Humidity Definitions
Vapor pressure Pw
Vapor pressure refers to the vapor pressure of water in
air or other gas. Water vapor has a partial pressure Pw which is part of the
total pressure (Ptot) of the gas according to Dalton's law
Ptot= Poxygen +
Pnitrogen ...+ Pw
Saturation vapor pressure
Pws
The saturation vapor pressure (Pws) is the equilibrium
water vapor pressure in a closed chamber containing liquid water. It is a
function only of temperature, and it indicates the maximum amount of water that
can exist in the vapor state. This amount increases with increasing temperature.
The "Wexler-Hyland" formula can be used to calculate the water vapor saturation
pressure. (Hyland,R., Wexler,A.: "Formulations of the
Thermodynamic Properties of the Saturated Phases of H2O from 173.15 K to 473.15
K", Ashrae transactions 1983, Part 2A. pp 500-513).
Relative humidity
Relative humidity is defined as the ratio of water
vapor pressure (Pw) to the saturation water vapor pressure
(Pws) at the gas temperature:
NOTE: Above the boiling point of water (100 °C/212 °F),
the saturation vapor pressure Pws is greater than 1013 hPa (normal
atmospheric pressure). Therefore relative humidity cannot reach 100%RH above 100
°C/212 °F in an unpressurized system.
Below the freezing point (0 °C/32
°F) the definition is also valid. Here 100 %RH is also impossible because
condensation will occur at a lower humidity than 100%RH (when the vapor is
saturated against ice). You can find out the maximum relative humidity
value by choosing Tfrost/Tdew for dewpoint parameter and
setting the frostpoint value equal to the ambient temperature. For example at an
ambient temperature of -20 °C and a frostpoint of -20 °C the maximum %RH value
is aproximately 82.2 %RH.
Dewpoint
The Dewpoint temperature (Td) of a moist air or other
gas sample is the temperature to which the sample must be cooled to reach
saturation with respect to liquid water.
At dewpoint
temperature,
Frostpoint
At temperatures above freezing (0 °C/32 °F), saturation
vapor pressure (Pws) is always calculated with respect to water vapor at
equilibrium over a water surface. The corresponding parameter is dewpoint
temperature. At temperatures below freezing, equilibrium can be over either an
ice surface or a water surface.
NOTE: For most calculations, equilibrium
over an ice surface (frostpoint) for test temperatures below 0 °C/32 °F should
be used. This is by convention, although in spoken language it is common to use
the word dewpoint also when referring to frostpoint.
In some
metrological reporting practices, it may be desirable to calculate equilibrium
over a water surface at temperatures below freezing. The water in liquid phase
below normal freezing point is called supercooled water.
Parts per million (ppm)
Parts per million values can be calculated either in
relation to volume (ppmv) or weight (ppmw). For gas measurements, the ppmv is
more commonly used, and is in many cases referred to as ppm.
I:
Volume/volume ppmv dry gas (standard parameter for gas humidity):
Where
Pw=water vapor
pressure
Ptot=total pressure
II: Mass/mass ppmw
dry gas (less commonly used than ppmv for gas humidity):
Where
Pw=water vapor
pressure
Ptot=total pressure
Mw=molecular mass of
water
Md=molecular mass of dry gas
Absolute humidity
Absolute humidity is defined as the mass of water vapor
in a certain volume. If ideal gas behavior is assumed the absolute humidity can
be calculated using:
Where
C= constant 216.679 gK/J
Pw=
vapor pressure in hPa
T= temperature in K
Mixing ratio
The mixing ratio (mass of water vapor/mass of dry gas)
is calculated using:
Where
B=621.9907 g/kg
The value of B depends on the gas. 621.9907 g/kg is valid for
air.
In general the constant can be calculated using:
Where
M(H2O)=molecular weight of
water
M(gas)=molecular weight of gas
Water content
Water content is defined as the absolute humidity of
the gas if it is brought to a standard pressure and temperature state. For
metric units this state is normal ambient pressure (101325 Pa) and 0 °C. For
nonmetric units (lb/MMscf) the standard temperature is 60 °F.