Vaisala DEWCAP DM500 Podręcznik Użytkownika Strona 134

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User's Guide ______________________________________________________________________
132 _________________________________________________________________ M210205EN-D
xxTh ++= 5.2)00189.001.1((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))-/(( =
wtotwv
PPPPPM (10)
where
P
w
= water vapor pressure
P
to
t
= 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:
2211
// PwPtPwPt = (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.
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