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(sīkrŏm`ĭtər), one of many instruments used for measuring the water vapor content or relative humidityhumidity,
moisture content of the atmosphere, a primary element of climate. Humidity measurements include absolute humidity, the mass of water vapor per unit volume of natural air; relative humidity (usually meant when the term humidity
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 of the atmosphere. It consists of two identical thermometers—the wet-bulb thermometer, so called because its bulb is covered with a jacket of tight-fitting muslin cloth that can be saturated with distilled water; and the dry-bulb thermometer. When the cloth is soaked and the thermometers are properly ventilated, the wet-bulb temperature will be lower than the dry-bulb temperature (actual air temperature) because of cooling due to the evaporation of water from the cloth. The drier the air is, the greater the evaporation and thus the more the wet-bulb temperature is depressed. Psychrometric tables list various humidity variables, such as relative humidity, according to dry-bulb temperature and wet-bulb depression at equilibrium. Ventilation is provided by whirling the thermometers at the end of a chain (sling psychrometer) or by a suction fan (aspiration psychrometer). Newer psychrometers use special electronic sensors. See hygrometerhygrometer
, instrument used to measure the moisture content of a gas, as in determining the relative humidity of air. The temperature at which dew or frost forms is a measure of the absolute humidity—the weight of water vapor per unit volume of air or other gas at the
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an instrument for measuring the humidity and temperature of the air. The psychrometer consists of a dry-bulb thermometer and a wet-bulb thermometer. The dry-bulb thermometer shows the temperature of the air, and the wet-bulb thermometer, whose bulb is wrapped with wet cambric, shows a temperature that is dependent on the amount of evaporation from the surface of its bulb. Because heat is used in evaporation, the drier the air whose humidity is being measured, the lower the reading of the wet-bulb thermometer.

The vapor pressure or relative humidity is determined from the readings of the dry- and wet-bulb thermometers with the aid of a psychrometric table, alignment charts, or slide rules derived from the psychrometric formula. For temperatures below — 5°C, where the moisture content of air is very low, the psychrometer does not give reliable results. Under these conditions, a hair hygrometer is preferred.

Weather-station, aspiration, and distant-recording psychrometers are the main types of psychrometers. In the weather-station

Figure 1. Aspiration psychrometer: (1) thermometers, (2) ventilating fan, and (3) radiation shields

psychrometer, the thermometers are fastened to a special frame at a weather shelter. One drawback of this type of psychrometer is the dependence of the wet-bulb thermometer reading on the velocity of the air flow at the station. In the aspiration psychrometer (Figure 1), the thermometers are placed in a special holder, which protects them against damage and the warming effect of direct solar rays; a fan is used to ventilate the wet bulb with the air being studied, at a constant rate of about 2 m/sec. At air temperatures above zero, the aspiration psychrometer is the most reliable instrument for measuring air humidity and temperature. The distant-recording psychrometer uses resistance thermometers, thermistors and thermocouples.


Sternzat, M. S. Meteorologicheskie pribory i nabliudeniia. Leningrad, 1968.


A device comprising two thermometers, one a dry bulb, the other a wet or wick-covered bulb, used in determining the moisture content or relative humidity of air or other gases. Also known as wet and dry bulb thermometer.


An instrument used to measure humidity in the atmosphere from two thermometers which are similar except that the bulb of one is kept wet, the bulb of the other being dry.

dry- and wet-bulb hygrometer

dry- and wet-bulb hygrometerclick for a larger image
An instrument to measure the dew point, relative humidity, and vapor pressure. It essentially consists of two ordinary, accurate, mercury thermometers. One has thin muslin wrapped around it and is kept wet; it is called a wetbulb thermometer. The other is a dry-bulb thermometer. The dry-bulb thermometer shows the current temperature, whereas the wet-bulb thermometer shows a somewhat lower temperature, the result of the latent heat of evaporation, which depends on the relative humidity of the air. The difference between the two thermometers is a measure of the relative humidity of the air, which is calculated by a formula. Also called a psychrometer.

sling psychrometer

An instrument used to measure the amount of water vapor or relative humidity in the air. It is a combination of wet- and dry-bulb thermometers mounted on a frame with a handle that allows it to be swung around to permit air to move across the psychrometer. See psychrometer.
References in periodicals archive ?
As noted previously, the aspirated psychrometer is a device that is used to measure the wet-bulb temperature.
Temperature and relative humidity were recorded using digital psychrometer units (Jenway-digital psychrometer 5105, U.
When the answer was otherwise, the room temperature was measured using sling psychrometer.
N=74 Thermometer 42 yes 18 no Hygrometer 32 yes 22 no Recording hygrothermograph 44 yes 18 no Sling psychrometer 16 yes 29 no Temperature/Humidity data logger 42 yes 20 no Other 4 yes 3.
Measuring relative humidity above 90 percent is best with a psychrometer using d.
We recommend that at least two of your staff members learn how to take the WBGT by using the sling psychrometer prior to all summer workouts and practices.
Temperature and air relative humidity were monitored with a psychrometer installed next to the trays containing the samples.
c] is the thermal capacity of dry air, v is the constant of psychrometer, and L is the parameter of thermal exchange of water evaporation.
The black globe thermometer and the psychrometer were located near the subject (about 1 m away from the body), and measurements were made every ten minutes during the experiment.
The relative humidity in the room, measured with a sling psychrometer, was approximately 76 percent, which minimized the desiccation that the relatively high air change rate could have caused.
When we compared our CaliMat with our usual method of IR thermometer validation (see the October 1999 issue of the Journal) and the ambient conditions as measured by our temperature-standard motorized psychrometer (see the January/February 2002 issue of the Journal) , we found the liquid-crystal thermometer to be remarkably accurate.
We have kept the pen-type electronic psychrometer, but we discarded the sling psychrometer when our last one broke, resulting in a minor mercury spill.