Dry bulb temperature measures air temperature without considering moisture content. It is important for industries that dry materials using heated or unheated air. Evaporative coolers require high dry bulb and low dew point temperatures. A thermometer should be placed away from heat sources to measure dry bulb temperature. Psychrometric graphs can determine relative humidity and dew point.
The dry bulb temperature is the air temperature that does not take into account any moisture content. A standard thermometer placed indoors or outdoors will measure dry bulb temperature. The amount of moisture in the air, called relative humidity, cannot be determined by dry bulb temperature alone.
Air temperature, humidity and dew point are important considerations for many industries. Materials that are dried using heated or unheated outside air will be affected by the relative humidity of the air. Products such as tobacco, dried fruits, dried vegetables and dried meats are sensitive to both dry bulb temperature and dew point. Home and commercial air conditioning is designed for both expected dry air temperatures and humidity, as both high temperatures and humidity can increase operating time and reduce cooling system efficiency.
Dry bulb should not be confused with air dry. Dry air contains very little moisture and therefore has a low dew point. Dry bulb temperature simply measures the temperature of the air without any measurement of the humidity in it, and is often referred to as ambient air temperature or simply room temperature.
In desert climates, evaporative coolers use the low relative humidity of the outside air to evaporate water and cool living spaces by passing large volumes of outside air over a moist cooling material. Evaporative coolers, often referred to as swamp coolers, require both high dry bulb temperatures and low dew point temperatures to operate efficiently. Determining evaporative cooling efficiency requires knowing three temperatures: the dry bulb temperature, the wet bulb temperature, and the dew point. Dew point temperatures can be obtained from local weather services and indicate the temperature at which the air is totally saturated. Fog visible on a humid day, when the air can no longer hold moisture, is an example of saturated air.
Dry bulb temperature measurement is relatively simple. A liquid or digital thermometer should be placed in a place out of the sun and away from heated or cooled buildings. The thermometer should also be kept away from driveways or dark streets that can radiate heat on sunny days. The thermometer must reach a constant temperature before taking any measurements. To avoid erroneous readings, the thermometer should not be held or manipulated during measurements.
Psychrometric graphs show the relationship between temperature and humidity and are useful for determining relative humidity and dew point. After measuring dry bulb temperature and wet bulb temperature, these two temperatures and a psychrometric chart can be used to determine relative humidity. A simple device for measuring dry-bulb and wet-bulb temperatures is a sling psychrometer, which is a pair of thermometers attached to a chain or handle that can be twisted by hand to obtain dry-bulb and wet-bulb temperatures simultaneously.
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