Wood moisture content: methods of determination, GOST, lumber moisture content

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 emits 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 mass. It is divided into:

• Absolute - has different meanings for each type of wood. Wood moisture content is determined according to GOST 8486-86.
• Relative - measurements are taken in a wet state and are divided into free and bound.

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In addition, humidity can be:

• Natural – 27–81%. Used for various auxiliary works: making formwork, rafters and sheathing.
• Furniture – 8–12%. It is used in furniture production and for the production of laminated veneer lumber.
• 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.

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The most suitable wood for construction work is transport wood. The material with a humidity of about 20% is easy to process, it does not warp and is not subject to fungal infections.

What temperature should be in a carpentry workshop?

The air temperature should be maintained at approximately 20 degrees, and the humidity should be between 40-60%.

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Humidity level

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 in open areas for a long time. Its humidity completely depends on natural conditions and the time of year, the degree of humidity is 15–20%.
• Room-dry - located in residential premises with a water content of no more than 10%.
• Absolutely dry – there is practically no moisture, its content is 0%. In this case, it is often subject to deformation.

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Knowing the degree of humidity is necessary for the practical use of wood.

Doesn't winter wood require drying?

There is a common myth about winter wood on the Internet. From a theoretical point of view, it looks like this: in winter, the leaves fall from the trees, and accordingly the process of photosynthesis and plant nutrition stops. As a result of the slowing down of the life processes of the tree, water stops flowing from the root system, and accordingly the moisture content of the wood is lower.

The moisture content of winter wood is higher than that of summer wood. In winter, the tree does not stop its vital activity, the movement of sap slows down, but the moisture remains in the trunk. In summer, the liquid constantly moves, evaporating under the influence of high temperature.

What is natural humidity?

Wood is a living material that constantly grows and breathes. Water is necessary for its life. Its content depends on the species, weather conditions and location of the tree. Immediately after cutting or sawing a log, the material has moisture, which is called natural. There are no exact standards for this value. It varies between 27–81%. Wood with natural moisture often becomes infected with fungi and bacteria and rots. It is not recommended to transport 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-critical 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 structures.

EB lumber

In the price lists of trading companies you can find lumber with the EB (natural moisture) category, but this is a completely different concept than the moisture content of freshly cut wood. Category EB denotes “raw” timber or board with a moisture content exceeding 22%. Lumber with a moisture content of freshly sawn wood (80-95%) cannot enter the market; such material during storage and transportation is easily affected by fungus and turns blue-gray, the effect is called “blueing.” In order to preserve its presentation, lumber after sawing is subjected to air drying by ventilating under natural conditions.

Despite the formal discrepancy, the term “natural moisture lumber” is widely used by specialists and denotes the equilibrium moisture content of wood acquired under conditions of prolonged exposure to normal atmospheric conditions. For softwood lumber, the natural moisture limit was determined by GOST 8486-86. According to the standard, lumber containing more than 22 percent moisture is damp; this value can be considered the threshold for determining the natural moisture content of a building material. Raw timber is classified as the last - fourth grade; price differences between dry and raw lumber can range from 50%.

Features of moisture content in wood

Everyone has known for a long time that air humidity has a significant impact on products made from wood. This is especially noticeable with sudden changes in moisture content in the air. The phenomenon is explained by the peculiarities of 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 physical and chemical bonds and greatly affects many properties of wood.
• Free – located in intercellular spaces and cell cavities, preserved due to mechanical connections, easily removed and has little effect on the quality of the material.

When wood dries, free moisture evaporates first, and bound moisture evaporates only after that. The process will continue until the water is evenly distributed throughout the material and equals the relative humidity of the air. The same thing happens when moisture is absorbed.

Types of wood for making furniture

When choosing a material for a particular job, craftsmen evaluate indicators such as hardness, stability, degree of oxidation (color changes under the influence of UV rays), resistance to stress, expressiveness of texture.

For each type of tree, and often for different parts of the same tree, the properties may be different. It all depends on the conditions in which he had to grow.

