Section 3
Theory of Operation
3-7
w
F
VG V
P
P
T
T
w
r
o
o
=
+
(
)
+
+
0 9
273
273
.
3-12
and
V
F
w
T
273
T
273
P
P
r
w
1
r
o
o
0.9
=
+
+
−
3-13
For computational speed, we make the following approximation:
P
P
P
P
o
o
≅
+
0 9
0 8845 0 1089
.
.
.
3-14
Vapor pressure e (kPa, FCT 38) is computed from the water vapor mole
fraction w and total pressure P (kPa).
e
wP
=
1000
3-15
The dewpoint temperature T
d
(
°
C, FCT 38) is computed from an equation
that was fit to the data of Goff and Gratch (1946), as given by List (1966),
giving saturation vapor pressure as a function of temperature over a range of
-50 to 50
°
C:
T
z
z
d
=
−
242 62
7 6448
.
.
3-16
where
e
z = log
10
.61083
and e is vapor pressure in kPa.
The LI-6262 computes the water vapor weight fraction w
g
(mg/g, FCT 36)
as
w
w
M
g
=
18
3-17
where M is given after equation 3-12.
