Psychrometry measures water vapor and heat in air, used in refrigeration, clean room design, and manufacturing. Dry and wet bulb temperatures and relative humidity are key concepts. Hygrometers measure wet bulb temperature, and relative humidity measures current humidity compared to saturation.
Psychrometry is the measurement of water vapor and heat in a sample of air. It is used in the refrigeration industry, in clean room design, in some manufacturing processes and for theoretical applications. Psychrometry is a branch of thermodynamics and deals with terms such as dry bulb temperature, wet bulb temperature, specific heat and relative humidity. Knowledge of psychrometrics is a prerequisite for precise control of humidity in contained environments. The key concept in psychrometrics is the relationship between wet bulb temperature, dry bulb temperature and relative humidity.
Psychrometry applies the well-known relationship between humidity and temperature in air to practical problems. In psychrometrics, dry bulb temperature refers to the air temperature as measured by a conventional thermometer. Wet bulb temperature is measured with a device called a hygrometer, which is designed to measure 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 in the air, usually using a hand-held rotary 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 psychrometry, two of the three values (dry bulb temperature, wet bulb temperature, and relative humidity) can be used to calculate the latter.
Relative humidity measures the current humidity in a space compared 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 the water vapor begins to condense back into water. The higher the relative humidity, the more similar the wet bulb temperature is to the dry bulb temperature. If the water does not evaporate during the rotation of the hygrometer due to supersaturation of the air, the wet bulb temperature cannot decrease. In contrast, absolute humidity is the measure of the actual density of water vapor in a given sample of air. Hygrometers are used in greenhouses, industrial spaces and some saunas, humidifiers and museums.
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