Percentage relative humidity

Percentage relative humidity is defined as the partial pressure of water vapor in air divided by the vapor pressure of water at the given temperature. Thus RH = lOOp/p,. 

The relationship between the percentage humidity of a gas and the percentage relative humidity may be derived as follows ... 

When (P - Pwq)/(P — Pw) 1, the percentage relative humidity and the percentage humidity are equal. This condition is approached when the partial pressure of the vapour is only a small proportion of the total pressure or when the gas is almost saturated, that is as Pw -> PiM. 

Thus, the water activity of a food is equal to the relative vapor pressurepv/p°. The relative vapor pressure is also related to percentage relative humidity (%RH) divided by 100. It is critical to bear in mind the assumptions underlying the development and thus the use of Eq. (5) for determining the aw of a food. These assumptions are examined in detail in the next section. 

FIG. 35 Percentage relative humidities of common foods at room temperature and typical steady-state moisture contents plotted as a universal sorption isotherm with portions of three glass curves (relatively positioned) for sorbitol, for a nonnetworked biopolymer, and for a permanent network [reproduced with permission from Slade and Levine (2003)]. 

The distinction between percentage humidity and percentage relative humidity is of significance though, the difference in the values of the two quantities does not usually exceed 7 to 8 per cent. Reference may be made here to Volume 1, Section 13.2.1. 

Relative humidity or relative saturation is the ratio of the prevailing humidity to the saturation humidity at the same temperature, or the ratio of the partial pressure to the vapor pressure expressed as a percentage,... 

Related Calculations. Do not confuse percentage humidity with relative humidity. Relative humidity is the ratio of the partial pressure of the water vapor to the vapor pressure of water at the temperature of the air, this ratio usually being expressed as a percent. Percentage humidity is the ratio of the actual humidity to the saturation humidity that corresponds to the gas temperature, which is also usually expressed as a percent. At all humidities other than 0 or 100 percent, the percentage humidity is less than the relative humidity. 

Percentage absolute humidity (percentage saturation) S Ratio of absolute humidity to saturation humidity, given by S = 100Y/Ys= lOOp (P - ps)/[ps(P - p)]. It is much less commonly used than relative humidity. 

Adsorption and desorption isotherms may be determined by monitoring equilibrium moisture uptake or loss in samples stored in desiccators with different relative humidities (saturated salt solution yielding different percentage relative humidity). The amount of water present is determined by loss of drying (LOD), TGA, Karl Fischer titration, the coulometric method, or near-IR spectroscopy. 

In a process in which it is used as a solvent, benzene is evaporated into dry nitrogen. At 297 K and 101.3 kN/m, the resulting mixture has a percentage relative humidity of 60. It is required to recover 80 per cent of the benzene present by cooling to 283 K and compressing to a suitable pressure. What should this pressure be The vapour... 

Table 9.3. Percentage relative humidities of the atmospheres above saturated solutions of various pure salts at 15 °C...

Substance Formula Stable phase at 15 °C Percentage relative humidity... 

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