Owner's manual

Appendix B _______________________________________________________ Calculation Formulas
VAISALA _______________________________________________________________________ 173
APPENDIX B
CALCULATION FORMULAS
This Appendix contains the formulas used for the calculated output
quantities.
The DMT346 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:
1
log
−






=
A
Pw
m
T
T
n
d
(1)
P
w
is the water vapor pressure.The parameters A, m, and T
n
depend on
temperature according to the following table:
t
A
m
T
n
<0 °C
1)
6.1134
9.7911
273.47
0 ... 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
1)
Used for frostpoint calculation if the dewpoint is negative
Mixing ratio:
w
w
Pp
P
x
-
99.621 ×=
(2)