Psychrometer

psychrometer

[sī′kräm·əd·ər]

(engineering)

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.

psychrometer

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

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.

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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

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 It might be outdated or ideologically biased.

Psychrometer

 

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.

REFERENCE

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.