# Wood moisture

As wood moisture content , or wood moisture content is the ratio of the wood contained water mass to the dry mass of wood in percent . It is not to be confused with the water content of the wood, which shows the ratio of the water mass contained in the wood to the total mass of the (moist) wood in percent. ${\ displaystyle u}$

The wood moisture is one of the most important parameters in wood processing as well as for energy wood . In the low range up to approx. 30% it can be easily determined with commercially available wood moisture measuring devices.

## Basics

The wood moisture is a decisive condition of the material wood for its technological and mechanical properties. If the moisture content of the wood changes below the fiber saturation , this has a decisive influence on its properties. B. to swell or to shrink . Wood can also be bent much more easily from a certain wood moisture content ( see bentwood ). With a moisture content above the wood species-specific fiber saturation, wood can only absorb additional moisture in the form of free water in the lumina of its cells , which has only a minor influence on its physical and mechanical properties.

Wood shrinks from a wood moisture content below the fiber saturation range , which varies depending on the type of wood . Fiber saturation range describes the moisture content at which all the water has escaped from the cell cavities and the water bound in the cell walls begins to dry out, causing the cell to contract. The shrinkage process is reversed by the absorption of water (e.g. when the air humidity rises), the wood swells . The Dimensions change below the fiber saturation point, which colloquially called the wood works is called, can be in use for furniture be inconvenient or for construction purposes. Therefore, it is important to adjust the target moisture content of the wood to be used to the ambient environment in which it will be used: outdoor humid in interior dry.

In wood that is not exposed to direct moisture (e.g. due to weathering or soil moisture ), a certain equilibrium moisture level develops over time, which is dependent on the relative humidity and the temperature . This condition is called air dry (for example with dry firewood ), further drying can only be achieved by technical means, for example in drying chambers .

The moisture range of the wood up to fiber saturation is also referred to as the hygroscopic range . In this, moisture is absorbed by the wood and stored as bound water in the cell walls . Depending on the type of binding of the water, sorption takes place in three phases, some of which overlap and are unevenly distributed in space:

• Chemisorption takes place when the moisture content of the wood is around 0 to 6%. Here, at a relative humidity of up to around 20%, moisture is bound to the cellulose micelles as a monomolecular layer . Water dipoles are aligned with the negative poles of the exposed cellulose OH groups and therefore take up a smaller volume than before. This chemical process takes place intermicellar, i.e. on the surface of the micelle framework, starting from its looseness. The process can be viewed as molecular sorption . As long as hardly any crystalline areas are shifted, the volume of the wood does not increase significantly.
• Adsorption occurs at 6 to 15% wood moisture and around 20% to 60% relative humidity. Due to electrostatic and van der Waals forces, the water molecules are now deposited in a polymolecular manner, i.e. in several layers, which, however, are not evenly distributed. With increasing layering, the water molecules detach from the boundary layers of the micelles, begin to flow and their surface tension becomes noticeable.
• Capillary condensation occurs from 15% up to wood species-specific fiber saturation between 24% and 32% wood moisture and from around 60% relative humidity. The condensation occurs in capillaries with radii of 50 nm to 1 µm due to the lower vapor saturation pressure there. These are also part of the cell wall structure. The intermicellar and interfibrillar cavities are now completely filled with liquid water. When the fibrils cannot expand further because of the relatively tight crystalline bonds, the fiber saturation point has been reached.

In nature, sapwood has a significantly higher moisture content than heartwood , as the tree's water transport takes place in the sapwood under the bark . The wood moisture also has a great influence on the risk of wood pests such as fungi and insects.

## Absolutely dry (atro), air dry (lutro), forest and sap or freshly felled

Designations in the sequence of the sequence (decreasing humidity):

• Freshly felled or freshly sap is the name given to the wood immediately after it has been cut , i.e. living wood - both are called green wood .
• Forest fresh is the forest language expression for the wood that is transported away after interim storage.
• Air dry refers to wood that has been stored dry (acclimatized) for several years.
• As darr dry refers to absolutely dry wood ( dry weight ).

