# Saturation vapor density

Humidity and hygrometry
Specific concepts
General concepts
Measures and Instruments

Saturation vapor density (SVD) is a concept closely tied with saturation vapor pressure (SVP). It is useful for getting an 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: PV = nRT 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 kelvin. 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, setting V to 1 cubic meter, we get Pm/RT = M/V = density.

The values shown at hyperphysics-sources[1] indicate that the saturated vapor density is 4.85 g/m3 at 273 K, at which the saturated vapor pressure is 4.58 mm of Hg or 610.616447 Pa (760 mm of Hg ≈ 1 atm = 1.01325 * 105 Pa).

Therefore, for particular mole number and volume the saturated vapor pressure will not change if the temperature remains constant.