Fh ik, Tc t – Vaisala HMP240 User Manual
Page 118
HMP240
Appendix 6: HMP243 wiring diagram
M210300en
116
APPENDIX 5: CALCULATION FORMULAS
The HMP240 transmitter measures relative humidity and temperature. From
these values dewpoint, mixing ratio and absolute humidity are calculated using
the following equations:
dewpoint:
T
T
m
P
RH
A
d
n
ws
=
⋅
⋅
FH
IK
−
log
100
1
(1)
mixing ratio:
x
RH
P
p
RH P
ws
ws
=
⋅
⋅
⋅ −
⋅
621 98
100
.
b
g
(2)
absolute humidity:
a
RH
P
t
ws
=
⋅
⋅
⋅ +
216 68
100
273 2
.
.
a
f
(3)
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)
The partial pressure of water P
ws
is calculated using equation:
∑
=
−
=
Θ
3
o
i
i
i
T
C
T
(4)
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
The vapour pressure (in Pascals) is calculated using equation: