Humidity, relative definition

In diying solids it is important to distinguish between hygroscopic and nonhygroscopic materials. If a hygroscopic material is maintained in contact with air at constant temperature and humidity until equilibrium is reached, the material will attain a definite moisture content. This moisture is termed the equilibrium moisture content for the specified conditions. Equilibrium moisture may be adsorbed as a surface film or condensed in the fine capillaries of the solid at reduced pressure, and its concentration will vaiy with the temperature and humidity of the surrounding air. However, at low temperatures, e.g., 15 to 50°C, a plot of equilibrium moisture content versus percent relative humidity is essentially independent of temperature. At zero humidity the equilibrium moisture content of all materials is zero. 

The partial pressure of water vapor can be calculated as a function of the dry-bulb and wet-bulb temperatures, Eq. (12.23), and the relative humidity from its definition ... 

The utilization of IR spectroscopy is very important in the characterization of pseudopolymorphic systems, especially hydrates. It has been used to study the pseudopolymorphic systems SQ-33600 [36], mefloquine hydrochloride [37], ranitidine HC1 [38], carbovir [39], and paroxetine hydrochloride [40]. In the case of SQ-33600 [36], humidity-dependent changes in the crystal properties of the disodium salt of this new HMG-CoA reductase inhibitor were characterized by a combination of physical analytical techniques. Three crystalline solid hydrates were identified, each having a definite stability over a range of humidity. Diffuse reflectance IR spectra were acquired on SQ-33600 material exposed to different relative humidity (RH) conditions. A sharp absorption band at 3640 cm-1 was indicative of the OH stretching mode associated with either strongly bound or crystalline water (Fig. 5A). The sharpness of the band is evidence of a bound species even at the lowest levels of moisture content. The bound nature of this water contained in low-moisture samples was confirmed by variable-temperature (VT) diffuse reflectance studies. As shown in Fig. 5B, the 3640 cm-1 peak progressively decreased in intensity upon thermal... 

If the definition of TOW established by ISO is used (TOW-ISO), a linear relationship between time and TOW is obtained, in spite of the different possible changes in corrosion rate caused by changes in the nature of TOW. It has to be remarked that it is not the same effect on corrosion rate caused by a heavy rain than dew, fog or water adsorption, so for the same interval of relative humidity (80-100%), notable changes in nature of TOW-ISO and consequently in corrosion rate could take place. 

Meteorological information which has been useful to receptor model studies includes temperature, relative humidity, mixing height, windspeed and wind direction. The wind direction is particularly important for the verification of certain receptor model source contribution predictions definite differences should exist between samples on which the source is upwind and downwind of the receptor. 

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). 

The overall effect of humidity on the smog reactions of propylene has been estimated, based on Reactions 48-54 and 58. Figure 9 shows a comparison of reaction profiles computed for zero and 50% relative humidity. According to the present kinetic model, the humidity effect seems to be rather slight. However, this prediction should be further verified experimentally. Existing smog chamber results on the humidity effect are not definitive and are frequently conflicting (66, 67, 68). 

In this era of automatic titrators, microprocessor-controlled thermal analysis, and definitive spectral techniques, one of the most powerful techniques, that is, optical microscopy, is frequently overlooked. The value of direct sample observation, preferably while it is exposed to different relative humidities, cannot be overstated. In the author s laboratory, a plexiglass chamber was constructed that can be placed on the stage of the microscope, through which air of known humidity can be circulated. This simple technique has been very useful in examining the swelling (or lack) of disintegrants and the influence of very hydrophilic excipients in combination with a moisture sensitive drug. ... 

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

The activity of water in the gas phase is defined as the ratio of the partial pressure of water to the vapor pressure of pure water, which is also related to the definition of relative humidity. 

Relative humidity (A ) same definition as relative saturation, but applied to the air-water system, i.e.. 

There is a difficulty with the definition of the fibre saturation point. Strictly it corresponds to the moisture eontent when the cell walls are fully saturated (which would occur where the relative humidity is 1.0) and all the lumens are free of water. However at high relative humidities, above about 0.98, the lumen tips and pits begin to fill by capillary condensation, eausing a sharp upward break in the sorption curve. In praetiee it is extremely hard to measure moisture content at these humidities and extrapolation from lower relative humidities is not particularly accurate. 

Before the ICH was formed, agreement on the appropriate storage conditions for stability studies was always controversial. The regulatory agencies in different countries did not always agree with each other. For example, the definition of room temperature or controlled room temperature was different globally. Currently, the world is divided into four climatic zones based on the mean kinetic temperature. The definition of room temperature in the four zones is given in Table 5 as is the relative humidity. Some of the countries that fall into these zones are also shown. 

Corrosion rate was evaluated with respect to, 1) flux of pollutants (sulfur oxides, nitrogen oxides, oxidants, and particles) to the steel during both wet and dry periods, 2) temperature, and 3) exposure history. Different definitions of when the steel was wet were evaluated to determine the most likely "critical relative humidity." Non-linear multiple regression techniques were used to determine the statistical significance of each factor and develop a theoretically consistent environmental damage function. 

Aerosol particles in the atmosphere usually carry with them some moisture. The amount of water associated with the aerosol depends on the relative humidity. Increasing the relative humidity condenses more water onto the particles, until finally, when the vapor pressure of water exceeds the saturation point, a certain number of particles grows into fog or cloud droplets. Meterologists call these particles condensation nuclei, or simply nuclei. Fogs and clouds are treated as separate systems and are not included in the normal definition of the atmospheric aerosol, even though they represent an assembly of particles suspended in air and thus constitute an atmospheric colloid. The smoothness of the transition from an assembly of aerosol particles to one of cloud elements makes it difficult to define a boundary line between both colloids. Due to the overlap of size ranges of the particles in both systems, any division will be rather arbitrary. 

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