In Russia, oak, beech, linden and maple are most often used for the production of furniture modules. Oak wood is highly valued because of its strength and beautiful texture. Furniture made from it lasts a very long time. But this material is difficult to process due to its fragility. When making furniture, it is best to take oak that is at least 150 years old.

Equilibrium moisture content of wood

Any wood is capable of absorbing water vapor and water and releasing the resulting moisture. Depending on the moisture content and ambient temperature and the length of time the wood is exposed to air, equilibrium moisture content is established. This concept is associated with the complete uniform distribution of moisture when the drying process of the tree stops and the percentage of moisture in the wood and the environment become equal.

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It turns out that the hygroscopic equilibrium of a region’s climate can be determined by the equilibrium humidity of a house built from 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 has been completely removed and the cells remain saturated with water. Regardless of the type of wood, the humidity at the saturation point is generally considered to be 23–31%.

Natural moisture lumber

What should be the natural moisture content of lumber? GOST 3808.1-80 regulates this indicator at 22%. Natural moisture content timber can be created from almost any type of wood. On the territory of the Russian Federation, such raw materials are produced mainly from coniferous wood, such as spruce, pine, cedar or larch.

Planed boards and timber made from these varieties are distinguished by an increased level of strength and resistance to the effects of the environment on them, for this reason they are excellent for internal and external work in a building. This material is produced by simply sawing logs.

Do not forget that lumber with natural moisture can rot and be damaged by microorganisms, so they must be subjected to protective treatment.

The timber is produced in a wide variety of sections, which makes it possible to purchase exactly the option that is best suited for solving the tasks at hand. The main advantages of natural moisture timber include high quality and affordable cost. Compared with the cost of dry timber, the price of wet material is almost 30% lower.

Operating humidity values ​​for lumber and wooden products

What is the standard humidity?

To use lumber in construction, it is necessary to sort it according to the degree of moisture. The standard wood moisture content is taken to be approximately 15% and is considered a constant indicator for any type of wood, which all manufacturers of wood products strive for. In this condition, the material is perfectly stored and used for construction and finishing work. The concepts of equilibrium and standard moisture content are familiar to any specialist working in the production of lumber.

Insufficiently dried wood is a real environment for the formation of mold and fungal growth, which leads to its destruction. Modern drying equipment allows you to achieve a humidity level of 6%. In this case, the material will be even more resistant to rotting, resistant to deformation and durable.

Does chamber drying guarantee results?

Kiln drying is a method of processing lumber where it is dried in a drying chamber. Forced drying methods allow you to get results faster, and the wood can be brought to a humidity level of 10 - 20%. Let's consider whether chamber-dried wood is so useful.

• The production process of such lumber is more expensive, resulting in a higher final cost of production.
• Over-drying wood by force can lead to cracks.
• When lumber is delivered to the site, it will gain moisture equilibrium with the environment.
• When chamber drying thick materials (timber), the core will remain wet.

Kiln-dried wood is suitable for certain types of work: the production of furniture, individual decorative elements made of wood, flooring, etc. They do not allow warping and shrinkage of the material and require ideal joining of wooden elements.

Wood properties related to moisture

The main deforming properties of wood whose moisture content changes are the following:

• 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 wood; free water has no effect. When sawing logs into boards, allowances are made for shrinkage of the material.
• Warping is a change in shape of lumber that occurs during drying and improper storage. This often occurs due to different shrinkage in the transverse or longitudinal direction.
• Moisture absorption is the ability of a wood material to absorb moisture from the air. This property does not depend on the type of wood and greatly impairs its properties, reducing biostability. To protect wooden surfaces, they are coated with paints and varnishes.
• Swelling – volume and linear dimensions increase, resulting in an increase in the amount of bound water in the cells. This phenomenon occurs when logs are floated on water or wood is kept in humid air. Moreover, swelling across the fibers is observed more strongly than along them.

The property of wood to absorb liquids is used when impregnating it with antiseptics, cooking it to obtain cellulose, and rafting wood.

