Where p simply notes the absolute pressure in the observed system. p d is found considering partial pressure, resulting in:.This will give a result in mb, 1 mb=100 Pa.A simplification of the regression used to find this, can be formulated as: The saturation vapor pressure of water at any given temperature is the vapor pressure when relative humidity is 100%. The vapor pressure of water may be calculated from the saturation vapor pressure and relative humidity. R v = Specific gas constant for water vapor, 461.495 J/(kg R d = Specific gas constant for dry air, 287.05 J/(kg Using this method, error in the density calculation is less than 0.2% in the range of −10 ☌ to 50 ☌. In this case, the partial pressure of water vapor is known as the vapor pressure.

The density of humid air may be calculated as a mixture of ideal gases. Hence the mass per unit volume of the gas (its density) decreases. So when water molecules (vapor) are introduced to the air, the number of air molecules must reduce by the same number in a given volume, without the pressure or temperature increasing. For any gas, at a given temperature and pressure, the number of molecules present is constant for a particular volume. This occurs because the molecular mass of water (18) is less than the molecular mass of air (around 29). The addition of water vapor to air (making the air humid) reduces the density of the air, which may at first appear contrary to logic. At standard ambient temperature and pressure (70 ☏ and 14.696 psia), dry air has a density of ρ SATP = 0.075 lb m/ft 3 ~ 1.2 kg/m 3.At standard ambient temperature and pressure (25 ☌ and 100 kPa), dry air has a density of ρ SATP = 1.168 kg/m 3.At standard temperature and pressure (0 ☌ and 101.325 k Pa), dry air has a density of ρ STP = 1.292 kg/m 3.The specific gas constant R for dry air is: Where ρ is the air density, p is pressure, R is the specific gas constant, and T is temperature in kelvins. The thermodynamic properties of air along the saturation curve are given in Table 1 these properties for the liquid and gaseous air-in Table 2.The formula for the density of dry air is given by:

The equilibrium dissociation degree can be calculated according to the Saha equation. This implies the transition of air into the plasma state. Their equilibrium concentration can be derived from the isotherm equations of the respective reactions.Īt temperatures higher than 2000 K and moderate pressures the nitrogen and oxygen start to dissociate, and at temperatures exceeding 4000 K and atmospheric pressure the ionization of oxygen, nitrogen, and other components becomes evident. The nitrogen reacts with oxygen producing various oxides: N 2O, NO, NO 2, NO 3. The normal condensation temperature of air is -191.4☌, the normal boiling temperature -194☌.Īt elevated temperatures air undergoes some physicochemical transformations. Liquid air at atmospheric pressure behaves practically as an ideal solution following the Raoult's Law.

The normal (at 0.1013 MPa) boiling (condensation) temperature of the oxygen is equal- 183☌, that of the nitrogen -195.8☌. Where v denotes specific volume u is specific internal energy R is the gas constant for air.Īt low temperatures the air is liquified.

Dynamic viscosity of air at 25 c