Wood is one of those materials that are sensitive to changes in external environment, primarily to fluctuations in temperature and humidity. One of the key properties of wood is the ability to absorb atmospheric moisture, i.e. hygroscopicity.
What is the natural moisture content of wood?
By natural moisture content of wood we mean the moisture content that is present in the still growing state of the tree or after it has been sawed and sawn into individual elements without any additional drying. This figure varies very widely - on average from 30% to 80%, specific numbers depend on the specific type of wood.
Coniferous species are characterized by the highest natural humidity:
- Spruce - 90%;
- Different types of pine trees - 88-92%;
- Fir - 90-92%;
- Larch - 80-82%
Softwoods:
- Willow - 85%;
- Aspen, alder - 80-82%;
- Linden - on average 60%.
Hardwoods:
- Different varieties of birch trees - 68-78%;
- Beech - 65%;
- Elm - 75-78%;
- Hornbeam - 60%;
- Oak - 50%.
At the same time, wood felled in winter period has a lower level of humidity than summer.
How and why is wood dried?
After sawing into individual boards/beams, the wood is dried under atmospheric conditions or using chambers, hydrophobic liquids, and various heating elements.
Drying wood protects or at least reduces the likelihood of its rotting, prevents deformation of shape and size, improves the quality of finishing of the finished product, and increases the strength of adhesive joints. During the drying process, not only weight loss occurs wooden element due to water loss, but also a slight change in size - up to 5-7% in length, width or height.
The main purpose of drying is to bring the wood to the so-called. equilibrium humidity, i.e. one that it would acquire after a certain period of operation under specific conditions. If this is not done artificially, then the process will occur naturally - for example, doors will begin to become damp and swell, parquet or lining may dry out, and as a result, at the joints individual elements cracks will appear, etc.
Depending on where and under what conditions a product made from this wood is subsequently used, it is dried to a certain level of humidity. So for floor coverings the optimal humidity will be 6-8%, for those items that will come into contact with atmospheric air(those. window frames, doors) - 11-12% or even more in the case of a humid climate in the region.
Wood is a fairly porous material containing a large number of capillaries filled with moisture. In practice, wood moisture content is defined as the ratio of the weight of water contained in the tree to the weight of absolutely dry wood. There is a concept of “free” and “bound” moisture. “Free” moisture is contained in the pores and capillaries of the tree. “Bound” moisture is that contained directly in the cells of the tree.
When drying, the tree shrinks - it decreases in size (volume). In this case, there is practically no decrease in size along the fibers (along the length of the board), but in the direction transverse to the grain, there is a significant change in size (along the thickness and width of the board). The magnitude of this change depends on the type of wood and the specific value of the change in wood moisture content. In life, the most unpleasant surprises are associated with changes in the width of the board.
For example, if you lay a floor with a board that has natural moisture, then the decrease in its width over time can be so significant that two adjacent boards will lose their grip on each other. In this case, to remove the cracks, you will have to tear off all the boards from the joists and lay them again, fitting them end to end.
“What humidity should the board have?” you ask. It’s simple - any wooden product, during its operation, tends to the so-called “equilibrium humidity”. “Equilibrium humidity” is determined by the temperature and humidity of the air in the environment where the board will be located. You can see the values of this humidity in the table. For a residential premises it averages 8-10%, for a street it averages 12-14%. It logically follows from this that a damp board will dry out indoors, losing its width, on the other hand, a dry board will be moistened outdoors, expanding.
Natural moisture content, final wood moisture content
Natural humidity- this is the moisture inherent in wood in a growing or freshly cut (sawn) state, without additional drying. Natural humidity is not standardized and can range from 30% to 80%. The natural moisture content of wood varies depending on growing conditions and time of year. Thus, the natural humidity of freshly cut trees in a “winter” forest is traditionally less than the humidity of freshly cut trees in a “summer” forest.
Initial humidity- the same as natural humidity. A freshly felled tree has a maximum moisture content, which for different species can even exceed 100%. Balsa wood can have a freshly cut moisture content of up to 600%. In practice, we deal with smaller values (30-70%), because After cutting, some time passes before the tree is sawed and placed in the dryer, and it, of course, loses a certain amount of water. We take the initial moisture content to be the moisture content of the wood that it has before being sent to the drying chamber.
