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psychrometer(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
..... Click the link for more information. 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
..... Click the link for more information. .
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
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.