Appendix A. Calculation Formulas
A.1 Calculation Formulas Overview
The transmitter receives the relative humidity and temperature values from the HMP110 probe.
Based on these temperature and relative humidity values, the transmitter calculates dewpoint,
frostpoint, absolute humidity, mixing ratio, enthalpy, wet bulb temperature, saturation vapor
pressure and vapor pressure in normal pressure.
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 Enthalpy (kJ/kg)
T
w
Wet bulb temperature (°C)
T
df
Dew/frostpoint temperature (°C)
A.2 Dewpoint Temperature
The dewpoint temperature (T
d
) 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 (T
d
) is calculated with the following formula:
=
log
1 °
Pw is the water vapor pressure (see the separate calculation formula for P
w
). The parameters
A, m, and Tn depend on the temperature as shown in Table 47 (page 77).
Table 47 Temperature Dependence for A, m, and Tn
T A m Tn
... 0 °C 6.119866 7.926104 250.4138
0 ... 50 °C 6.1078 7.5000 237.30
Appendix A – Calculation Formulas
77
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