Calculation formulas, Dewpoint temperature, Appendix a – Vaisala HMT130 User Manual
Page 87: Calculated output quantities
Appendix A _______________________________________________________ Calculation Formulas
VAISALA ________________________________________________________________________ 85
APPENDIX A
CALCULATION FORMULAS
This appendix contains the formulas used for the calculated output
quantities.
HMT130 transmitter receives relative humidity and temperature from the
HMP110 probe. From these values dewpoint, frostpoint, absolute
humidity, mixing ratio, enthalpy, wet bulb temperature, saturation vapor
pressure and vapor pressure in normal pressure are calculated using the
following equations.
Symbols:
T
d
= Dewpoint temperature (°C)
P
w
= Water vapor pressure (hPa)
P
ws
= Saturation vapor pressure (hPa)
RH = Relative humidity (%)
x = Mixing ratio (g/kg)
p = Atmospheric pressure (hPa)
a = Absolute humidity (g/m3)
T = Temperature (°C)
h
Tw
Tdf
=
=
=
Enthalpy (kJ/kg)
Wet bulb temperature (°C)
Dew/frostpoint temperature (°C)
Dewpoint Temperature
The Dewpoint temperature (Td) of a moist air sample is the temperature
to which the sample must be cooled to reach saturation with respect to
liquid water.
Dewpoint temperature is calculated using:
1
log
−
=
A
P
m
T
T
w
n
d
[ ]
C
(1)