Vaisala PTU300 - Manual (Page 207)

Appendix B _______________________________________________________ Calculation Formulas
VAISALA _______________________________________________________________________ 205
∑
− =
Θ + Θ =
3
1
4
ln ln
i
i
i ws
b b P
(6)
where:
bi = coefficients
b-1 = -0.58002206 * 104
b0 = 0.13914993 * 101
b1 = -0.48640239 * 10-1
b2 = 0.41764768 * 10-4
b3 = -0.14452093 * 10-7
b4 = 6.5459673
The water vapor pressure is calculated using:
100
ws
w
P
RH P
⋅ =
(7)
Parts per million by volume is calculated using:
( )
w
w
v
P p
P
ppm
−
⋅ = 6
10
(8)
Symbols:
T
d
= dewpoint temperature (°C)
P
w
= water vapor pressure (hPa)
P
ws
= water vapor saturation pressure (Pa)
RH = relative humidity (%)
x = mixing ratio (g/kg)
p = atmospheric pressure (hPa)
A = absolute humidity (g/m3)
T = temperature (K)
h = enthalpy (kJ/kg)