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INDIANA UNIVERSITY, DEPARTMENT OF PHYSICS, P309 LABORATORY Auxiliary Lab #90: Humidity Goal: Measure the water content of the air in the lab Equipment: Dew-point hygrometer. The lab is filled with air of temperature T (K, Kelvin, obtained by adding 273K to the temperature in o C). Air usually contains a certain amount of water in solution. Absolute humidity is defined as the partial pressure p w of the water vapor. As the temperature decreases, the solubility also decreases. At a certain temperature T D (called the dew point ) the air is saturated with water. With further cooling, liquid water (condensation, fog) appears. The H 2 O partial pressure of a saturated air mass is called the saturation pressure p s . The saturation pressure is a function of the temperature T . Since in saturated air gaseous and liquid water co-exist, the saturation pressure p s ( T ) is the same as the vapor pressure p v ( T ) of water at that temperature. The vapor pressure of water can be obtained from a table
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