
Appendix A. Calculation formulas
HMP155 measures relative humidity and temperature. The following equations are used for
calculating dew point and frost point, mixing ratio, wet bulb temperature, absolute humidity,
and enthalpy in normal pressure.
Dew point / frost point (1)
Td/f =
237.3
7.5
log
10
Pw
6.1078
− 1
+ 273.15
Parameter
Description
P
w
Water vapor pressure. Dew point/frost point is measured in Kelvin.
Mixing ratio (2)
x = 621.99 ×
Pw
p − Pw
Absolute humidity (3)
A = C ⋅
Pw
T
Parameter
Description
C
216.679
Enthalpy (4)
ℎ = T ⋅ 1.01 + 0.00189 ⋅ x + 2.5 ⋅ x
The water vapor saturation pressure at temperature T
pws
is calculated over water and over ice
by using 4 equations (5-8). These formulas are taken from Hyland, R.,Wexler, A.:
Formulations
of the Thermodynamic Properties of the Saturated Phases of H2O from 173.15 K to 473.15 K
,
Ashrae transactions 1983, Part 2A. pp. 500-513.
Appendix A – Calculation formulas
89
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