In gases and liquids, the molecules or ions in constant, statistically
not directional heat movement. If a mixture or in a solution for a substance a
concentration gap, it will be offset by this movement, which are addressed to a
statistically movement, for the diffusion. A substance dissolved diffuses along
his own gap.
Gas exchange of components found in the rule only by diffusion. "Diffusion"
called for the gradual mixing of different gases (and even liquids and solids)
without external influences, but by motion, until the distribution of various
molecules is the same everywhere.
The diffusion is a balancing process, the increase in entropy to a less orderly,
therefore more likely condition leads; finds therefore determined by the II. Law of
thermodynamics necessarily.
The resistance of a material of the diffusion of water vapor and other gases
counteract depends mainly on its pore-structures; more open pores, the lower the
resistance. The pore diameter plays for water vapor molecules virtually no role,
they are smaller and lighter than nearly all other air molecules; oxygen
molecules, for example, have 60% more mass and carbon dioxide molecules are
almost three times as much. The diffusion speed of a substance is approximately
inversely proportional to the root of their molecular weight M (diffusion
coefficient D ~ 1 / √W); large molecules diffuse so slowly. Therefore, the
diffusion rate of carbon dioxide molecules are already in the air is much lower
than that of water vapor, and there are layers component, although the diffusion
of carbon dioxide almost completely absperren, water vapour diffusion but not
too much resistance oppose. This can be possible protection of concrete surfaces
used to see here formation of Carbonat. The comparison with water vapor, oxygen or
carbon dioxide molecules usually huge "residential-poisons" by diffusion certainly not
from the air removed, as in some residential-healthy writings from time to time to
read. They also forms the wall diffusionsfähigste a virtually insurmountable
obstacle.
Diffusion resistance values are the water vapor diffusion resistance figures and
the calculated "diffusionsäquivalenten air layer thickness" (sd-value). In analogy to these values, only the diffusion of water vapor through a material or
component, some type of building catalogues especially in paints and other
coatings also resistance values for carbon dioxide diffusion.
Supplement: The distance travelled by diffusion is proportional to the root of the period, doubling the distance is already four times the time needed.
The proof of internal condensation example, is still after Glaser's diffusion scheme (proof of the usability of components in accordance with DIN 4108-3:2001-07), but neither the hygroscopic up and discharge or the relaxation of the capillary condensate. Problems with the diffusion-technical calculation emerge then if practical realities are not exclusively diffusion-processes. Capillary water transport in multi-wall construction in certain building materials occur beyond calculation.
More under steam diffusion resistance number
Quelle: Kur, Friedrich; Wohngifte, Handbuch für gesundes Bauen und Einrichtungen, 3. Aufl. Verlag Eichborn, 1993, S. 536
Libbert, Eike; Allgemeine Biologie , VEB Gustav Fischer Verlag Jena 1986, S. 153
Haupt, P.; Bauphysik 5/94, Stand Deut. Bücherei (ZB 75 909) 24-5
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