Final humidity- this is the humidity that we want to get after a full drying cycle. In this case, the purpose of the product made from dried wood is taken into account.
First of all, wood drying is the process of removing moisture from wood by evaporation.
Drying wood is one of the most important operations in the wood processing process. The wood is dried after sawing, but before wood processing. The wood is dried in order to protect it from damage by wood-staining and wood-decaying fungi during its further storage and transportation. Drying prevents wood from changing shape and size during the manufacturing and use of products made from it, improves the quality of wood finishing and gluing. The humidity to which wood is dried depends on its area further application. The whole point is to bring the moisture content of the board to the same value that a product made from this board would reach over time during operation under these conditions. This humidity value is called “equilibrium humidity”; it depends on the humidity and temperature of the surrounding air. For example, the board from which parquet and other products used indoors will be made should have a humidity of 6-8%, since this is the humidity that will be equilibrium. For products that will be used in contact with the atmosphere (for example: wooden windows, outer skin at home) the equilibrium humidity will be 11-12%.
You ask: “What will happen otherwise?” We answer: Otherwise, what happens all the time in Russia will happen, namely, the consumer will face problems. Imagine that you bought lining to cover the walls inside your country house or dacha. If you buy clapboard made from raw boards from a careless manufacturer and cover the walls of your house with it, it will begin to slowly dry out naturally in its already installed state. Let us turn to the table of equilibrium humidity and experience. If you heat a room in winter to 25 degrees Celsius, then with a typical indoor air humidity of 35% for winter, the equilibrium humidity value for a board in such a room will be 6.6%. At bases and markets, lining can very often have a humidity of 14% or higher (we have encountered 30%). Next, imagine that your lining begins to dry out, losing water from its pores. Drying out the process is underway, called “shrinkage” and is expressed in a reduction in the size of the wooden product. The amount of shrinkage depends on the type of wood, the direction of the fibers in the product, etc. The main shrinkage occurs across the fibers (according to the thickness and width of your lining). When your lining dries in the installed state to equilibrium moisture, you, in the worst case, risk not only seeing that the lining has come apart in places, but getting gaps between the boards, almost the width of a finger.
The industry uses various technologies for drying wood, differing both in the equipment used and in the characteristics of heat transfer to the dried material.
The classification of types and methods of drying is usually based on heat transfer methods, according to which four wood drying technologies can be distinguished:
- convective drying technology;
- conductive drying technology;
- radiation drying technology;
- electric drying technology;
Each type of drying can also have several varieties depending on the type of drying agent and the characteristics of the equipment used for drying wood. There are also combined technologies for drying wood, in which they simultaneously use different kinds heat transfer (for example, convective-dielectric) or other characteristics are combined various technologies drying wood.
Independent drying technologies
Chamber drying
Chamber drying. This is the main industrial technology drying of wood, carried out in wood drying chambers various designs, where lumber is loaded in stacks. Drying occurs in gaseous environment(air, flue gases, superheated steam), which transfers heat to the wood by convection. To heat and circulate the drying agent, drying chambers are equipped with heating and circulation devices.
With chamber wood drying technology, the drying time for lumber is relatively short (from tens of hours to several days), the wood dries to any given final moisture content at the required quality, and the drying process can be reliably regulated.
Atmospheric drying
The second most important and widespread method at sawmills is the industrial drying of wood, carried out in stacks placed in a special open area (warehouses), washed by atmospheric air without heating. The advantage of atmospheric wood drying technology is its relatively low cost. In addition, this method is the most gentle. Disadvantages: seasonality (drying practically stops in winter); long duration; high final humidity. Atmospheric wood drying technology is used mainly for drying lumber at sawmills to transport moisture and at some woodworking enterprises for drying and leveling the initial moisture content of lumber before drying in wood drying chambers.
