The humidity of a vapor-gas mixture is defined as the mass of vapor associated with unit mass of the gas.
VAPOR AND GAS MIXTURES
The
humidity of a vapor-gas mixture is defined as the mass of vapor associated with
unit mass of the gas. This principle is generally applicable to any vapor
present in any noncondensable gas. In this section, however, only water vapor
in air is considered. The percent humidity is the ratio of ambient humidity to
the humidity of the saturated gas at the same temperature, expressed as a
per-centage. These terms should be carefully distinguished from the relative
humidity with which they are distantly related. The relative humidity is the
ratio of the partial pressure of the vapor in the gas to the partial pressure
when the gas is saturated. This is also usually expressed as a percentage. The
relative humidity of a given vapor-gas mixture changes with temperature, but
the humidity does not.
The
study of the properties of the air–water vapor mixture is called psy-chrometry,
and data is presented in the form of psychrometric charts. These take various
forms and present various data (Perry and Chilton, 1999). In Figure 6.1,
humidity is plotted as the ordinate and temperature as the abscissa. Percent
relative humidity is then plotted as a series of curves running across the
chart. The use of this simplified chart is demonstrated later in this chapter
and in the succeeding chapter.
FIGURE 6.1 A psychrometric chart that can be used to determine the humidity.
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