Vaisala HMT130 - Manual (Page 89)

Appendix A _______________________________________________________ Calculation Formulas
VAISALA ________________________________________________________________________ 87
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
Entalpy can be calculated from mixing ratio using:
x x T h ⋅ + ⋅ + ⋅ = 5 . 2 ) 00189 . 0 01 . 1 ( [ ]
kg kJ /
(5)