Vaisala DEWCAP DM500 Guia do Utilizador descarregar pdf (Página 134)

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

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:

10))-/(( ⋅=

wtotwv

PPPPPM (10)

where

= water vapor pressure

= 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

= total pressure of the gas

= 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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