Drying in liquids
Drying in liquids is carried out in baths filled with a hydrophobic liquid (petrolatum, oil) heated to 105-120 °C. Intensive heat transfer from liquid to wood allows the drying time to be reduced by 3-4 times compared to chamber drying, all other conditions being equal. This method is used in wood preservation technology to reduce its moisture content before impregnation. Attempts to dry lumber in petrolatum at woodworking enterprises have not yielded positive results due to the fact that lumber after such drying does not meet the requirements for wood for furniture and joinery and construction products.
Conductive drying technology
Conductive (contact) wood drying technology is carried out by transferring heat to the material through thermal conductivity upon contact with heated surfaces. It is used in small volumes for drying, thin wood materials- veneer, plywood.
Radiation drying
Radiation drying of wood occurs when heat is transferred to the material by radiation from heated bodies. The effectiveness of radiation drying is determined by the flux density of infrared rays and their permeability in solid wet bodies. The intensity of the radiant energy flow weakens as it goes deeper into the material. Wood is a low-permeability material for infrared radiation (penetration depth 3-7 mm), therefore this method is not used for drying lumber. It can be used for drying thin-sheet materials (veneer, plywood), in addition, this method is widely used in the technology of finishing wood products for drying paint coatings. Electric stoves, electric heating elements, gas (flameless) burners, and incandescent lighting lamps with a power of 500 W and above are used as emitters.
Rotary drying
Rotational drying of wood is based on the use of the centrifugal effect, due to which free moisture is removed from the wood when it is rotated in centrifuges. Mechanical removal of free moisture is achieved with a centripetal acceleration value of at least 100-500g (g - acceleration free fall). Such accelerations have not yet been achieved in practice due to the difficulty of accurately balancing a centrifuge with a stack; only experimental development of corresponding devices is underway. In known industrial rotary dryers, the centripetal acceleration does not exceed 12g. Under these conditions, mechanical dehydration occurs to a small extent. However, intensification of the drying process in the humidity range above the hygroscopic limit is observed.
When installing the carousel in drying chamber The technology for drying lumber is the same as in conventional batch kilns. The duration of drying at the first stage (from the initial moisture content to the hygroscopic limit) is reduced several times depending on the thickness, species and initial moisture content of the wood compared to conventional convective drying under the same conditions. Although rotary dryers are economical and provide high quality drying, the rotary method has not yet found industrial use for drying lumber.
Vacuum drying
Vacuum drying at reduced pressure in special sealed drying chambers. Due to the complexity of the equipment and the impossibility of obtaining low final moisture content of wood vacuum drying has no independent meaning. It is used in combination with other drying methods and as an auxiliary operation in preparing wood for impregnation.
Dielectric drying
Dielectric drying is the drying of wood in an electromagnetic field of high frequency currents, in which the wood is heated due to dielectric losses. Due to the uniform heating of wood throughout its entire volume, the emergence of a positive temperature gradient and excess pressure inside it, the duration of dielectric drying is tens of times less than convective drying. Due to the complexity of the equipment, high energy consumption and insufficient quality of drying, dielectric drying itself is not widely used.
Combined wood drying technologies
It is more effective to use combined wood drying technologies, for example convective-dielectric and vacuum-dielectric. For mass drying, the use of these methods is uneconomical, but in some cases, especially when drying expensive, critical lumber and blanks made from hard-to-dry wood species, these methods can be used.
Convective-dielectric drying
With a combined convective-dielectric technology for drying wood, high-frequency energy from a special high-frequency generator is also supplied to a stack loaded into a chamber equipped with thermal and fan devices through electrodes located near the stack.
The heat consumption for drying in the drying chamber is mainly compensated by the thermal energy of steam supplied to the heaters, and high-frequency energy is supplied to create a positive temperature difference across the cross section of the material. This difference, depending on the characteristics of the material and the rigidity of the given mode, is 2-5°C. The quality of convective-dielectric drying of lumber is high, since drying is carried out with a small difference in humidity across the thickness of the material.
Vacuum dielectric drying
This is another way of drying wood using high-frequency energy. This technology uses the advantages of both vacuum and dielectric drying. By heating wood in a high-frequency field at reduced pressure, boiling of water in wood is achieved at low wood temperatures, which helps preserve its quality. The movement of moisture in wood during vacuum-dielectric drying of wood is ensured by all the main driving forces of moisture transfer: moisture content gradient, temperature, excess pressure, which reduces the drying time.
