Calculation formulas – Vaisala HMP230 User Manual
Page 139
HMP230 SERIES
M210225en-B
Appendix 6: Calculation formulas
133
CALCULATION FORMULAS
The HMP230 series transmitters measure relative humidity and temperature.
From these values dewpoint, mixing ratio, absolute humidity and enthalpy in
normal pressure are calculated using the following equations
dewpoint:
T
T
m
P
RH
A
d
n
ws
=
⋅
⋅
ж
из
ц
шч
−
log
100
1
(1)
mixing ratio:
(
)
x
RH
P
p RH P
ws
ws
=
⋅
⋅
⋅ −
⋅
621 98
100
.
(2)
absolute humidity:
(
)
a
RH
P
t
ws
=
⋅
⋅
⋅ +
216 68
100
273 2
.
.
(3)
enthalpy:
x
x
T
h
⋅
+
⋅
+
⋅
=
5
.
2
)
00189
.
0
01
.
1
(
(4)
where
T
d
=
dewpoint temperature (°C)
P
ws
=
partial pressure of saturated water vapour (mbar)
RH
=
relative humidity (%)
x
=
mixing ratio (g/kg)
p
=
atmospheric pressure (mbar)
a
=
absolute humidity (g/m
3
)
T
=
temperature (°C)
h
=
enthalpy (kJ/kg)
The partial pressure of water P
ws
is calculated using equation:
å
=
−
=
Θ
3
o
i
i
i
T
C
T
(5)
where:
T
= temperature in K
Ci
= coefficients
C0 = 0.4931358
C1 = -0.46094296 * 10-2
C2 = 0.13746454 * 10-4
C3 = -0.12743214 * 10-7