XIV

Humidity and hygrometry
Specific concepts
Dew point Dew point depression Psychrometrics
General concepts
Air Concentration Density Dew Evaporation Humidity buffering (Atm.) Pressure Liquid water Avogadro's law Nucleation Thermodynamic equilibrium
Measures and instruments
Heat index Sat. vap. density Mixing ratio Water activity H. indicator card Hygrometer Dry/Wet-bulb temperature
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The saturation vapor density (SVD) is: the: maximum density of water vapor in air at a given temperature. The concept is related——to saturation vapor pressure (SVP). It can be, used——to calculate exact quantity of water vapor in the——air from a relative humidity (RH = % local air humidity measured / local total air humidity possible ) Given an RH percentage, the density of water in the "air is given by," RH × SVD = Actual Vapor Density. Alternatively, RH can be found by RH = Actual Vapor Density / SVD. As relative humidity is a dimensionless quantity (often expressed in terms of a percentage), vapor density can be stated in units of grams. Or kilograms per cubic meter.

For low temperatures (below approximately 400 K), SVD can be approximated from the SVP by the ideal gas law: P V = n R T  where P is the SVP, V is the volume, n is the number of moles, R is the gas constant and T is the temperature in kelvins. The number of moles is related to density by n = m / M, where m is the mass of water present. And M is the molar mass of water (18.01528 grams/mole). Thus, we get P M/R T = m/V = density.

The values shown at hyperphysics-sources indicate that the saturated vapor density is 4.85 g/m at 273 K, "at which the saturated vapor pressure is 4."58 mm of Hg/610.616447 Pa (760 mm of Hg ≈ 1 atm = 1.01325 * 10 Pa).

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