During vacuum-dielectric drying, a stack of lumber is placed in an autoclave or a sealed chamber, where a vacuum pump creates a reduced pressure of the environment (1-20 kPa). The lower the environmental pressure, the lower the evaporation temperature of moisture and wood during drying. Heat consumption for drying is provided by the supply of high-frequency energy to the wood. When using this wood drying technology, operational difficulties also arise - the complexity of the equipment, especially the setup and operation of high-frequency generators, and the high energy consumption for drying. Therefore, when deciding on the use of vacuum-dielectric chambers, it is necessary to first develop a feasibility study based on the conditions of a particular enterprise.
Induction or electromagnetic drying of wood
The method is based on the transfer of heat to the material from ferromagnetic elements (steel mesh) stacked between rows of boards. The stack together with these elements is in an alternating electromagnetic field industrial frequency(50 Hz), formed by a solenoid mounted inside the drying chamber. Steel elements(grids) are heated in an electromagnetic field, transferring heat to wood and air. In this case, a combined transfer of heat to the material occurs: by conduction from the contact of heated meshes with wood and convection from circulating air, which is also heated by the meshes.
For regular readers, customers and visitors to my website, I am posting GOST regulations (%) for moisture content of planed dry products. I specially systematized and combined into one table where and in what kind of construction and carpentry work these products are used. see table below.
Humidity standards for planed dry products. Use in construction and carpentry work.
| Planed dry product | Purpose | Usage | GOST | Humidity, % |
| board, block | interior work | production of floors, plinths, platbands, window sills | GOST 8242 |
12 ± 3 |
| board, block | interior work | roof truss system, lathing, counter lathing, furniture | 12 ± 3 | |
| beam, board, block | external works | roof truss system, frame manufacturing wooden structure and its lining. | 15 ± 3 | |
| board, block | other external works | production of fences, cladding of small wooden buildings | 12 ± 3 | |
| solid wood | external and internal works | load-bearing structures - floor beams, floor joists, frames (casing) of windows and doors, etc. load-bearing elements | GOST 4981 | up to 20 |
Simply it looks like this:
It is simply necessary for the customer to check these regulations, because In my opinion:
To check by eye the characteristics declared by the supplier, and accordingly trust the supplier with your money, an action, in my opinion, borders on real madness!
How I carry out a full check of the declared characteristics on the website before delivery to my customers, I invite the reader to visit the entire section of my articles dedicated to each section separately. In which the details and conditions are discussed in more detail: - how the customer should check his supplier of planed dry products.
Study of the characteristics of sections of planed dry lumber, carried out by me before delivery to customers of the TorgLes Moscow website-
The photo shows the customer checking the site's moisture content of the finished planed dry product at the TorgLes Moscow retail space before delivery. The test is carried out with a German needle moisture meter, because... a needle moisture meter gives a minimum error in the measurements taken, i.e. allows you to determine how well the GOST regulations for chamber drying are implemented.
Let's look at why this GOST is needed using an example.
For clarity, I am publishing a photo of the material that I've never been in a drying chamber. Those. The entire technological production cycle was disrupted.
Namely, the board natural humidity was planed on a machine, violating an important technological cycle -chamber drying!
In the photo below, in front of the reader batten section 140x35x6000mm. natural humidity. I marked with red circles the defects due to which this material, after processing on the machine, cannot be used for its intended purpose - laying the finished floor in the house.
Some fragments of this profiled board show significant defects:
| Defects | Cause |
|
Bullying and difficult material. |
the machine blades cannot properly process the surface of the material due to its high, non-uniform humidity |
| - tears of individual fragments of wood on the front part of the board | the machine blades cannot properly process the surface of the material due to its high, non-uniform humidity; |
| - the “comb” from the operation of an unevenly operating machine is very noticeable | the machine cannot properly process the surface of the material due to its high, non-uniform humidity |
| - the tongue is curved and “walks” in size, sometimes larger, sometimes smaller. it is simply not possible to connect the floorboards into a shim groove in the future | It is here that you can clearly see that the board was not in the drying chamber, because... the machine cannot properly process thin elements of a profiled board |
| - the rapid process of warping in wood has begun | indicates that the board was not in the chamber at all, because the board began to bend like a "Z" |
The tree is natural material, susceptible to fluctuations in temperature and humidity. Its main properties include hygroscopicity , that is, the ability to change humidity in accordance with environmental conditions. They say that wood “breathes,” that is, it absorbs air vapor (sorption) or releases it (desorption), reacting to changes in the microclimate of the room. The absorption or release of vapors occurs due to the cell walls. Given a constant state of the environment, the moisture level of wood will tend to a constant value, which is called equilibrium (or stable) moisture.
