
Appendix A __________________________________________________________ Humidity Theory
VAISALA ______________________________________________________________________ 131
Absolute Humidity (a)
Absolute humidity (a) is the mass of water vapour present in unit
volume of moist air of a given temperature and pressure.
15.273
16679.2
+
⋅=
P
a
w
(6)
where
T = temperature (ºC)
a = absolute humidity (g/m
3
)
P
w
= water vapor pressure
Absolute Humidity at NTP (aNTP)
NTP
NTP
T
Pw
P
P
a ⋅⋅= 16679.2(7)
where
a
NTP
= absolute humidity at NTP (g/m
3
)
P
w
= water vapor pressure (Pa)
P = pressure (Pa)
Normal conditions are defined as: P
NTP
= 101 325 (Pa)
T
NTP
= 273.15 (K)
Wet Bulb Temperature
)-(000662.0-
wtotwsw
TTPPP ⋅⋅= (8)
where
P
w
= water vapor pressure (Pa)
P
ws
= saturation water vapor pressure at temperature T
w
(Pa)
Enthalpy (h)
Enthalpy (h) is measure of the total energy in a humid gas. Enthalpy is
a function of the gas temperature and pressure, and of the moisture
content, since water absorbs energy on changing from condensed state
to vapour.
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