Vaisala HMP243 Guide de l'utilisateur Page 118

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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
=
F
H
I
K
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
oi
i
i
TCT
(4)
where: T = temperature in K
C
i
= coefficients
C
0
= 0.4931358
C
1
= -0.46094296 * 10
-2
C
2
= 0.13746454 * 10
-4
C
3
= -0.12743214 * 10
-7
The vapour pressure (in Pascals) is calculated using equation:
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