers are designed to work with fans that ventilate the wet bulb thermometer. This also speeds up evaporation and is designed to allow the water to evaporate more evenly so the reading is more accurate. Care should be taken to avoid heating the thermometer or placing it in a location that could distort the reading.
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