Mixing ratio, Absolute humidity, Enthalpy – Vaisala HMT120 User Manual
Page 81
Appendix A _______________________________________________________ Calculation Formulas
VAISALA ________________________________________________________________________ 79
Mixing Ratio
The mixing ratio (mass of water vapour/mass of dry gas) is calculated
using:
w
w
p-P
P
B
x
×
=
[
]
kg
g /
(3)
where:
B = 621.9907 g/kg
The value of B depends on the gas. 621.9907 g/kg is valid for air.
Absolute Humidity
Absoute humidity is defined as the mass of water vapour in a certain
volume. If ideal gas behaviour is assumed the absolute humidity can be
calculated using:
(
)
15
.
273
+
⋅
=
T
P
C
a
w
[
]
3
/ m
g
(4)
where:
C = 216.679 gK/J
Enthalpy
Enthalpy is the sum of the internal energy of a thermodynamic system.
It can be calculated from mixing ratio using:
x
x
T
h
⋅
+
⋅
+
⋅
=
5
.
2
)
00189
.
0
01
.
1
(
[
]
kg
kJ /
(5)