Wood is one of the most popular materials used in construction, in the manufacture of furniture, carriage building and many other sectors of the economy. The main advantages are high strength, environmental friendliness, good machinability, chemical resistance. One of the important properties that must be taken into account is the moisture content of the wood. How to determine it and what it should be in different types of wood, we will consider in this article.
Types of humidity
Wood has a natural origin, therefore it is very sensitive to fluctuations in air temperature and atmospheric pressure. Submitting to the state of the environment, it changes the percentage of humidity. Experts say that wood breathes, absorbs or releases air vapor and this phenomenon is called hygroscopicity. It is believed that the most important characteristic is humidity, which determines the ratio of moisture to dry weight. It is divided into:
- Absolute - has different meanings for each tree species. The moisture content of wood is determined in accordance with GOST 8486-86.
- Relative - measurements are taken in the wet state and are divided into free and bound.
In addition, humidity is:
- Natural - 27–81%. It is used for various utility work: the manufacture of formwork, rafters and lathing.
- Furniture - 8-12%. It is used in furniture production and for glued beams.
- Transport - 18-22%. Used for all other types of work, except those listed in the first two paragraphs: for the production of lumber, construction, sheet piling.
For construction work, the most suitable wood is transport. Material with a moisture content of about 20% is easily processed, it does not warp and is not exposed to fungal infections.
Humidity degree
According to the degree of moisture content in wood, it is divided into the following types:
- Wet - it contains more than 100% moisture. This condition occurs when the material is present in water for a long time.
- Freshly chopped - the percentage of wood moisture in this case ranges from 50 to 100.
- Air-dry - for a long time contained in open areas. Its humidity completely depends on natural conditions and the season, the degree of humidity is 15–20%.
- Room-dry - located in residential premises with a water content of not higher than 10%.
- Absolutely dry - almost no moisture, its content is 0%. In this case, it is often subjected to deformation.
Knowledge of the degree of humidity is necessary in the practical use of wood.
What is natural humidity?
Wood is a living material that constantly grows and breathes. For his life needs water. Its content depends on the breed, weather conditions and the place of growth of the tree. Immediately after cutting or sawing logs, the material has moisture, which is called natural. Exact standards for this value do not exist. It varies within 27–81%. Wood with natural moisture is often infected with fungi and bacteria, and rot. It is not recommended to transport the logs over long distances immediately after cutting down, after a few days they become unusable.
Material with natural humidity is used only for the construction of low-liability facilities. Before long-term transportation and subsequent use of wood, felled logs are subjected to atmospheric or forced drying. Only after this procedure can it be safely used for the manufacture of various designs.
Features of moisture content in wood
Everyone has known since ancient times that air humidity has a significant effect on products made from wood. This is especially noticeable with sharp changes in the moisture content in the air. The phenomenon is explained by the peculiarities of the water content in tree species.
There are two forms in which it is found in the material:
- Bound - stored in the cell membrane, held by physico-chemical bonds and greatly affects many properties of wood.
- Free - located in the intercellular spaces and cell cavities, preserved due to mechanical bonds, is easily removed and has little effect on the quality of the material.
When the wood dries, the first evaporation of free moisture occurs and only after it is bound. The process will continue until the water is evenly distributed throughout the material, and is not equal to the relative humidity. Similarly, when absorbing moisture.
Equilibrium wood moisture
Any wood is capable of absorbing water vapor and water and returning the resulting moisture. Depending on the moisture content and ambient temperature and the length of time the wood has been in the air, equilibrium humidity is established. This concept is associated with a complete uniform distribution of moisture, when the process of drying the tree stops, and the percentage of moisture in the wood and the environment become equal.
It turns out that the hygroscopic equilibrium of the climate of the region can be determined by the equilibrium humidity of the house, built of natural wood. The moisture contained in the wood, as indicated earlier, consists of free and constituent. The boundary of these concepts is the point of saturation. This is the state of wood when the free liquid is completely removed and the cells remain saturated with water. Regardless of the type of wood, humidity at the saturation point is considered to be 23–31%.
What humidity is accepted as a standard?
For the use of lumber in construction, it is necessary to sort them according to the degree of humidity. The standard wood moisture content is assumed to be approximately 15% and is considered a constant indicator for any wood species that all wood product manufacturers seek. In this state, the material is perfectly stored, used for construction and finishing works. The concepts of equilibrium and standard moisture are familiar to any specialist working in the production of lumber.
