Equilibrium moisture defined

Relative humidity method. Water absorption is defined as the water absorbed by a dried protein powder with equilibration against water vapor at a known relative humidity. This method, also known as the equilibrium moisture content (EMC) method, was described first by Mellon et al. (10). Huffman et al. (11) used... 

In practice, the diffusion constant is modified to reflect the complex nature of the biofilm. In the development of synthetic hydrogels, the hydrophilicity of the polymer in part defines the concept of equilibrium moisture. We discussed this concept earlier when we described determination of equilibrium moisture. The first practical application is the diffusion constant. As the equilibrium moisture approaches 100%, the diffusion constant approaches that of water. 

If polyurethanes are used to entrap cells, the diffusion wiU depend on the polyol used to build the polyurethane since the polyol defines equilibrium moisture. Later in this chapter, we will discuss a number of entrapment systems, including acrylates and polysaccharides. Each has its own equilibrium moisture and therefore unique diffusion constant. Only polyurethanes, however, offer the opportunity to affect changes in the constants. Conventional hydrophilic polyurethanes have equilibrium moisture levels around 70%. It is possible, however, to increase the molecular weight of a polyol (an ethylene glycol of 1000 molecular weight) to 3000 or more. This increases the equilibrium moisture to greater than 90%. 

Fiber-saturation point is the moisture content of cellular materials (e.g., wood) at which the cell walls are completely saturated while the cavities are liquid-free. It may be defined as the equilibrium moisture content as the humidity of the surrounding atmosphere approaches saturation. 

Equilibrium Moisture Content. If a material is exposed to air at a given temperature and humidity, it will gain or lose moisture until equilibrium is reached. The moisture present at this point is defined as the equilibrium moisture content for the given exposure conditions. At a given temperature, it will vary with the partial pressure of the water vapor in the surrounding atmosphere. 

Moisture content slightly hygroscopic. A well-defined crystalline hydrate is not formed although surface moisture may be picked up or contained within small pores in the crystal structure. At relative humidities between about 15% and 65%, the equilibrium moisture content at 25°C is about 2.0%. At relative humidities above about 75%, tribasic calcium phosphate may absorb small amounts of moisture. Particle size distribution Tribasic calcium phosphate powder typical particle diameter 5-10 pm 98% of particles <44 pm. 

Air dry tonne is defined as pulp with 10% moisture content, which is below the normal equilibrium moisture content of pulp exposed to ambient air. 

Consider the drying of a wet solid under fixed drying conditions. In the most general cases, after an initial period of adjustment, the dry-basis moisture content X decreases linearly with time t following the start of the evaporation. This is followed by a nonlinear decrease in X with t nntil, after a very long time, the solid reaches its equilibrium moisture content X and drying stops. In terms of free moisture content, defined as... 

Chang et al. (1991) have studied the dehydration process in relation to IM meats. They stated that if the unbound moisture in meats is defined as that which exerts water vapor pressure like that of pure water, all unbound moisture must evaporate before equilibrium can be achieved with air that is less than saturated. In other words, the water will evaporate until the water vapor pressure of the meat is equal to the partial water vapor pressure in the air. Data on the equilibrium moisture content-relative humidity (i.e., isotherms) of meat or meat mixtures are needed over a wide range of temperatures for dehydration applications. For example, isotherms for meat/meat mixtures below ambient temperature are needed for salami and raw ham, and isotherms above steam temperatures are needed for high-temperature finished dried IM meats (Chang et al, 1991). 

A bibliography on sorption isotherms of food materials is presented in Ref. [161]. The collection comprises 2200 references, including about 900 papers with information on equilibrium moisture content of foods in defined environments. The papers are listed alphabetically according to the names of the first author, but they are also grouped according to product. 

Brown coals, like the hard coals, are also defined in terms of their calorific value (see the previous text) and were also recognized as potential fuels. The International System for the classification of brown coals is based on two inherent characteristics which indicate the value of brown coals as fuels (1) the total moisture (ash-free basis) and (2) the yield of tar (daf basis). Thus, the six classes of brown coal based on the ash-free, equilibrium moisture content are divided into groups according to the yield of tar on a dry, ash-free basis (Table 2.14). This system indicates... 

A high water level is defined as when the water level is 300 mm beneath the formation level and is consistent with effective sub-soil drainage a low water level is when the water level is 1 m below the formation level. Good construction conditions result in subgrades never getting wetter than their equilibrium moisture contents beneath the finished road. A thick pavement is 1200 mm deep, including a 650 mm capping layer, and is typical of motorway construction a thin pavement is 300 mm deep. For pavements of intermediate thickness founded on plastic soils, an equilibrium value of GBR may be interpolated (Powell et al. 1984). 

