Humid air volume/dry air volume ppm, Effect of pressure on dewpoint, Using a condensation hygrometer – Vaisala DM500 User Manual
Page 134
User's Guide ______________________________________________________________________
132 _________________________________________________________________ M210205EN-D
x
x
T
h
⋅
+
⋅
+
⋅
=
5
.
2
)
00189
.
0
01
.
1
(
(9)
where
x
=
mixing ratio (g/kg)
T
=
temperature (ºC)
h
=
enthalpy
Humid Air Volume/Dry Air Volume PPM
v
Parts per million (PPM) by volume is the ratio of the partial pressure
of the water vapor to the partial pressure of dry gas. This can be
calculated with a reasonable accuracy with following formula:
6
10
))
-
/(
(
⋅
=
w
tot
w
v
P
P
P
PPM
(10)
where
P
w
=
water vapor pressure
P
tot
=
total pressure
Effect of Pressure on Dewpoint
Changing the total pressure of a gas changes the partial pressures of
the component gases according to Dalton’s law, and therefore changes
the dewpoint/frostpoint temperature of the gas. This can be
represented as follows:
2
2
1
1
/
/
Pw
Pt
Pw
Pt
=
(11)
where
P
t
=
total pressure of the gas
P
w
=
partial pressure of water vapor
Using a Condensation Hygrometer
It is useful to consider the behavior of water vapor when working with
any condensation hygrometer. Keep the following in mind when
making measurements:
Contamination – Water soluble contaminants, such as salt, can have a
significant effect on saturation vapor pressure. Some gases will also
go into solution with water. Water soluble contaminants cause a
lowering of vapor pressure known as Raoult Effect, and a
corresponding measurement error.