Insufficiently dried wood is a real environment for the formation of mold and the growth of fungi, which leads to its destruction. Modern drying equipment allows to achieve a moisture content of 6%. In this case, the material will be even more resistant to decay, resistant to deformation and durable.
Moisture-related properties of wood
The main deforming properties of wood, the moisture content of which varies, are as follows:
- Shrinkage - characterized by a decrease in the volume and linear dimensions of wood. The drying process is associated only with the removal of bound water from the wood, free has no effect. When sawing logs on boards make allowances for shrinkage of the material.
- Warping is a change in lumber in shape that occurs during drying and improper storage. Often this occurs due to different shrinkage in the transverse or longitudinal direction.
- Moisture absorption - the ability of wood material to absorb moisture from the air. This property is independent of the wood species and greatly degrades its properties, reducing biostability. For protection, wooden surfaces are coated with paints and varnishes.
- Swelling - the volume and linear dimensions increase, as a result, the amount of bound water in the cells increases. This phenomenon occurs when the logs are fused in water or in wood in moist air. Moreover, swelling across the fibers is observed stronger than along.
The property of wood to absorb liquids is used when impregnating it with antiseptics, cooking to produce cellulose, and alloying wood.
Wood specific gravity
To determine the weight of wood, a value called density or specific gravity is used. It is calculated by the ratio of the mass of the material to the unit volume. This parameter is very variable and depends on the moisture and wood species, therefore it is customary to use its average value. To calculate this value, proceed as follows:
- Determine the mass of wood by weighing.
- Calculate the volume. It is calculated according to a special technique, taking into account only the volume of pure wood without voids that are formed during installation.
Density is usually calculated at the natural moisture content of a particular material. The specific gravity of any wood is measured in kg / m 3 and is entered in the table.
Determination of wood weight of natural moisture
The consumer needs to know the weight of wood for:
- Transportation of purchased raw materials - the choice of transport and payment for delivery.
- Determination of the weight of the finished structure - installation of the foundation and installation of ceilings.
- Determining the storage location of the material.
Calculating the weight of the material you buy is very easy. To do this, you need to determine the specific gravity of a particular wood from the table and multiply by the number of cubes of purchased material.
Determination of the degree of humidity in domestic conditions
There are several ways to determine how much water is contained in wood. In everyday life, an electric moisture meter is often used to measure wood moisture. Its action is associated with a change in the electrical conductivity of the material from its moisture content. Needles are mounted in the body of the device, to which an electric current is supplied. To carry out the measurement, it is enough to insert them into the studied wood and turn it into an electric network. The scale displays the moisture value exactly in the place where the needles are inserted. The disadvantage of the electrovlagometer is that it only determines humidity in a specific place. Experienced craftsmen who work with wood for a long time can determine the water content in it in appearance. For this, the color of the crust, the presence of cracks at the end and along the fibers are taken into account.
Moisture Measurement Methods
In industrial conditions, to determine the moisture content of wood, use the direct and indirect method. Direct refers to the weighted method of determination. For this, a certain number of boards are selected from a controlled batch. A sample is cut from each and weighed on a balance. Then they begin to dry it in the oven, continuing weighing every two hours until the readings become the same. Then, using all the data, the result is calculated using a special formula. This method requires a lot of time, about nine hours, so it is rarely practiced and an indirect method is adopted. To do this, use a device for measuring wood moisture, which is based on measuring the conductivity of the material. The conductivity sensor is a three-needle probe. It is stuck along the fibers to the entire depth of the electrodes.
The readings are viewed on a milliampere scale, which is graduated in percent. A split cup is used to measure the moisture content of the chips. In it between two electrodes in the form of disks a certain portion of pressed material is placed. The moisture content of particle boards is determined by a four-needle probe. This method is easy to use, gives a quick result, but errors are possible, especially with high humidity of the material. Another instrument for measuring wood moisture is the Testo 616 hygrometer. It allows you to quickly perform the procedure for measuring the water content in wood without destroying it. The measurement depth is 5 cm. Data is displayed as a percentage of weight to dry wood mass. The device is very convenient for obtaining data in dynamics, for example, when drying walls or floors.
Conclusion
Wood is a natural product and is very susceptible to temperature and humidity. Due to its hygroscopicity, it constantly changes its moisture content due to environmental conditions. This process is called tree respiration; it can absorb water from the air or secrete it. A similar phenomenon is a reaction to changes in the indoor climate. When the atmosphere (air) remains unchanged indoors, the moisture content of the wood assumes a constant value, called equilibrium.