The ability to absorb moisture is affected not only by the microclimate of the room, but also by the type of wood. The most hygroscopic species include beech, pear, and kempas. They respond most quickly to changes in humidity levels. In contrast, there are stable species, for example, oak, merbau, etc. These include the bamboo stem, which is very resistant to unfavorable climatic conditions. It can even be installed in the bathroom. Different types of wood have different levels humidity. For example, birch, hornbeam, maple, and ash have low humidity (up to 15%) and, when dry, tend to form cracks. The moisture content of oak and walnut is moderate (up to 20%). They are relatively resistant to cracking and dry less quickly. Alder is one of the most drying-resistant species. Its humidity is 30%.
Humidity is one of the main characteristics of wood. Under humidity wood is understood as a percentage ratio of the mass of water to the dry mass of wood.
Absolute humidity wood is the ratio of the mass of moisture contained in given volume wood, to the mass of absolutely dry wood. According to GOST, the absolute humidity of parquet should be 9% (+/- 3%).
Relative humidity wood is the ratio of the mass of moisture contained in wood to the mass of wood in a wet state.
There are two forms of water found in wood - bound and free. These add up to the total amount of moisture in the wood. Bound (or hygroscopic) moisture is contained in the cell walls of wood, and free moisture occupies the interior of the cells and intercellular spaces. Free water is removed more easily than bound water and has less effect on the properties of wood.
Based on the degree of moisture content, wood is classified into the following types::
Wet wood. Its humidity is more than 100%. This is only possible if the wood for a long time was in the water.
Freshly cut. Its humidity ranges from 50 to 100%.
Air dry. Such wood is usually stored in air for a long time. Its humidity can be 15-20%, depending on climatic conditions and time of year.
Room-dried wood. Its humidity is usually 8-10%.
Absolutely dry. Its humidity is 0%.
During prolonged drying, water evaporates from the wood, which can lead to significant deformation of the material. The process of moisture loss continues until the moisture level in the wood reaches a certain limit, which directly depends on the temperature and humidity of the surrounding air. A similar process occurs during sorption, that is, absorption of moisture. A decrease in the linear volume of wood when bound moisture is removed from it is called shrinkage. Removing free moisture does not cause shrinkage.
Shrinkage is not the same in different directions. On average, complete linear shrinkage in the tangential direction is 6-10%, and in the radial direction - 3.5%. With complete drying (that is, one in which all bound moisture is removed), the moisture content of the wood is reduced to the limit of hygroscopicity, that is, to 0%. If moisture is unevenly distributed during drying of wood, internal stresses can form in it, that is, stresses that arise without the participation of external forces. Internal stresses can cause changes in the size and shape of parts when machining wood.
The properties of wood directly determine the properties wooden products. When there is excess or insufficient humidity, wood usually absorbs or releases moisture, increasing or decreasing in volume accordingly. At high humidity indoors, wood can swell, and if there is a lack of moisture, it usually dries out, so all wooden products, including floor coverings, require careful care. To prevent deformation of the floor covering in the room, it is necessary to maintain constant temperature and humidity. This has a beneficial effect not only on the quality and durability of floor coverings and wooden furniture, but also on people's health. With a sharp change in the temperature and humidity conditions in a room, internal stresses arise in the wood, which lead to cracks and deformations. Optimal temperature in a room with parquet flooring should be approximately 20 0 C, and optimal humidity air - 40-60%. Hydrometers are used to monitor room temperature, and room relative humidity is maintained using humidifiers.
