Absolute humidity, definition

If equilibrium has not been reached between a mixture of components, the condition is referred to as partial saturation. At partial saturation the gas mixture obeys real gas laws. There are several ways to express the concentration of a vapor in a mixture of gases. Most often, weight or mole fraction is used. Other definitions are relative saturation (relative humidity), molal saturation (molal humidity) and absolute saturation (absolute humidity). 

Definitions related to humidity, vapor pressure, saturation, and volume are as follows the most useful are absolute humidity, vapor pressure, and relative humidity. 

Table 12-1 gives formulas for conversion between absolute humidity, mole fraction, vapor pressure, and volumetric humidity for the air-water system, and Table 12-2 does likewise for a general gas-vapor system. Where relationships are not included in the definitions, they are given below. 

Absolute humidity (hj same definition as absolute saturation but applied to the air-water system, i.e.,... 

Figure 23.1 is a psychrometric chart for the air-water system. It shows the relationship between the temperature (abscissa) and absolute humidity (ordinale, in g water per kg dry air) of humid air from 0°C to 130°C at one atmosphere absolute pressure. Line as representing percent humidity and adiabatic saturation are drawn according to the thermodynamic definitions of these terms. Equations for the adiabatic saturation and wet-bulb temperature lines on the chart are as follows (Geankoplis 1983) ... 

Based on these definitions and the data provided, we compute the enthalpy of the air at the inlet and outlet of the column (Figure 3.20). Alternatively, we can use the functions defined in the exercise for mixtures of air streams to estimate absolute humidity and enthalpy. 

Prom the definition of relative humidity it is seen that the partial pressure of the water vapour p in fact assumes the value ps when RH = 100 %. To calculate the absolute humidity of a mass of air it is necessary to know both the relative humidity RH and the temperature 6. 

Moist-heat sterilization is achieved when water vapor (or, more generically, moist heat, i.e., a suitable combination of temperature and humidity) at a definite temperature is introduced or generated (even indirectly) at the level of the micro-organisms to be inactivated and is maintained in such conditions for a definite time. As explained in detail hereafter, moist-heat sterilization proceeds as an inverse logarithmic progression. Therefore, only a treatment of infinite duration provides absolute certainty that all micro-organisms have been inactivated. 

Water content is defined as the amount of water lost by a food when it reaches the true equilibrium against zero water vapor pressure. From this definition arises a quantification method of water content called the absolute reference method, a determination that is only possible in specialized laboratories. The food industry only uses practical reference methods, calibrated against the absolute reference method. Moreover, water content standards are not available because a product s water content depends on the humidity and temperature of the environment, which makes participating in intercomparison exercises between laboratories essential for detecting possible experimental errors. 

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