| Hygrometer |
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| CATEGORIES ABOUT HYGROMETER | |
| atmospheric thermodynamics | |
| meteorological instrumentation and equipment | |
| navigational equipment | |
| psychrometrics | |
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Hygrometers are instruments used for measuring Humidity . A simple form of a hygrometer is specifically known as a "psychrometer" and consists of two Thermometer s, one of which includes a dry bulb and the other of which includes a bulb that is kept wet to measure '' Wet-bulb Temperature ''. Evaporation from the wet bulb lowers the Temperature , so that the wet-bulb thermometer usually shows a lower temperature than that of the dry-bulb thermometer, which measures '' Dry-bulb Temperature ''. When the air temperature is below freezing, however, the wet bulb is covered with a thin coating of ice and yet may 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 temperatures as shown by the wet-bulb and dry-bulb thermometers. Relative humidity can also be determined by locating the intersection of the wet- and dry-bulb temperatures on a Psychrometric Chart . 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. Accurate calibration of the thermometers used is of course fundamental to precise humidity determination by the wet-dry method; it is also important for the most accurate results to protect the thermometers from radiant heat and ensure a sufficiently high speed of airflow over the wet bulb. One of the most precise types of wet-dry bulb psychrometer was invented in the late 19th century by Adolph Richard Aßmann (1845-1918) in English-language references the device is usually spelled "Assmann psychrometer." In this device, each thermometer is suspended within a vertical tube of polished metal, and that tube is in turn suspended within a second metal tube of slightly larger diameter; these double tubes serve to isolate the thermometers from radiant heating. Air is drawn through the tubes with a fan that is driven by a clockwork mechanism to ensure a consistent speed (some modern versions use an electric fan with electronic speed control)[http://www.sil.si.edu/SmithsonianContributions/HistoryTechnology/text/SSHT-0002.txt . for outdoor use]]Other types of hygrometers are also commonly used to determine the ambient humidity. Such devices frequently use a human or animal hair under tension. The traditional folk art device known as a " Weather House " works on this principle. In order to see changes that occur over time, several hygrometers record the value of humidity on a piece of graduated paper so that the values can be read off the chart. DIFFICULTY OF ACCURATE HUMIDITY MEASUREMENT Humidity measurement is among the more difficult problems in basic metrology. Two thermometers can be compared by immersing them both in an insulated vessel of water and stirring vigorously to minimize temperature variations. A high-quality liquid-in-glass thermometer if handled with care should remain stable for some years. Hygrometers must be calibrated in air, which is a much less effective heat transfer medium than is water, and many types are subject to drift so need regular recalibration. A further difficulty is that most hygrometers sense relative humidity rather than the absolute amount of water present, but RH is a function of both temperature and absolute moisture content, so small temperature variations within the air in a test chamber will translate into RH variations. DEWPOINT HYGROMETERS Dewpoint is the temperature at which a sample of moist air (or any other water vapor) at constant pressure reaches water vapor saturation. At this saturation temperature, further cooling results in condensation of water. ''Cooled mirror dewpoint hygrometers'' are the most precise instruments available. They use a chilled mirror and optoelectronic mechanism to detect condensation on the mirror surface. The temperature of the mirror is controlled by electronic feedback to maintain a dynamic equilibrium between evaporation and condensation on the mirror, thus closely measuring the dewpoint temperature. 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 Sauna s, Humidor s and museums. In residential settings, hygrometers are used to aid humidity control (to eliminate mildew or to reduce dust mite allergen). The sling or motorized psychrometer is used in Meteorology . SEE ALSO EXTERNAL LINKS |
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