Hygrometer
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Image:Sling psychrometer.JPG Image:Stevenson screen interior.JPG Image:Indoor hygrometer.JPG Hygrometers are instruments used for measuring humidity. The simplest form of a hygrometer consists of two thermometers, one of which has its bulb constantly kept wet. Evaporation from the bulb lowers the temperature so that this thermometer usually shows a lower temperature. However, when the air temperature is below freezing, it is possible for the "wet bulb" (actually a thin coating of ice) to be warmer than the dry bulb. Relative Humidity is computed from the ambient temperature as shown by the "dry bulb thermometer" and the difference in temperature shown by the "wet bulb" and dry bulb thermometers. One device that uses the wet/dry bulb method is the sling psychrometer, where the thermometers are attached to a handle or length of rope and spun around in the air for a few minutes.
There are several other types of hygrometers that are commonly used. Frequently, the devices use the stretching of a human or animal hair under tension to determine the ambient humidity. In order to see changes that occur over time, many hygrometers record the value of humidity on a piece of graduated paper so that the values can be read off the chart.
Modern instruments use electronic means of recording the information. The two most common electronic sensors are capacitive or resistive. The capacitive sensors sense water by applying an AC signal between two plates and measuring the change in capacitance caused by the amount of water present. The resistive sensors use a polymer membrane which changes conductivity according to absorbed water. They can be read by common meters or data acquisition boards. Temperature must also be measured, as it affects the calibration of all these sensors.
Besides green houses, and industrial spaces, hygrometers are also used in some saunas, humidors and museums. The sling or motorized psychrometer is used in meteorology. Template:Commons
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