Density basic

The basic wood densities (dry) for different species were obtained from Ref. [36]. A basic density value obtained from the weighted average of the densities of each site s species was used for the species that for various reasons could not be identified. For estimation of SOC (soil organic carbon), equation (6) was used [30]. For data analysis, the nonparametric type test was chosen. We used the INFOSTAT software, and a value of 0.05 was considered significant. 

Comparison of the Kohn-Sham and Skyrme functionals leads to a natural question why these two functionals exploit, for the time-dependent problem, so different sets of basic densities and currents If the Kohn-Sham functional is content with one density, the Skyrme forces operate with a diverse set of densities and currents, both T-even and T-odd. Then, should we consider T-odd densities as genuine for the description of dynamics of finite many-body systems or they are a pequliarity of nuclear forces This question is very nontrivial and still poorly studied. We present below some comments which, at least partly, clarify this point. 

In closing, it is emphasized that we are basing this work upon the simplified single-density formula for information, as is permitted by Liouville s measure-preserving theorem, which gives unity for the second (basic) density.11... 

Lastly, it is generally assumed that 0.5eV is the best possible resolution for solid state XPS measurements and the experimental resolution function is reasonably well reproduced by a Gaussian of full width f at half maximum of 0.7eV. A final "theoretical XPS spectrum" is obtained after correction of the basic density of states function by cross-section effects and convolution by the experimental resolution function (16) ... 

The specific gravity of a material is defined as the ratio of the density of the material to the density of water (2). If this concept is broadened to incorporate the parameter of basic density, another term is obtained basic specific gravity, which is defined as ... 

Consequently, the terms basic density and basic specific gravity give the same information, and they are different only in the fundamental sense that basic specific gravity is a pure number and basic density is not. The choice of one term over the other for descriptions of wood quality is a matter of preference and varies with particular authors or investigators. However, the assignment of units to these terms is appropriate only for basic density. 

In softwoods, basic density is strongly related to the volume proportion of latewood and its average fiber wall thickness. However, hardwood basic density depends not only on fiber wall thickness but also involves the volume ratio of fibers to vessels. Native commercial woods fall mostly in the basic density range of 0.35-0.65 g/cm, although native species can be as low as 0.21 g/cm (corkwood) and as high as 1.04 g/cm (black ironwood) (2). 

Woods with basic density values (means for the species) that fall in the range of <0.36, 0.36-0.50, and >0.50 g/cm are considered light, moderately light to moderately heavy, and heavy, respectively, and include both temperate and tropical woods (2). However, for a given species, there is considerable variability about any published and accepted mean. Specifically, at least for most North American woods, the expected coefficient of variation (i.e., standard deviation divided by the mean) is about 10% (24). Thus, if the 95% probability level is to be considered, a reasonable estimate of the total expected range of variability would be the mean basic density (10% x 1.96 X mean basic density). Table I presents the ranges of basic density that might be anticipated for several important U.S. woods. 

The importance of wood moisture and basic density in determining wood behavior will become more evident in subsequent chapters. Suffice it here to say that variation in the amount of cell wall substance at a given moisture content that must be traversed by a penetrating liquid or chemical, microbe, etc., can determine the rate of reaction as well as the extent of reaction or the change in the character of the wood in question. 

PARHAM AND GRAY Fomuition and Structuve of Wood 41 Table I. Basic Density (BD) of Some Important U.S. Woods and the Expected Range of Variability ... 

The reduced vessel volume of tension wood, together with thickened fiber walls, can lead to a higher than normal basic density. This general situation, coupled with a difference in wood chemistry, could cause a variable response of such tissue to both chemical and physical treatments or to microbial degradation when compared to normal hardwood xylem. 

Basic density relates to a specific condition, derivable from the previous definition ... 

The term basic emphasizes that both the parameters measured, the oven-dry mass and the swollen volume, have constant and reproducible values. Basic density is the most useful descriptor of wood density. 

Maximum moisture content and estimation of basic density... 

The maximum moisture content for a wood can be estimated easily. In a timber having a basic density of 300 kg m the oven-dry cell tissue (density 1500 kg m )... 

In the case just examined the basic density is, as expected ... 

Wood only shrinks when water is lost from the cell walls and it shrinks by an amount that is proportional to the moisture lost below fibre saturation point. To a first approximation the volumetric shrinkage is proportional to the number of water molecules that are adsorbed within the cell wall, and that in turn is related to the number of accessible hydroxyls on the cellulose, hemicelluloses and lignin, and to the amount of cell wall material, i.e. the basic density of the wood (Figure 4.2). 

Volumetric shrinkage (%) = MC at fibre saturation (%) X basic density x 10 (2)... 

Figure 4.3. External volumetric shrinkage of 106 different North American hardwoods from the green to oven-dry state plotted against their respective basic densities (Stamm, 1964).

Consider the following example of a timber having an unextracted basic density of 530 kg m, comprising 500 kg m of wood and 30 kg m of hydrophilic (water-soluble) extractives deposited within the cell wall... 

However, the extractive-free basic density of this wood is actually 500 kg m"", so if it were not bulked by extractives it should only shrink by 15% on oven-drying and by 7.5% on drying to 15% moisture content. 

This wood has 30 kg of extractives in a cubic metre of green wood and, assuming a density of 1400 kg m for these extractives (Tarkow and Krueger, 1961), they will occupy 0.021 m of the swollen cell wall per m of swollen wood. If the extractives were not present, the extractive-free basic density would be 500 kg m and the wood would shrink by 15%, i.e. the cell wall would shrink by 0.15 m per m of swollen wood. However the water soluble extractives occupy 0.021 m of the cell wall for every m of swollen wood, so the wood and the cell wall can only shrink by 0.150-0.021 m, i.e. 0.129 m per m of swollen wood rather than by 0.150 m per m which would be expected of an extractive-free wood having a basic density of 500 kg m. The oven-dry volumetric shrinkage will be only 12.9% rather than 15.0%. 

Clearly shrinkage is dependent on the extractive-free basic density, the amount of extractives in the wood, and the distribution of the extractives between the cell wall and the lumen. The bulking of the cell wall with extractives explains part of the shrinkage variation observed in Figure 4.3. 

In field-based research measurements of basic density are based frequently on unextracted wood and the mass of extractives is lumped in with the wood tissue. 

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