Use a moisture tester to examine the properties and moisture content of wood.
1. The source and state of moisture in wood
When a living tree grows, its roots continuously absorb water from the soil, and the xylem of the trunk sends water to various organs of the tree, and at the same time, the nutrients produced by photosynthesis of leaves are transported from the phloem of the trunk to various organs of the tree. part. Moisture is not only the material for tree growth, but also the carrier for trees to transport various substances. After the living tree is felled and sawn into boards and squares of various specifications, most of the water still remains inside the wood, which is the amount of water in the wood. At the same time, wood will also absorb some moisture into its interior during storage, transportation or use.
For different tree species, the water content of xylem in the trunk is different. Even the same tree has different water content in its xylem in different growing seasons. Each part of xylem, such as heartwood, sapwood, root, trunk and treetop, has different water content, so the water distribution in wood is very uneven. When the atmospheric conditions around the wood change, its moisture content will also change accordingly. The moisture can be divided into three types: free water, adsorbed water, and combined water because of its different positions in the wood:
(1) Free water exists in the large capillary system composed of the pits on the wood cell wall or the perforation at the end of the conduit, the cell cavity and the intercellular space. The free water is physically combined with the wood, and the combination is not tight. This part of the water It is easy to escape from the wood, and it is also easy to inhale. When wet wood is placed in dry air, the first thing to evaporate is free water. For freshly felled raw wood, the free water content of different tree species varies greatly, generally Between 60~70% and 200~250%.
(2) Adsorbed water exists in the micro-capillary system formed between the microfibrils and macrofibrils in the wood cell wall or adsorbed on the free radicals of cellulose molecules on the surface of microcrystals and in the amorphous region. Adsorbed water in wood There is little difference in the content among tree species. The maximum content of water absorbed by wood is generally between 23% and 31%, and the average is about 3%. The water absorbed is closely combined with the wood substance, and this part of water is not easy to escape from the wood. Only When the free water in the wood evaporates completely, and the partial pressure of water vapor in the wood is greater than the partial pressure of water vapor in the surrounding wall air, it can evaporate from the wood.
(3) Combination water and cell wall components are in a strong chemical combination state. The content of this part of water in wood is very small and can be ignored, and it cannot be removed under normal dry conditions.