DETERMINATION OF WOOD MOISTURE
There are several ways to determine wood moisture content. IN living conditions 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. Electric moisture meter needles with electrical wires connected to them are inserted into the wood and passed through them electricity, while the moisture content of the wood is immediately marked on the instrument scale in the place where the needles are inserted. Many experienced carvers 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 mass, by the presence of cracks at the end or along the wood fibers, by warping and other signs. By the color of the bark, its size and the color of the wood, you can recognize ripe or freshly cut wood and the degree of its moisture content. When processing a semi-finished plane 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. Very wet wood is easy to cut, and a wet mark from the chisel is noticeable at the cutting site. But in the end it is unlikely that it will be possible to obtain a high-quality thread, since cracking, warping and other deformations cannot be avoided.
DRYING WOOD
Drying wood - the process of removing moisture from wood to a certain percentage of moisture content.
Dry wood has high strength, warps less, is not susceptible to rotting, is easy to glue, is better finished, and is more durable. 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. In the room, the wood humidity is sufficient up to 10%, and under open air- no more than 18%. There are many ways to dry wood. The simplest and most accessible - natural look drying - atmospheric, airy . Wood should be dried 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. Boards, timber, etc. are stacked on metal, wooden or other supports with a height of at least 50 cm. The boards are stacked with their inner layers facing up to reduce their warping. It is believed that boards placed on the edges dry faster, since they are better ventilated and moisture evaporates more intensely, but they also warp more, especially material with high humidity. It is recommended to compact a stack of p/m, prepared from freshly cut and live trees, with a heavy load on top to reduce warping. During natural drying, cracks always form at the ends; to prevent cracking and preserve the quality, it is recommended to carefully paint the ends of the boards oil paint or soak in hot drying oil or bitumen to protect the pores of the wood. The ends must be processed immediately after cross-cutting into the cut. If the wood is characterized by high humidity, then the end is dried with a flame. blowtorch, and only then paint over it. The trunks (ridges) must be debarked (cleared of bark), only small collars-muffs 20-25 cm wide are left at the ends to prevent cracking. The bark is cleaned so that the tree dries out faster and is not affected by beetles. A trunk left in the bark in relative heat with high humidity quickly rots and is affected by fungal diseases. After atmospheric drying in warm weather, the wood moisture content is 12-18%.
There are several other ways to dry wood.
Way evaporation Or steaming has been used in Rus' since ancient times. The blanks are sawn into pieces, taking into account the size of the future product, placed in ordinary cast iron, sawdust from the same blank is added, filled with water and placed in a heated and cooling Russian oven for several hours, “languishing” at t = 60-70 0 C. In this case, “leaching” - evaporation of wood; Natural juices come out of the workpiece, the wood is painted, acquiring a warm, thick chocolate color, with a pronounced natural texture pattern. Such a workpiece is easier to process, and after drying is less likely to crack and warp.
Way waxing . The blanks are dipped into melted paraffin and placed in an oven at t=40 0 C for several hours. Then the wood dries for a few more days and acquires the same properties as after steaming: it does not crack, does not warp, the surface becomes tinted with a distinct texture pattern.
Way steaming in linseed oil. Wooden utensils steamed in linseed oil are very waterproof and do not crack even with everyday use. This method is still acceptable today. The workpiece is placed in the container and poured linseed oil and steamed over low heat.
Wood is a “living” material; structures made from it breathe and can change their humidity. This is its main difference from brick, concrete, metal... This feature causes some problems when using wood building materials, especially with regard to changes in the percentage of humidity.
Humidity: concept of free and bound moisture
In wood, the main part of the water is found in cell cavities, intercellular spaces, channels, voids, cracks - this is free moisture. A certain amount of water is present in the thickness of the cell membranes - bound moisture.
Free (capillary) moisture in wood is retained due to simple physical and mechanical bonds; it easily evaporates during normal drying. This is the water that wood can absorb and release. When it comes to the moisture content of lumber, we mean the amount of free moisture.
Bound (microcapillary) moisture is retained in wood by complex physicochemical processes; its removal involves enormous energy costs. In nature, it evaporates from wood during combustion or natural aging, that is, when the cells are completely destroyed.
