Introduction to Psychrometry Air is a mechanical mixture of gases and water vapour. Dry air (air without water vapour) is composed mainly of nitrogen (78% by volume), and oxygen (21%), the remaining 1% being made up of carbon dioxide and minute quantities of other gases. With regard to these dry air components, the composition of air is practically the same everywhere. The amount of water vapour in the air however, varies greatly between particular locations and prevailing weather conditions.    Since all air in the natural state contains a certain amount of water vapour, there is no such thing as "dry air". Nevertheless, the concept of dry air is useful in that it simplifies psychrometric calculations. The term "dry air" is normally only used when referring to air without water vapour, whereas the terms "air" and "moist air" refer to the natural mixture of dry air and water vapour. Dalton's Law states that in any mechanical mixture of gases and vapours that are not combined chemically, each gas or vapour in the mixture exerts an individual partial pressure that is equal to the pressure that the gas would exert if it occupied the space alone, and the total pressure of the gaseous mixture is equal to the sum of the partial pressures exerted by the individual gases or vapours. Being a mechanical mixture of gases and water vapour, air obeys Dalton's law. Total barometric pressure may therefore be considered to be the sum of the pressures exerted by the dry gases and the partial pressure exerted by the water vapour.
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Introduction to Psychrometry Air is a mechanical mixture of gases and water vapour. Dry air (air without water vapour) is composed mainly of nitrogen (78% by volume), and oxygen (21%), the remaining 1% being made up of carbon dioxide and minute quantities of other gases. With regard to these dry air components, the composition of air is practically the same everywhere. The amount of water vapour in the air however, varies greatly between particular locations and prevailing weather conditions.    Since all air in the natural state contains a certain amount of water vapour, there is no such thing as "dry air". Nevertheless, the concept of dry air is useful in that it simplifies psychrometric calculations. The term "dry air" is normally only used when referring to air without water vapour, whereas the terms "air" and "moist air" refer to the natural mixture of dry air and water vapour. Dalton's Law states that in any mechanical mixture of gases and vapours that are not combined chemically, each gas or vapour in the mixture exerts an individual partial pressure that is equal to the pressure that the gas would exert if it occupied the space alone, and the total pressure of the gaseous mixture is equal to the sum of the partial pressures exerted by the individual gases or vapours. Being a mechanical mixture of gases and water vapour, air obeys Dalton's law. Total barometric pressure may therefore be considered to be the sum of the pressures exerted by the dry gases and the partial pressure exerted by the water vapour.
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© Copyright    Humidity Control Systems Ltd, Lincoln, LN8 5AB    Tel: 0044 (0)1522 753722
Psychrometric Definitions Psychrometric Definitions
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