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October 2007
Humidity
B-2
Relative Humidity is defined as the ratio of the water vapor pressure
(e) to the saturation vapor pressure (e
dry bulb temperature (Ta):
=
100
%RH
Absolute humidity is expressed as water vapor density: water vapor
mass per unit volume of dry air, according to the following:
------ -
M
Water vapor content expressed as parts per million by volume is given
by the following:
Expressing water vapor content as parts per million by weight (or
mixing ratio) requires multiplication of the above by the ratio of the
molecular weight of water to that of air as given by the following:
E
6
PPM
=
---------- -
10
W
P E
–
See Figure B-1 on page B-3 for a graphical humidity conversion
chart.
) at the prevailing ambient or
S
(
)
E
T
E
⎛
⎞
W
D
---- -
=
100
------------------- -
⎝
⎠
E
(
)
E
T
S
WS
A
(
)
216.7E T
G
D
=
------------------------- -
T 273.16
+
3
(
)
E T
6
D
PPM
=
10
------------ -
V
P
18
×
--------------------------------------------------------------------------------------- -
mw of carrier gas for air (24 g/mole)
Humidity Equations and Conversion Chart
(B-3)
(B-4)
(B-5)
(B-6)
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