Special units of measurement have become established in the grinding and pulpwood industry ( industrial wood ) as well as in the bioenergy sector :

• the absolutely dry ton (t-atro, “atro-ton”) is the unit of measurement for the mass of a ton of absolutely dry wood
• the air -dry bin (t-lutro, "lutro bin") takes into account the respective water content. Air-dry for industrial wood can be significantly above the usual use for construction and firewood and extend to freshly forest , because this is how the raw material for industrial production and processing is usually supplied.
Atro mass to solid cubic meters by type of wood
Wood species AMO / FMO FMO / AMO AOO / FOO FOO / AOO
Oak 0.750 t-atro / m³ 1.334 m³ / t-atro
Beech (common beech ) 0.707 t-atro / m³ 1.414 m³ / t-atro 0.650 t-atro / m³ 1.538 m³ / t-atro
Spruce / fir 0.475 t-atro / m³ 2.100 m³ / t-atro 0.427 t-atro / m³ 2.342 m³ / t-atro
poplar 0.400 t-atro / m³ 2,490 m³ / t dry mass

With

• AMO: Atro-ton, m it bark delivered, o sons bark reused
• AOO: Atro-ton, o delivered teeth bark, o sons bark reused
• FMO: cubic meters , m it bark delivered, o sons bark reused
• FOO: F estmeter, o delivered teeth bark, o sons bark reused

From one ton of theoretical dry mass of the spruce tree completely (tabulated mean values ​​according to laboratory analysis), 2.1 solid cubic meters of dry wood-based material can be obtained in practice.

## definition

The wood moisture is defined as a percentage unit value from the mass of the water contained in the wood sample ( water mass  m w ) and the mass of the anhydrous (kiln- dry ) wood sample ( dry mass  m 0 ):

${\ displaystyle u = {\ frac {m_ {w}} {m_ {0}}} \ cdot 100 \, \%.}$
DIN 52183
Area Timber industry
title Testing of wood; Determination of the moisture content
Latest edition 1977-11 (withdrawn 2006-07)
ISO -

This definition of wood moisture can be found, for example, in

• EN 13183 - moisture content of a piece of sawn timber (especially part 1)
• of EN 14298 sawn timber - determination of the drying quality
• the - now withdrawn - European Standard EN 844-4 Round and Sawn Timber - Terminology - Part 4: Terms relating to the moisture content
• the German DIN 52183 test of wood - withdrawn without replacement in July 2006 ; Determination of the moisture content .

${\ displaystyle m_ {w} = m_ {u} -m_ {0} \ quad \ Leftrightarrow \ quad m_ {u} = m_ {0} + m_ {w} \ quad \ Rightarrow \ quad u = {\ frac {m_ {u} -m_ {0}} {m_ {0}}} = {\ frac {m_ {u}} {m_ {0}}} - 1}$

With

• m u : total mass of the wet sample (wet weight)
• m 0 : dry mass of a standard sample of the same size (kiln weight).

If you know the standard weight ( specific weight , based on the bulk density ) of the kiln-dried wood of a certain type of wood and quality, you can determine the original water content using the amount of water removed or the wet weight . ${\ displaystyle m_ {w}}$${\ displaystyle m_ {u}}$${\ displaystyle u}$

Dehumidified wood has 0% wood moisture, air- dry wood roughly 10–20% (at best around 8–10%); Wood with 100% wood moisture has just as much water as wood mass, which is typical for fresh wood. In living wood with bark, the wood moisture can be over 100%, so the tree can store more water than its wood mass makes up.

Typical moisture levels of timber (guide values):
Status Humidity
freshly felled up to 150%
water-saturated 100%
forest fresh approx. 60%
Fiber saturation range 28-32%
externally stored 15-18%
Windows and front doors 12-15%
Interiors without heating 10-12%
Interiors with stove heating 8-10%
Interiors with central heating 6-8%
Drought 0%

Examples of wood types:
property Spruce fir Oak (europ. ) Black locust
Wood moisture
(in%)
u max (fresh from the juice) 140 (1) 165 (1) 73 50
net, fresh from the forest 55-70 75 39
Normal humidity u 65 11.9 12.4 13.0 13.0
Density  ("weight")
(in kg / m 3 )
debarked, fresh from the forest 750-850 800-980 1180-1170
fresh 1100 930
Sawn timber, air dry 480 460 870
Bulk density ρ H 430 470 650 730
(1)for sapwood ; Heartwood and mature wood about half to a third

## Wood moisture and water content

The wood moisture is related to the dry mass, the water content ( moisture content ), however, to the total mass: ${\ displaystyle w}$

${\ displaystyle w = {\ frac {m_ {w}} {m_ {0} + m_ {w}}} \ cdot 100 \, \%}$

Kiln-dry wood has a water content of 0%, air-dry wood is still about the same values ​​as the wood moisture, water-saturated wood has 50% water content, and the water content can not reach 100% (that would be pure water without wood content).

