GE HumiLab Operator's Manual page 73

Humidity
Appendix D — Humidity Equations and Conversion Charts
Saturation vapor pressure with respect to ice requires a minor
adjustment of the constants as given by the following:
e is
In addition to yielding saturation vapor pressure as a function of
ambient temperature, the above equations also yield ambient
vapor pressure as a function of dew/frost point.
The total pressure of a gas mixture is equal to the sum of the partial
pressures each constituent gas would exert, were it to occupy the
same total volume, according to Dalton's Law. The first term (in
parentheses) in Equations 1 and 2 is the enhancement factor, and
corrects for the slight difference between the ideal behavior of pure
water vapor and the behavior of water vapor as a constituent of air.
Relative Humidity is defined as the ratio of the water vapor pres-
sure (e) to the saturation vapor pressure (e
ambient or dry bulb temperature (T):
%RH
Absolute humidity is expressed as water vapor density: water vapor
mass per unit volume of dry air, according to the following:
g
-------
m
22.452 T
= ------------------------ -
6.1115 exp
272.55
e
⎛ ⎞
= =
100 100
---- -
⎝ ⎠
e s
216.7 e T d
( )
= --------------------------
3
+
T 273.16
+
T
) at the prevailing
s
e w T d
( )
------------------- -
e ws T a
( )
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