Detecting Wood Moisture and Wood Properties with a Moisture Tester
The source and state of moisture in wood
When a living tree grows, its roots continuously absorb water from the soil, which is then transported to various organs of the tree by the xylem of the trunk. At the same time, the nutrients produced by photosynthesis in the leaves are transported from the phloem of the trunk to various parts of the tree. Water is not only the substance for tree growth, but also the carrier for transporting various substances. After a living tree is cut down and sawn into various specifications of timber, most of the water remains inside the wood, which is the main source of water in wood. At the same time, wood also absorbs some water into its interior during storage, transportation, or use.
For different tree species, there are differences in the moisture content of the xylem in the trunk. Even within the same tree, the moisture content of its xylem varies in different growing seasons. The moisture content of various parts of the xylem, such as heartwood, sapwood, roots, trunk, and treetops, is also different, so the distribution of moisture in wood is very uneven. When the atmospheric conditions around the wood change, its moisture content also changes accordingly. Moisture can be divided into three types: free water, adsorbed water, and combined water due to its different positions in the wood: (1) Free water exists in the large capillary system composed of pores on the wood cell wall or the end of the conduit, the cell cavity, and the intercellular space that communicate with each other. Free water is physically bound to the wood, and the binding is not tight. This part of water is easy to escape from the wood and is also easy to be inhaled. When wet wood is placed in dry air, the first thing that evaporates is free water. For newly felled raw wood, the free water content of different tree species varies greatly, generally between 60-70% and 200-250%.
(2) Water is adsorbed in the microcapillary system composed of microfibrils and microfibrils that exist within the cell walls of wood, or adsorbed on the surface of microcrystals and free control groups of cellulose molecules in amorphous regions. The difference in water adsorption content in wood is relatively small among tree species, and the maximum water adsorption content in wood is generally between 23-31%, with an average of about 3%. The absorption of water is closely bound to the wood material, 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, can it evaporate from the wood.
(3) The combined water and the substances that make up the cell wall form a strong chemical bond. The content of this water in wood is very small and can be ignored, and it cannot be removed under normal drying conditions.
