User's Guide ______________________________________________________________________132 _________________________________________________________________ M210205EN-DxxTh ⋅+⋅+⋅= 5.2)00189.001.1( (9)wherex = mixing ratio (g/kg)T = temperature (ºC)h = enthalpyHumid Air Volume/Dry Air Volume PPMvParts per million (PPM) by volume is the ratio of the partial pressureof the water vapor to the partial pressure of dry gas. This can becalculated with a reasonable accuracy with following formula:610))-/(( ⋅= wtotwv PPPPPM (10)wherePw = water vapor pressurePtot = total pressureEffect of Pressure on DewpointChanging the total pressure of a gas changes the partial pressures ofthe component gases according to Dalton’s law, and therefore changesthe dewpoint/frostpoint temperature of the gas. This can berepresented as follows:2211 // PwPtPwPt = (11)wherePt = total pressure of the gasPw = partial pressure of water vaporUsing a Condensation HygrometerIt is useful to consider the behavior of water vapor when working withany condensation hygrometer. Keep the following in mind whenmaking measurements:Contamination – Water soluble contaminants, such as salt, can have asignificant effect on saturation vapor pressure. Some gases will alsogo into solution with water. Water soluble contaminants cause alowering of vapor pressure known as Raoult Effect, and acorresponding measurement error.
