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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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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



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.
The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


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.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


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.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.

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.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved
References in periodicals archive ?
After doing some research on the measurement of humidity--and particularly on measuring units that would meet our needs--we settled on the Psychro-Dyne, a battery-operated, motor-driven, wet-and-dry-bulb thermometer psychrometer manufactured by Industrial Instruments & Supplies.* Our decision was predicated on the following: First, the difference between the wet- and dry-hulb measurements is the most accurate means of measuring the quantity of moisture in the air.
Soil water potential was measured weekly using thermocouple psychrometers at the 45-cm depth to provide an indication of the soil water stress the plants were exposed to in each water treatment throughout the growing season.
In the 150% water treatment, soil water potentials determined with thermocouple psychrometers at the 10 and 134 kg N [ha.sup.-1] rates were similar for both the diploid and tetraploid entries (Fig.
Psychrometers were maintained in a temperature controlled, insulated box.
The simplest procedure was to put salt solution standards into the psychrometers and repeatedly change the temperature to take readings at 15, 25, and 35 [degrees] C.
This was determined by dipping filter paper discs in the PEG solution and measuring wet-bulb depression on six individually calibrated thermocouple psychrometers.
After steady-state conditions were achieved on a given leaf blade, which usually took about 15 min, the blade area inside the reaction chamber was removed, two 0.24-[cm.sup.2] samples were taken with two psychrometers, and total blade area determined.
Soil water potential was measured weekly at the 45-cm depth using thermocouple psychrometers with results standardized to 25 [degrees] C.
Measurements of water potential ([Psi]) and solute potential ([Psi]s) of the same terminal leaflets of mainstem leaves in the upper canopy were conducted at approximately 0830 h using dewpoint thermocouple psychrometers attached to a Wescor HR 33-T dewpoint microvoltmeter (Beadle et al., 1993; Ferris and Taylor, 1995).
Root water potential was measured using thermocouple psychrometers and a HR-33T dewpoint microvoltmeter (Wescor, Logan.
In situ stem psychrometers (PWS Instruments, Guelph, ON, Canada) were installed on each maize plant at 12 to 15 cm from the base to measure stem water potential.
For OP measurement, a sample consisting of three 1-cm-long midleaf segments was sealed in a thermocouple psychrometer cup (2-mL volume) and freeze-killed at -20 [degrees] C.