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What is dry air and How it is measured ?

 



Dry air is the mechanical mixture of gases oxygen, nitrogen, carbon dioxide, hydrogen, argon, neon, krypton, helium, ozone, and xenon. However oxygen and nitrogen make up the major part of the combination. Dry air is considered to consist of  21% oxygen and 79% nitrogen by volume, and 23% oxygen and 77% nitrogen by mass. Completely dry air does not exist in nature. Water Vapor in varying amounts is diffused through it. If pa and pw are the partial pressures of dry air and water vapor respectively, then by Dalton's law of partial pressures, 

                                  pa +pw =p 

where p is the atmospheric pressure. 

mole -fraction of dry air, xa  = pa / p = pa 

and mole fraction of water vapor , xw = pw / p = pw 

Since pw is very small, the saturation temperature of water vapor at pw is less than atmospheric temperature , ta tm . So the water vapor in air exists in superheated state, and air is said to be unsaturated.









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