Vaisala HUMICAP HMT330 SERIES - MODBUS Status Registers (Read-Only); MODBUS Configuration Registers

Appendix B ______________________________________________________ Calculation Formulas
VAISALA______________________________________________________________________ 191
APPENDIX B
CALCULATION FORMULAS
This Appendix contains the formulas used for the calculated output
quantities.
The HMT330 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 Tn
<0 °C * 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)
Absolute humidity:
T
P
a
w
 68.216
(3)