Wood is one of the most demanded materials used in construction, furniture manufacturing, carriage building and many other sectors of the national 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 wood. How to determine it and what it should be for different types of wood, we will consider in this article.
Types of humidity
Wood is of natural origin, so it is very sensitive to fluctuations in air temperature and atmospheric pressure. Subject 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 generally accepted that the most important characteristic is humidity, which determines the ratio of moisture to dry matter. It is subdivided into:
- Absolute - hasdifferent values for each type of tree. Wood moisture is determined according to GOST 8486-86.
- Relative - measurements are carried out in a wet state and are divided into free and bound.
Besides this, humidity happens:
- Natural - 27-81%. Used for various ancillary work: making formwork, rafters and battens.
- Furniture - 8-12%. It is used in furniture production and to obtain glued beams.
- Transport - 18-22%. Use for all other types of work, except for 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 subject to fungal infections.
Degree of humidity
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 cut - the percentage of wood moisture in this case ranges from 50 to 100.
- Air-dry - kept outdoors for a long time. Its humidity completely depends on natural conditions and the season, the degree of humidity is 15–20%.
- Room-dry - located in residential areas with a water content of no more than 10%.
- Absolutely dry - practically no moisture, its content is 0%. In thatcase, it is often deformed.
Knowing the degree of moisture is necessary for the practical use of wood.
What is natural moisture?
Wood is a living material that constantly grows and breathes. Water is essential for its life. Its content depends on the species, weather conditions and the place where the tree grows. Immediately after cutting or sawing a log, the material has moisture, which is called natural. Exact norms for this value do not exist. It varies within 27–81%. Wood with natural moisture is often infected with fungi and bacteria, and rots. It is not recommended to transport the logs over long distances immediately after felling, after a few days they become unusable.
Material with natural moisture is used only for the construction of low-responsibility structures. 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 impact on products made from wood. This is especially noticeable with sudden 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:
- Linked - stored incell membrane, is held by physico-chemical bonds and strongly affects many properties of wood.
- Free - located in the intercellular spaces and cell cavities, is preserved due to mechanical bonds, is easily removed and has little effect on the quality of the material.
When wood dries, first of all, free moisture evaporates and only after it - bound moisture. The process will last until the water is evenly distributed throughout the material, and equals the relative humidity of the air. The same happens with the absorption of moisture.
Equilibrium wood moisture content
Any wood is able to absorb water vapor and water and release the resulting moisture. Equilibrium humidity is established depending on the moisture content and ambient temperature and the duration of the wood exposure to air. This concept is associated with a complete uniform distribution of moisture, when the drying process of the wood stops, and the percentage of moisture in the wood and the environment become equal.
It turns out that the hygroscopic equilibrium of the region's climate can be determined by the equilibrium humidity of a house built of natural wood. The moisture contained in wood, as mentioned earlier, consists of free and component. The boundary of these concepts is the saturation point. 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, the moisture content at the saturation point is considered to be equal to23–31%.
What is the standard humidity?
For the use of lumber in construction, it is necessary to sort them according to the degree of moisture. The standard moisture content of wood is assumed to be approximately 15% and is considered a constant value for any type of wood, which all manufacturers of wood products strive for. In this state, the material is perfectly stored, used for construction and finishing work. The concepts of equilibrium and standard moisture are familiar to any professional working in the lumber industry.
Insufficiently dried wood is a real environment for mold and fungus to grow, causing it to break down. Modern drying equipment allows you 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 properties of wood
The main deforming properties of wood, the moisture content of which changes, 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 water has no effect. When sawing logs into boards, allowances for material shrinkage are made.
- Warping - a change in shape of lumber that occurs during drying and improper storage. Often this is due to different shrinkage in the transverse or longitudinal direction.
- Moisture absorption - abilitywood material to absorb moisture from the air. This property does not depend on the type of wood and greatly impairs 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 logs are rafted on water or wood is kept in moist air. Moreover, swelling across the fibers is observed more than along.
The property of wood to absorb liquids is used when impregnating it with antiseptics, cooking to obtain cellulose, rafting of wood.
Specific weight of wood
To determine the weight of wood, a quantity 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 humidity and type of wood, so 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 method, taking into account only the volume of clean wood without voids that form during installation.
Density is usually calculated based on the natural moisture content of a particular material. The specific gravity of any wood is measured in kg/m3 and entered into the table.
Determining the weight of natural moisture wood
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 floors.
- Determining where to store material.
Calculate the weight of the material you buy is very easy. To do this, you will need to determine the specific gravity of a particular wood from the table and multiply it by the number of cubes of purchased material.
Determining the degree of humidity at home
Determining how much water is contained in wood can be done in different ways. In everyday life, an electric moisture meter is often used to measure the moisture content of wood. 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 wood under study and turn it on to the electrical network. The humidity value is displayed on the scale exactly in the place where the needles are inserted. The disadvantage of the electric moisture meter is that it determines the humidity only in a particular place. Experienced craftsmen who have been working with wood for a long time can determine the water content in it by its appearance. For this, the color of the bark, 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 a direct and indirect method. The direct method includes the weight method of determination. To do this from a controlled batchchoose a certain number of boards. A sample is cut from each and weighed on a scale. Then they begin to dry it in an oven, continuing to weigh it every two hours until the readings become the same. Then, using all the data, the result is calculated using a special formula. This method takes a very long time, about nine hours, so they rarely practice and adopt an indirect method. To do this, use a device for measuring the moisture content of wood, which is based on measuring the electrical conductivity of the material. The probe of the conductometric device is a three-needle probe. It is stuck along the fibers to the entire depth of the electrodes.
Indications look at the milliamp scale, which is graduated in percentage. A split glass is used to measure the moisture content of chips. A certain portion of the compressed material is placed in it between two electrodes in the form of disks. The moisture content of particle boards is determined with a four-needle probe. This method is easy to use, gives a quick result, but errors are possible, especially with high material moisture. Another device for measuring the moisture content of wood 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. The data is displayed as a percentage of the dry weight of the wood. 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 sensitive to temperature and humidity. Due to its hygroscopicity, it constantly changes its moisture content due to environmental conditions. This process is called the breathing of a tree, it can absorb water from the air or release it. A similar phenomenon is a reaction to changes in the microclimate of the room. With a constant state of the atmosphere (air) in an enclosed space, the moisture content of wood takes on a constant value, called equilibrium.