Determination of moisture content and drying of wood

Wood moisture determination

There are several ways to determine wood moisture content. To determine humidity, you can use a special device - an electric moisture meter. The operation of the device is based on changes in the electrical conductivity of wood depending on its humidity. The needles of an electric moisture meter with electrical wires connected to them are inserted into the wood and an electric current is passed through them, while the moisture content of the wood is immediately marked on the scale of the device in the place where the needles are inserted.

Electric moisture meters EVA-2M are widely used, determining humidity in the range of 7 - 60%.

Many experienced carpenters determine the moisture content of wood by eye. Knowing the types of wood, its density and other physical properties, it is possible to determine the moisture content of wood by weight (weighing several identical pieces of the same species in turn), by the presence of cracks at the end or along the grain of the wood, by warping and other signs.

With the weight method, a section of moisture 10-12 mm thick is sawed off from a board (control sample) at a distance of 300 - 500 mm from the end, thoroughly cleaned of burrs, sawdust and weighed, the result is recorded in a journal, and the section is placed in a drying cabinet with a temperature of up to 103 °C. After 6 hours of drying, the section is weighed and the mass is recorded in a journal, then dried again and weighed every 2 hours after drying. If after repeated weighings the mass of the section does not change, this means that the section has been dried to an absolutely dry state with humidity W0 = 0% and mass P.

The initial moisture content of the sample wood is determined by the formula: W = (Pн - Рс): Рс * 100%, where W is the initial moisture content, %; Рн and Рс - initial mass and mass in an absolutely dry state of the sample.

Also, checking the current humidity during the drying process can be carried out by weighing control samples with a length of at least 1000 mm, which are also cut from the boards to be dried at a distance of 300 - 500 mm from the end, cleaned of bark, burrs, sawdust, after which the ends are painted. . The sample is weighed to the nearest 5 g.

When processing lumber with a plane, its thin shavings, compressed by hand, are easily crushed, which means the material is wet. If the chips break and crumble, this indicates that the material is dry enough. When making transverse cuts with sharp chisels, also pay attention to the shavings. If they crumble or the wood of the workpiece itself crumbles, this means that the material is too dry.

The complete saturation of wood with water is called the hygroscopic limit. This stage of humidity, depending on the type of wood, is 25-35%. In practice, wood is distinguished: room-dry (with a humidity of 8-12%), air-dry artificially dried (12-18%), atmospheric-dry wood (18-23%) and damp (humidity exceeds 23%).

Wood from a tree that has just been cut down or has been in water for a long time is called wet, its moisture content is up to 200%. There is also a distinction between operational humidity, which corresponds to the equilibrium moisture content of wood under specific conditions.

Requirements for wood moisture content in products Table 1.

The moisture content of freshly cut wood (which has the moisture content of a growing tree) depends on the species and the location of sampling along the cross section of the trunk. In coniferous species, the moisture content of the wood in the peripheral part of the trunk (sapwood) is greater than the moisture content of the wood in the central part of the trunk (core). In deciduous species, the moisture content throughout the entire section of the trunk is approximately the same.

The moisture content of driftwood is usually higher than that of wood delivered overland, and the moisture content of driftwood is higher than that of freshly cut wood. Thus, the moisture content of the sapwood part of pine logs after rafting increases to 150%, and the core part of the logs - to 50%.

As you know, wood has a cellular structure. Moisture in wood can fill cell cavities, intercellular spaces and permeate cell walls. Moisture that fills cell cavities and intercellular space is called free, and moisture that permeates cell walls is called bound, or hygroscopic.

Freshly cut wood has both free and bound moisture. When drying wood, free moisture is first removed, and then bound moisture.

Moisture content of freshly cut wood Table 2

Drying wood

When making any type of joinery, the wood must be dry. Dry wood has high strength, warps less, is not susceptible to rotting, is easy to glue, is better finished, is more durable, and finished products do not crack. Any wood of various species reacts very sensitively to changes in environmental humidity. This property is one of the disadvantages of timber. At high humidity, wood easily absorbs water and swells, but in heated rooms it dries out and warps. Therefore, for joinery products, wood must be dried to the degree of humidity that is expected in the future during their use. Indoors, wood moisture content of up to 10% is sufficient, and outdoors - no more than 18%.