Bound moisture is associated with the hygroscopic nature of the woody components. There are some uncertainties about the limits of hygroscopic behavior, particularly with woods of high extractives content but it is useful to define a maximum sorptive moisture content, called the fiber saturation point (FSP). If the capillary condensation effects in pores greater than 0.1 xm in equivalent cylindrical diameter are ignored, FSP of the wood may be defined as the equilibrium moisture content (EMC) in an environment of 99% relative humidity. This yields a value of 30 to 32%i for most commercial species (Keey et al., 2000) at room temperature. FSP falls with increasing temperature. For a softwood such as Sitka spruce (Picea sitchensis), FSP falls from about 31%i at 25° C to 23% at 100°C (Stamm, 1964). 

The water vapor transmission rate (WVTR], often erroneously called water vapor permeability (WVP], is defined as a mass of water vapor transmitted through a unit area in a unit time under specified conditions of temperature and humidity. The water vapor barrier properties for the packaged product, whose physical or chemical deterioration is related to its equilibrium moisture content, are of great importance for maintaining or extending its shelf life. The water vapor barrier is quantified by the water vapor permeability coefficients (WVPC] which indicate the amount of water vapor that permeates per unit of area and time in packaging materials (kgm/m s Pa] (Bangyekan etal., 2006]. 

Briggs and Shantz defined an upper limit of hygroscopic moisture which is determinable by experiment. If a soil is placed in a water-saturated atmosphere, it will absorb water vapor until a condition of approximate equilibrium is reached. The moisture content of a soil under these conditions is called the1 hygroscopic coefficient of that soil. Determination of the hygroscopic coefficient is subject to error unless carried out with considerable care. It has been pointed out that hygroscopic moisture is tenacious and difficult to drive off even at elevated temperatures. For this reason, and also because the equilibrium... 

It should be noted that the limits so defined, which bound the region of apparent equilibrium, are by no means fixed. In the phosphorescence of phosphorus, e. g., they depend on the amount of moisture present. The combination of oxygen and hydrogen, again, is ultimately dependent on the substance with which the gases are in contact. Thus, Mitscherlich gives the temperature of explosion in glass vessels as 674° instead of 845 . 

Saturated salt solutions and sulfuric acid solutions establish relative humidity by reducing the vapor pressure above an aqueous solution (a colligative effect). Saturated salt solutions at controlled temperature maintain a constant relative humidity as long as excess salt and bulk solution are present. As water is added or removed from the solution, moisture from the head-space will either condense or evaporate (as appropriate), with subsequent dissolution or precipitation of salt to maintain the equilibrium vapor pressure. Because the degree of vapor pressure depression is dependent on the number of species in solution and, further, since the solubility of most salts is somewhat dependent on temperature, the relative humidity generated is also temperature dependent. Hence, use of the same salt at different temperatures can result in different relative humidities. Refs. can be consulted for specific saturated salt solutions that result in defined relative humidities as a function of... 

Hailwood-Horrobin Solution Sorption Theory. The Hailwood-Horrobin (57) model treats moisture sorption as hydration of the polymer, taken here to be dry wood, by some of the sorbed water called water of hydration, m. The hydrate forms a partial solution with the remaining sorbed water, called water of solution, m,. An equilibrium is assumed to exist between the dry wood and water and the hydrated wood with an equilibrium constant K. Equilibrium is also assumed to exist between the hydrated wood and water vapor at relative vapor pressure h, with equilibrium constant K2. A third constant is defined as the moisture content corresponding to com-... 

Microorganisms and enzymes need water in order to be active preservation of foods can be sought by reducing their moisture content to a point where the food-spoilage of pathogenic microorganisms are inhibited. Scott [ 1,2] called the equilibrium relative humidity (ERH), which is the availability of water in a food medium. Water Activity (a, ). a, is defined as the ratio of vapour pressure (p) of solution to the vapour pressure (Po) of the solvent, usually water ... 

These dehydration steps are carried out at the temperatures indicated above, but at any temperature, some moisture can escape from a hydrate. For the complete dehydration of the pentahydrate we can define an equilibrium constant... 

Humidity is the level of water vapour in the atmosphere. It is usually defined as the relative (RH) and absolute humidity. The effect of a temperature change is that in saturated air, if the temperature drops there is precipitation of water, e.g. rain or conden sation onto exposed surfaces. Bear in mind the concept of equilibrium between atmosphere and water-absorbing products, e.g. cellulosic materials which may be considered as both moisture-absorbing and water-losing packaging. 

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