Vaisala HMP155 - Manual (Page 81)

Appendix A ______________________________________________________ Calculation Formulas
VAISALA _______________________________________________________________________ 79
APPENDIX A
CALCULATION FORMULAS
This Appendix contains the formulas used for the calculated output
quantities.
The HMP155 series probe measures relative humidity and temperature.
From these values dew point and frost point, mixing ratio, wet bulb
temperature, absolute humidity and enthalpy in normal pressure are
calculated using the following equations:
Dew point/frost point:
15 . 273
1
1078 . 6
log
5 . 7
3 . 237
10
/





















w
f d
P
T
(1)
P
w
is the water vapor pressure. Dew point/frost point is measured in
Kelvin.
Mixing ratio:
w
w
P p
P
x
-
99 . 621
 
(2)
Absolute humidity:
T
P
C A w
 
(3)
where
C = 216.679