Drying is the process of removing moisture from wood by evaporation. Drying of lumber can be natural or artificial.

Natural drying

Natural drying occurs under the influence of atmospheric circulating air, which evaporates moisture from the wood. Natural drying of lumber is combined with storage. It is necessary to dry the wood in the shade, under a canopy and in a draft. When dried in the sun, the outer surface of the wood quickly heats up, but the inner surface remains damp. Due to the difference in stress, cracks form and the wood quickly warps. Wet lumber is dried immediately after sawing. This prevents the appearance of wormholes and rot.

Stacked materials dry worse in spring than in summer. This process occurs more intensively in June. The time for drying coniferous lumber under natural conditions to 18 - 22% humidity is given in the table.

Time required to dry lumber stacked with spacers to 18-22% moisture content:

Table 3

Note: For larch, drying times increase by 60%. Climate zones

1st - Arkhangelsk, Murmansk, Vologda, Perm, Sverdlovsk, Sakhalin, Kamchatka, Magadan regions, the northern half of Western and Eastern Siberia and Komi, the northern part of the Khabarovsk Territory and the eastern part of the Primorsky Territory. 2nd - Karelia, Leningrad, Novgorod, Pskov regions, the southern part of the Khabarovsk Territory and the western part of the Primorsky Territory. 3rd - Smolensk, Kaliningrad, Moscow, Tver, Oryol, Tula, Ryazan, Ivanovo, Yaroslavl, Nizhny Novgorod, Bryansk, Chelyabinsk, Vladimir, Kaluga, Kostroma, Amur regions, the southern part of Western and Eastern Siberia, the Republic of Chuvashia, Mari El, Mordovia, Tatarstan, Bashkotorstan, Udmurtia. 4th - Kursk, Astrakhan, Samara, Saratov, Volgograd, Orenburg, Voronezh, Penza, Tambov, Rostov, Ulyanovsk regions, North Caucasus.

Natural drying of lumber decreases sharply from mid-August. Spruce lumber dries faster than pine lumber. Thin-size materials dry faster than thick-size materials. Sawn softwood 16 mm thick loses half of the initial moisture content after 4 days of drying, then the drying intensity drops sharply. Lumber with a thickness of more than 20 mm evaporates most of the moisture after 20 - 30 days of drying.

Laying the stack begins with the construction of the base, the height together with the logs is at least 50 cm. The top of the base must be horizontal. The base supports are placed in increments of 1.5 m to prevent deflection of the lumber. The shape of the stacks is square or rectangle.

Stacks of lumber are protected by a roof that protects the material from precipitation, direct exposure to sunlight and dust.

The lumber is laid on dry softwood pads measuring 25x40 mm. The outermost spacers are laid flush with the ends of the boards, and the rest at a distance between them of no more than 70 cm. To create better ventilation of the stack, all spacers are laid in a strictly vertical row along a plumb line. Between the boards or bars stacked in stacks, equal-width gaps (splits) are left, forming vertical channels along the entire height of the stack. The width of the spacing, depending on climatic conditions and the cross-section of the boards, is set for lumber with a thickness of up to 45 mm from 1/2 to 3/4 of the width of the lumber and for lumber with a thickness of over 45 mm from 1/5 to 1/3 of the width of the lumber. To ensure uniform drying of lumber along the height of the stack, vents 150 mm high are installed at a distance of 1 and 2 m from the bottom row of boards. The boards are laid with the inner faces up to reduce their warping. To prevent cracking, it is recommended to carefully paint the ends of the boards with oil paint or soak them several times with hot drying oil to protect the pores of the wood. The ends should be processed immediately after cross-cutting to size. If the wood is characterized by high humidity, then the end is dried with a blowtorch, and only then painted over.

