
HMP260-U017en-1.5 Appendix 6: Calculation formulas
1 CALCULATION
The HMP260 series transmitters measure relative humidity and temperature. From these
values dewpoint, mixing ratio and absolute humidity are calculated using the following
equations
dewpoint: T
m
P
RH
d
n
ws
=
⋅
F
H
I
K
−
log
1
(1)
mixing ratio: x RH
p RH P
ws
ws
= ⋅ ⋅
⋅ − ⋅
621 98
100
.
b g
(2)
absolute humidity:
a RH
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
P A
ws
m t
t T
n
= ⋅
⋅
+
H
G
K
J
10
. (4)
The parameters A, m, and T
n
depend on temperature according to the following table
t A m T
n
-40 ... 50 °C 6.1078 7.5000 237.3
50 ... 100 °C 5.9987 7.3313 229.1
100 ... 150 °C 5.8493 7.2756 225.0
150 ... 180 °C 6.2301 7.3033 230.0
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