Absolute humidity (a), Absolute humidity at ntp (antp), Wet bulb temperature – Vaisala DM500 User Manual
Page 133: Enthalpy (h)
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
+
⋅
=
T
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
-
w
tot
ws
w
T
T
P
P
P
⋅
⋅
=
(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.