The moisture content of lumber is one of the most important technical characteristics, affecting the quality and scope of products. In addition, the humidity indicator can convert a board or timber into one of five grades. Thus, GOST 8486-86 states that selected and first-third grades include lumber with a moisture content of up to 22 percent (dry) or from 22% (raw, treated with an antiseptic), and only the 4th grade is not standardized for this indicator.
Absolute and relative humidity are distinguished. In construction practice, attention is paid mainly only to the absolute value, which is defined as the ratio of the mass of moisture contained in the tree to the mass of dry wood.
It is customary to distinguish several types of humidity:
- wet wood (floated) - 100 percent or more;
- freshly sawn - from 50 to one hundred percent;
- air-dry - up to 20 percent;
- room-dry - 7-10 percent;
- absolutely dry - 0 percent.
The concepts of “semi-dry” lumber and wood products with “transport humidity” are also distinguished - about 22%.
Why do you need to know the moisture content of lumber?
The moisture content of wood is an unstable value. Wood is hygroscopic; it always tends to enter with environment into balance. Moisture exchange occurs constantly; when air humidity increases, lumber absorbs water from it, and when it decreases, it releases it. This interaction causes a number of processes to change the structure and shape of the material, such as:
- swelling;
- shrinkage;
- warping;
- cracking.
When drying, lumber decreases in volume due to the evaporation of moisture from the wood. Moreover, the volume decreases in direct proportion to the decrease in humidity. Wood dries out differently in different directions, the least along the fibers (0.1-0.3%), in the radial direction - 4-8%, in the tangential direction - 6-10%. Volumetric shrinkage can average 12-15 percent. To ensure that the manufactured lumber has the required dimensions after drying, allowances are made when sawing logs, for example, into timber or boards. It is taken into account that shrinkage depends on the density of the wood - the higher it is, the more it shrinks. By the way, different breeds react differently to changes in humidity conditions, some of them are more or less resistant, and some are characterized by carpenters and joiners as “nervous”.
Swelling is an increase in volume of wooden products, that is, a process opposite to shrinkage. It occurs when wood is exposed to high humidity conditions. An increase in volume in itself does not affect the strength of wood as such, but can lead to a violation of the shape/integrity of structures assembled from it.
The property of wood to swell and shrink with a radical change in humidity is considered negative. However, in some cases, for example, swelling can be useful - it ensures the tightness of the connection of parts, for example, during manufacturing wooden boats, barrels, etc.
Street wooden structures They undergo swelling and shrinkage processes periodically, with the changing seasons. Therefore, they are particularly susceptible to processes such as cracking and warping, which are the result of constantly changing humidity.
During drying, uneven distribution of water in the layers of wood can occur, resulting in stress in different directions, leading to the formation of cracks. Due to shrinkage, curvature of the product - warping - is often observed. It can be longitudinal or transverse and appears unevenly in different directions. Curved lumber complicates installation and may lose load-bearing capacity. They even highlight (GOST 2140 81) a specific visible defect of the board - “winging”, that is, a helical longitudinal bend.
Selecting lumber based on moisture content
Lumber of a certain moisture content is intended for specific purposes. It is allowed to use boards/timbers with a moisture content of 20-30 percent for the construction of any street structures - fences, gazebos, canopies, fences for animals, and the like. In addition, timber and boards with natural moisture are suitable for arranging some structural elements during the construction of houses and repair work. For example, to arrange rafter system or log under flooring. In this case, the reliability of the fastening will not allow cracks and warping to appear during drying. And to protect against fungus, products are treated with special antiseptic compounds.
Planed small and large moldings (lining, block house, skirting boards, corners, casing) external and internal are made from dry lumber (room-dry humidity at the level of 7-10 percent). door leaves, window transoms and frames, parquet, furniture.
Woodworkers have such a concept as “transport humidity”. Its figure should not be more than 20-22 percent. If the moisture content of the lumber is higher, then transporting it over long distances is not permissible, since transportation will take a long time, during which the wood may rot.
Absolutely dry wood with an indicator of 0 percent is not found in practice. This concept applies only when using one of the methods for determining humidity - gravimetric.
Basic methods for determining humidity
To determine the moisture content of lumber, two main methods are used today - by weight and using a moisture meter.