Chamber drying of lumber

Chamber drying is the main method in which lumber is dried in drying chambers equipped with the necessary equipment and devices. The chambers regulate temperature, humidity and the degree of air circulation.

Atmospheric drying is used for preliminary drying of lumber and, as a rule, is combined with a wood drying chamber.

Lumber can be stacked in pieces or in batches. When forming a stack individually, dry (with a moisture content of no more than 18%) calibrated softwood and hardwood pads with a cross-section of 25 x 40 mm and a length equal to the width of the stack are laid between the rows of boards. Spacers along the height of the stack must be laid perpendicular to the boards and strictly vertically one above the other.

The stack is formed from boards of the same type and thickness. The number of spacers laid along the length of the stack is given in the table:

Number of spacers laid along the length of the stack Table 4

Note: The numerator is the number of spacers for stacks made of softwood, the denominator is the number of spacers made of hardwood.

Methods for stacking lumber depend on the direction (circulation) of the drying agent. For drying chambers with countercurrent circulation, lumber is laid at intervals (splits), and for chambers with transverse reverse and countercurrent rectilinear circulation - densely.

Drying modes

Drying of lumber occurs under a certain temperature and humidity regime, which is understood as a natural alternation of processes of temperature and humidity effects on wood in accordance with its humidity and drying time.

During the drying process in the chamber, the air temperature gradually increases (in stages) and the relative humidity of the drying agent decreases. Drying modes are prescribed taking into account the type of wood, the thickness of the lumber, the final moisture content, the quality category of the materials being dried and the design (type) of the chambers.

Quality categories of dried wood Table 5.

Drying modes, depending on the purpose of the lumber, provide for two processes - low temperature and high temperature. In low-temperature conditions, moist air with a temperature of less than 100°C is used as a drying agent in the first drying stage.

Depending on the requirements for lumber, modes are divided into:

• soft M, with soft modes defect-free drying is obtained while preserving the physical and mechanical properties of wood and color;
• normal H, under normal conditions defect-free drying is obtained with a possible slight change in color of coniferous wood, but maintaining strength;
• forced F, with forced drying modes, wood is obtained with preservation of bending, tensile and compressive strength, but with a decrease in chipping and splitting strength by 15 - 20% and with possible darkening of the wood.

These modes provide for a three-stage change in the parameters of the drying agent, and the transition from each stage of the mode to the next can be made only when the material reaches a certain humidity specified in the mode.

The modes of the high-temperature drying process for batch chambers provide for a two-stage change in the parameters of the drying agent, and the transition from the first stage to the second is made when the wood reaches a moisture content (transition) of 20%. The high temperature regime is determined depending on the species and thickness of the lumber.

High-temperature conditions may be used for drying wood used for the manufacture of non-load-bearing elements of building structures, in which a decrease in strength and darkening of the wood is allowed.

Wood drying process

Before the drying process is carried out according to the selected mode, the wood is heated with steam supplied through humidifying pipes, with heating devices turned on, fans running and the sugar-exhaust ducts closed. At the beginning of heating, the temperature of the drying agent should be 5°C higher than the first stage of the mode, but not more than 100°C. The degree of saturation of the environment should be in the range of 0.98 - 1 for wood with an initial moisture content of more than 25%, and 0.9 - 0.92 for wood with a moisture content of less than 25%.

The duration of the initial heating of wood depends on the type of wood and for lumber of coniferous species (pine, spruce, fir and cedar) at an outside temperature of more than 0°C is 1 - 1.5 hours at a temperature less than 0°C - 1.5 - 2 hours for every centimeter of thickness. The duration of heating of lumber of soft deciduous species (aspen, birch, linden, poplar and alder) increases by 25%, and for lumber of hard deciduous species (maple, oak, ash, hornbeam, beech) increases by 50% compared to the duration of heating of coniferous wood breeds

After warming up, the parameters of the drying agent are adjusted to the first stage of the mode and then they begin to dry the lumber, observing the established mode. The temperature and humidity of the air are regulated by valves on the steam lines and gates of the sugar-exhaust channels.

