Saturation moisture content

Saturated moisture content equals porosity of soil... 

Volumetric shrinkage data on other woods have also indicated that the ratio Shf/Gg tends to approximate the fiber-saturation moisture content Mf. For example, a mean ratio was found for Shf/Gg of 27 for 170 Australian woods (33). Data on tropical woods suggest somewhat lower values for this same ratio. The mean value for 140 Indian woods was approximately 20, considerably lower than the values for U.S. woods. This may indicate that tropical woods are less hygroscopic than temperate-zone woods, possibly because of their higher mean extractive contents. 

The relationship between i/j and the water content of a porous medium is also determined empirically Fig. 3-23 shows typical ijs — 0 curves. When is zero, the porous medium is at its saturated moisture content, which is equal to porosity (assuming that there is negligible trapped gas). The pressure head can become somewhat negative before air enters (and water leaves) some soils the value ijja at which air begins to enter is called the air entry value. Water leaves the matrix as i/j (defined as pressure head) becomes more negative than ifsa as water leaves the soil, the hydraulic conductivity also decreases. 

FIGURE 5. Examples of soil moisture retention curves for different textured soils. Heavily textured soils such as clay soils will, in contrast to light textured sandy soils, have a larger total porosity and saturated moisture content, but also large residual moisture contents. 

Experiments have been carried out to study the effect of water saturation (moisture content, humidity, etc.) on the stability of various rock types including shale (Colback Wild, 1965), chalk (Papamichos et al., 1997), and sandstone (Bruno et al., 1996 Hawkins McConnell, 1992). The main possible reasons for strength loss are ... 

Rock classification tests Saturation moisture content (alteration index)... 

Other properties of Pis were also improved by the diimide additives [135]. The dielectric constant, e of PI from BDSDA and 2,2 -bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (BDAF) reduced appreciably by additives shown in Fig. 44. The results are as listed in Table 5. Furthermore, the addition of 10 wt% AN-BDSDA-AN into BDSDA-ODA m-PDA polyimide caused a decrease in the saturation moisture content from 1.52% for the neat PI to 1.27%. Positron annihilation spectroscopy proved that a decreased free volume is responsible for the reduced water uptake. Also, a slightly increased in modulus and a decreased CTE were caused by the 5% addition of AN-6FDA-A into LARC-TPI in both the undrawn and drawn states. 

Unlike the water absorption characteristics, in moisture absorption by superabsorbent polymers these characteristics do not appear because these polymers are crosslinked. The saturation moisture content is basically the same as for the hydrophilic polymers prior to crosslinking. However, due to the crosslinked structure, adhesion is minimized. Thus, even after moisture absorption/drying cycles, the surface area is kept relatively constant and the rate of moisture absorption/drying will also be kept relatively constant. 

The saturation moisture content of poly(acrylic acid) salt-type superabsorbent polymers depends on the relative humidity as shown in Fig. 6. 

Example 4.9. Estimation of the Saturation Moisture Content of Polymers... 

Estimate the saturation moisture content of poly(methyl acrylate) (PMA) and polystyrene (PS) at 25 °C and relative humidity of 50%. Compare these values with experimental values from Crank and Park (1968, p. 263). 

The key to understanding dewatering by air displacement is the capillary pressure diagram. Figure 6 presents an example typical for a fine coal suspension there is a minimum moisture content, about 12%, called irreducible saturation, which cannot be removed by air displacement at any pressure and a threshold pressure, about 13 kPa. 

At equihbrium with relative humidity below 100%, the moisture ia wood is present primarily ia the cell wads. The moisture content at which the ceU wads would be saturated and the ced cavities empty is caded the fiber saturation poiat. Actuady, such distribution is impossible. Beginning at - 90% relative humidity, some condensation may occur ia smad capidaries. The determination of the fiber saturation poiat is based on the fact that certain properties of wood (eg, strength and volume) change uniformly at first with increasing moisture content and then become iadependent of the moisture content (Fig. 2). The equdibrium moisture content (usuady determined by extrapolation), at which the property becomes constant at 25 to 30% moisture, is represented by the fiber saturation poiat. 

Fiber saturation point is the bound moisture content of ceUular materials such as wood. 

In porous and granular materials, Hquid movement occurs by capillarity and gravity, provided passages are continuous. Capillary flow depends on the hquid material s wetting property and surface tension. Capillarity appHes to Hquids that are not adsorbed on capillary walls, moisture content greater than fiber saturation in cellular materials, saturated Hquids in soluble materials, and all moisture in nonhygroscopic materials. 

AH = moisture-content correction of air saturated at wet-bulb temperature when barometric pressure differs from standard barometer, gr/lb dry air NOTE To obtain AH reduce value of AH by 1 percent where t — t, = 24 F and correct proportionally when t — is not 24 F h = enthalpy of moist air, Btu/lb dry air... 

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

Unbound moisture in a hygroscopic material is that moisture in excess of the equihbrium moisture content corresponding to saturation humidity. All water in a nonbygroscopic material is unbound water. 

Capillary Flow Moisture which is held in the interstices of solids, as liquid on the surface, or as free moisture in cell cavities, moves by gravity and capiUarity, provided that passageways for continuous flow are present. In diying, liquid flow resulting from capiUarity appUes to liquids not held in solution and to aU moisture above the fiber-saturation point, as in textiles, paper, and leather, and to all moisture above the equiUbrium moisture content at atmospheric saturations, as in fine powders and granular solids, such as paint pigments, minerals, clays, soU, and sand. 

Liquid Diffusion The movement of liquids by diffusion in soUds is restricted to the equihbrium moisture content below the point of atmospheric saturation and to systems in which moisture and solid are mutually soluble. The first class apphes to the last stages in the diying of clays, starches, flour, textiles, paper, and wood the second class includes the diying of soaps, glues, gelatins, and pastes. 

When a solid is dried experimentally, data relating moisture content to time are usually obtained. These data are then plotted as moisture content (dry basis) W versus time 0, as shown in Fig. 12-41 7, This curve represents the general case when a wet sohd loses moisture first by evaporation from a saturated surface on the sohd, followed in turn by a period of evaporation from a saturated surface of gradually decreasing area, ancf finally, when the latter evaporates in the interior of the solid. 

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