Weight method
Humidity is determined as follows: a small sample (control sample) 20-25 mm wide is cut out of a timber or board. It is very important to take it not from the very edge, but from the middle, since the end parts always have less moisture. The sample is cleaned of sawdust and weighed technical scales, capable of giving ultra-precise indicators (up to one hundredth of a gram). The resulting weight is recorded - this will be the initial mass of the sample (ISM).
Next, the sample is dried in a special drying cabinet at 100-105 degrees. After five hours, it is taken out and weighed, recording the weight, dried again, checking the indicators every 1-2 hours. When the weight stops changing, absolutely dry wood is obtained - the final sample mass (FSM) is recorded. Next, the moisture content of lumber is determined as follows: the difference between the NMP and the KMP is divided by the KMP index, the resulting figure is multiplied by 100 - the initial moisture content is obtained.
The main advantage of the method is that it gives very accurate indicators (error no more than 1 percent). Minuses:
- analysis may take a long time;
- you will need to cut out a sample of the material, which is unacceptable for finished products.
Using a moisture meter
A moisture meter is an electrical device specially designed for measuring humidity. There are two varieties of it:
- contact (needle) - the work is based on the conductometric method;
- non-contact - the work is based on the dielcometric method.
A needle moisture meter has two sharp metal needles that plunge into the wood. Then the button is pressed, completing the circuit. The device measures electrical resistance, which changes depending on the moisture level in the material. Next, using a special formula stored in the moisture meter’s memory, the percentage of humidity is calculated. In this case, measurements are carried out locally, so the procedure for obtaining accurate results must be carried out in several places on the product.
The main working element of a non-contact moisture meter is a radio frequency generator. Measurements are carried out using built-in contact pads, therefore, unlike the needle model, the device does not leave marks on the product. The work is based on measuring the dielectric constant of wood - water itself is characterized by a high dielectric constant, which makes it possible to obtain accurate indicators of the percentage of moisture content of the material.
The main advantages of using moisture meters are ease of use and the ability to quickly obtain results. Non-contact devices are also ideal for measuring the moisture content of finished products. The main disadvantage is that moisture meters are not very accurate; the error can range from 2 to 7 percent.
Basic methods of drying lumber
Drying wood is the most important operation aimed at improving its technological and consumer properties. Even if the product is over-moistened, the lumber that was dry is much less likely to warp and crack, and is easier to process and install. Dry wood perfectly resists infection by harmful fungi. The weight of the products is less, while the strength and hardness increase, and the thermal insulation qualities also noticeably improve.
Today, the woodworking industry uses two main drying methods - natural (atmospheric) and forced (chamber).
Natural drying
During atmospheric drying, lumber is stacked under a canopy on outdoors. When forming a stack between rows of boards, beams, etc. put gaskets. The stacks are installed with gaps for air circulation. The ends of the products, in order to avoid the appearance of cracks, are treated special compounds. In addition, hardwood lumber must be subjected to antiseptic treatment before stacking.
The drying agent in this case is air, although, unlike the forced method, it is not possible to control its parameters (temperature, humidity). It all depends on the weather and time of year. An important point is how tightly the stacks are stacked. The denser it is, the higher the relative humidity and lower the temperature, which means the wood will dry out more slowly.
Atmospheric drying allows you to obtain products with a moisture content of 18-20 percent. The speed of the process will depend on the initial moisture content of the tree, time of year, type of wood and cross-section of lumber.
The main advantage is the relative cheapness of the process. No special equipment or costs for air heating are required. In addition, residual stresses during drying will not be so strong, cracks will not form - this is especially true when drying wood with initially high humidity.
The downside is that lumber takes a long time to dry, and it is impossible to control temperature and humidity. With prolonged increases in ambient humidity, there is a risk of fungal infection.
Forced drying
The most technologically advanced, highly effective method that allows you to obtain wood with a moisture content of 7-12 percent in a short period of time. Its essence lies in the fact that specially formed stacks of lumber are placed in special chambers in which the required temperature and temperature are maintained for a certain time. humidity conditions. The drying agent can be steam, heated air or flue gases; their movement can be either natural or forced.