During the drying process, residual internal stresses arise in the wood; to eliminate them, intermediate and final moisture-heat treatment is carried out in an environment of high temperature and humidity. In this case, lumber is processed, dried to operational humidity and subject to further mechanical processing.

Intermediate moisture-heat treatment is carried out during the transition from the second to the third stage or from the first to the second when drying at high temperatures. Lumber of coniferous species with a thickness of 60 mm and above and deciduous trees (depending on the species) with a thickness of 30 mm and above are subjected to moisture-heat treatment. In the process of heat and moisture treatment, the temperature of the environment should be 8°C higher than the temperature of the second stage, but not more than 100°C, with a degree of saturation of 0.95 - 0.97. The final moisture-heat treatment is carried out only when the wood reaches the required final average moisture content. During the final heat and moisture treatment, the temperature of the environment is maintained 8°C above the last stage of the regime, but not more than 100°C. At the end of the final moisture-heat treatment, the dried lumber is kept in the chambers for 2 - 3 hours at the parameters provided for by the last stage of the regime, after which the chambers are stopped.

Specific gravity 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 a unit volume. This parameter is very variable and depends on humidity and wood type, so it is customary to use its average value. To calculate this value proceed as follows:

• The mass of wood is determined by weighing.
• Calculate the volume. It is calculated using a special method, taking into account only the volume of clean wood without voids that are formed during installation.

Density is usually calculated using the natural moisture content of a particular material. The specific gravity of any wood is measured in kg/m3 and entered into the table.

Incorrect laying to dry

The drying result depends on the correct stacking. We list the most common mistakes when stacking.

• Vapor-tight material for protection from rain - such “protection” leads to the formation of fungus on the material and slows down the drying process. This mistake is made especially often in the summer. Instead of film, it is better to build a simple canopy over the stacks.
• No vertical gaps - adjacent stacks should not be close together.
• Spacers are made through the row - wooden blocks should separate each row of lumber from each other.

Determination of wood weight at natural moisture content

The consumer needs to know the weight of wood for:

• Transportation of purchased raw materials – selection of transport and payment for delivery.
• Determination of the weight of the finished structure - installation of the foundation and installation of floors.
• Determining the storage location of the material.

Calculating the weight of the material you buy is very easy. To do this, you will need to determine the specific gravity of specific wood from the table and multiply it by the number of cubes of purchased material.

Determining the degree of humidity in domestic conditions

There are different ways to determine how much water is contained in wood. 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 due to its humidity. Needles are installed in the body of the device, to which electric current is supplied. To carry out the measurement, it is enough to insert them into the wood under study and connect it to the electrical network. The scale displays the humidity value exactly in the place where the needles are inserted. The disadvantage of an electric moisture meter is that it only determines humidity in a specific location. Experienced craftsmen who have worked with wood for a long time can determine its water content by its appearance. To do this, take into account the color of the bark, the presence of cracks at the end and along the fibers.

Humidity measurement methods

In industrial conditions, direct and indirect methods are used to determine wood moisture content. The direct method includes the weight method of determination. To do this, a certain number of boards are selected from the controlled batch. A sample is cut out of each and weighed on a scale. Next, 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 lot of time, about nine hours, so it is rarely practiced and the indirect method is used. To do this, use a device for measuring wood moisture, which is based on measuring the electrical conductivity of the material. The sensor of the conductometric device is a three-needle probe. It is stuck along the fibers to the entire depth of the electrodes.

The readings are shown on a milliampere scale, which is graduated as a percentage. To measure the moisture content of the chips, use a split glass. A certain portion of compressed material is placed in it between two electrodes in the form of disks. The humidity of particle boards is determined with a four-needle probe. This method is easy to use and gives quick results, but errors are possible, especially with high moisture content of the material. Another device for measuring wood moisture is the Testo 616 hygrometer. It allows you to quickly measure the water content in wood without destroying it. The measurement depth is 5 cm. The data is displayed as a percentage of weight to dry wood mass. The device is very convenient for obtaining dynamic data, for example, when drying walls or floors.