This quantity is also known as the water vapor density. The mass of water vapor per unit mass of dry air containing the water vapor. Specific humidity is defined as the mass of water vapor as a proportion of the mass of the moist air sample (including both dry air and the water vapor) it is closely related to humidity ratio and always lower in value. Main article: Humidity Specific Humidity Knowing both the dry-bulb temperature (DBT) and wet-bulb temperature (WBT), one can determine the relative humidity (RH) from the psychrometric chart appropriate to the air pressure.
#ASHRAE PSYCHROMETRIC CHART PRINTABLE MANUAL#
A sling psychrometer requires manual operation to create the airflow over the bulbs, but a powered psychrometer includes a fan for this function. Errors up to 15% can occur if the air movement is too slow or if there is too much radiant heat present (from sunlight, for example).Ī wet bulb temperature taken with air moving at about 1–2 m/s is referred to as a screen temperature, whereas a temperature taken with air moving about 3.5 m/s or more is referred to as sling temperature.Ī psychrometer is a device that includes both a dry-bulb and a wet-bulb thermometer. Speeds up to 5,000 ft/min (~60 mph) are best but it may be dangerous to move a thermometer at that speed. The accuracy of a simple wet-bulb thermometer depends on how fast air passes over the bulb and how well the thermometer is shielded from the radiant temperature of its surroundings. The value indicated by a wet-bulb thermometer often provides an adequate approximation of the thermodynamic wet-bulb temperature.
The thermodynamic wet-bulb temperature is a thermodynamic property of a mixture of air and water vapor. Psychrometric properties Dry-bulb temperature (DBT) Wood destroying fungi generally do not grow at relative humidities below 75%. Molds and fungi can be controlled by keeping relative humidity low. In many industrial applications it is important to avoid condensation that would ruin product or cause corrosion. In industrial drying applications, such as drying paper, manufacturers usually try to achieve an optimum between low relative humidity, which increases the drying rate, and energy usage, which decreases as exhaust relative humidity increases. Relative humidity is often controlled in manufacturing areas where flammable materials are handled, to avoid fires caused by the static electricity discharges that can occur in very dry air. Industries that use these materials are concerned with relative humidity control in production and storage of such materials. Such substances include cotton, paper, cellulose, other wood products, sugar, calcium oxide (burned lime) and many chemicals and fertilizers.
Many substances are hygroscopic, meaning they attract water, usually in proportion to the relative humidity or above a critical relative humidity. In human terms, our thermal comfort is in large part a consequence of not just the temperature of the surrounding air, but (because we cool ourselves via perspiration) the extent to which that air is saturated with water vapor.
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