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Psychrometrics is the measurement of water vapor and heat in an air sample. It is used in the refrigeration industry, the design of clean rooms, certain manufacturing processes, and for theoretical applications. Psychrometry is a branch of thermodynamics and deals with terms like dry-bulb temperature, wet-bulb temperature, specific heat, and relative humidity. An understanding of psychrometry is a prerequisite for precise control of the humidity of contained environments. The key concept in psychrometrics is the relationship between wet-bulb temperature, dry-bulb temperature, and relative humidity.
Psychrometrics applies the well-understood relationships between humidity and temperature in air to practical problems. In psychrometrics, dry-bulb temperature refers to the temperature of air as measured by a conventional thermometer. Wet-bulb temperature is measured with a device called a hygrometer, designed to measure the temperature in a way that reflects the cooling properties of evaporating water.
A hygrometer consists of two thermometers, one attached to a wick soaked in distilled water, the other left alone. The two thermometers are spun through the air, usually using a hand-operated spinning device. The movement through the air causes the water on the wick to evaporate, lowering the temperature of the wet-bulb thermometer. The difference between the two thermometer measurements is then used to determine the wet-bulb temperature. Due to the nature of psychrometrics, any two of three values -- dry-bulb temperature, wet-bulb temperature, and relative humidity -- can be used to compute the last.
Relative humidity measures the current humidity in a space relative to the highest possible humidity the space can have before becoming saturated with water vapor. When a space is saturated with water vapor, it is said to reach its dew point, the point at which water vapor starts to condense back into water. The greater the relative humidity, the more similar the wet-bulb temperature is to the dry-bulb temperature. If no water evaporates when the hygrometer is spun due to the air being supersaturated, then the temperature of the wet-bulb cannot decrease. By contrast, absolute humidity is the measure of the actual density of water vapor in a given sample of air. Hygrometers are used in green houses, industrial spaces, and some saunas, humidors and museums.