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hygrometer
- Definição1
- An instrument for determining the humidity of the air. The dew-point hygrometer developed byDaniell, Regnault and others aims at determining the dew-point by artificially cooling, until dew is deposited, a polished surface, whose temperature can be measured. Since dew-point hygrometers depend only on the measurements of a temperature and involve no assumption as to the properties of the materials or the laws of evaporation from a wet surface, they may be classed as " absolute " instruments. To the same class belong chemical hygrometers in which the quantity of moisture in a given mass of air is determined by direct weighing. Unfortunately, neither dew-point nor chemical hygrometers are suitable for routine use in meteorology and it is, therefore, necessary to use methods depending on (a) the change of length of hairs with varying relative humidity, or (b) the lowering of temperature due to evaporation. In the hair hygrometer, of which Lambrecht's polymeter and Richard's hair hygrograph are applications, use is made of the fact that human hair expands with increasing relative humidity. Hair hygrometers are incapable of high precision, but they possess the advantage that they operate equally well above or below the freezing point. Wet and dry bulb hygrometers (or " psychrometers ") are ordinarily used for routine observations. They depend on the accurate determination of the depression of the wet bulb reading below the dry bulb and give sufficient precision for ordinary needs. Their accuracy is considerably increased if arrangements are made, as in the Assmann PSYCHROMETER, for " aspirating " the air at a known speed.
- Fonte1
- GRÃ-BRETANHA. Meteorological Office. The meteorological glossary. 2nd ed. London: H. M. Stationery Office, 1930.
- Definição2
- An instrument used for the measurement of humidity. The most common type of hygrometer is the wet and dry bulb thermometer arrangement.
- Fonte2
- CROCKER, David. Dictionary of aviation. 2nd ed. London: A&C, 2005.
- Fonte3
- WORLD METEOROLOGICAL ORGANIZATION. Guide to meteorological instruments and methods of observation. Geneva, 2012. (WMO, n.8).
- Contexto
- Both types of EMR absorption hygrometer require frequent calibration and are more suitable for measuring changes in vapour concentration rather than absolute levels. The most widespread application of the EMR absorption hygrometer is in the monitoring of very high-frequency variations in humidity, since the method does not require the detector to achieve vapour pressure equilibrium with the sample. The time constant of an optical hygrometer is typically just a few milliseconds. The use of optical hygrometers remains restricted to research activities.
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- hygromètre