Conversion between wood moisture and water content ( attention , both values ​​as decimal fractions, e.g. ): ${\ displaystyle u}$${\ displaystyle w}$${\ displaystyle u = 0 {,} 1 = 10 \, \%}$

${\ displaystyle u = {\ frac {w} {1-w}} \ quad \ Leftrightarrow \ quad w = {\ frac {u} {1 + u}}}$
 w in% 05 10 15th 20th 25th 30th 35 40 45 50 55 60 u in% 05 11 18th 25th 33 43 54 67 82 100 122 150

### example

If a delivery of spruce industrial wood is weighed a lutro weight (air dry) of 25,000 kg

${\ displaystyle m _ {\ text {lutro}} = m _ {\ text {0}} + m _ {\ text {w}} = 25,000 \, {\ text {kg}} = 25 \, {\ text {t} }}$

and a dryness of 50% determined (water content 50% corresponds to wood moisture 100%)

${\ displaystyle w = {\ frac {m _ {\ text {w}}} {m _ {\ text {0}} + m _ {\ text {w}}}} = 0 {,} 5 \ quad \ Rightarrow \ quad m _ {\ text {w}} = w \ cdot (m _ {\ text {0}} + m _ {\ text {w}}) = 12 {,} 5 \, {\ text {t}},}$

so the atro weight is calculated to

${\ displaystyle m _ {\ text {atro}} = m _ {\ text {0}} = m _ {\ text {w}} \ cdot \ left ({\ frac {1} {w}} - 1 \ right) = 12 {,} 5 \, {\ text {t}}.}$

This weight divided by the guideline value for spruce delivered for forestry (see table above "Atro-mass ...") results in a wood quantity of

${\ displaystyle V = {\ frac {m _ {\ text {atro}}} {0 {,} 475 \, \ mathrm {\ tfrac {t} {FMO}}}} = m _ {\ text {atro}} \ cdot 2 {,} 1 \, \ mathrm {\ tfrac {FMO} {t}} = 26 {,} 3 \, {\ text {FMO}}}$ (Solid cubic meters for further processing).

## Wood moisture balance and normal moisture

Wood is hygroscopic and therefore reacts to fluctuations in air humidity: if the air humidity falls, the wood moisture also falls, and vice versa. So there is a constant relationship between air humidity and wood moisture, which is called the wood moisture or sorption equilibrium (input / output equilibrium). This equilibrium does not come about spontaneously, but takes some time, depending on the thickness of the wood.

The normal humidity u N (also: u 65 ) is the wood humidity that is set in a normal climate (index  N ) of 20 ° C and 65% relative humidity. It is a material characteristic of the type of wood.

The equilibrium moisture content of construction timber is according to DIN 1052

• at 12 to 24% for structures that are exposed to the weather (service class 3)
• at 10 to 20% for roofed open structures (NKL 2)
• at 5 to 15% for closed, heated rooms (NKL 1).

## Determination methods

• The kiln method is the only method that was standardized (DIN 52183) and is therefore also used as a calibration method for the other methods. The sample to be examined is weighed and then dried to constant weight (kiln-dry), weighed again and the wood moisture content is determined according to the above equation. The disadvantage here is that the measurements are tedious and the sample is destroyed.
• The determination by means of infrared reflection is mainly used in industry and takes advantage of the fact that every material absorbs electromagnetic radiation of a certain wavelength. Wood does this particularly well at a wavelength in the infrared range of λ = 1.93 μm and λ = 2.9 μm, water and the like. a. at λ = 1.4 μm. Since this radiation is reflected by the wood and only penetrates a little, this method can only be used to measure surface moisture, or the moisture of very thin materials such as e.g. B. Veneers.

Other methods are e.g. B.

The disadvantage of all thermal drying processes is that other volatile organic compounds ( e.g. terpenes and essential oils ) can also outgas, which is sometimes desirable for the end product of wood drying, but can falsify the result of a measurement sample when measuring the wood moisture.

New on the market are electrical wood moisture measurement methods that utilize either the ohmic resistance or the dielectric properties of wood. The disadvantage of the resistance measurement method, however, is that with a wood moisture content of u <5% the resistance is very high and can only be measured with difficulty and the resistance changes only slightly at u> 25%, which leads to measurement inaccuracy. In the dielectric process , the different relative dielectric constants of water (ε r = 80) and wood (ε r = 2… 3.5) are used. Here, the raw density of the wood to be measured must be taken into account; the fiber orientation between the electrodes or the penetration depth of the electrodes also influence the measurement results in both methods.

## Influence on combustion

Increase in the calorific value through drying
Water content calorific value
50% 100%
40% 126%
30% 152%
20% 178%
10% 204%
Source: Holzforschung Austria

The wood moisture (or the water content) has a fundamental influence on the calorific value :

• Forest-fresh firewood has a calorific value of 6.8 MJ / kg
• air-dry firewood 14.4-15.8 MJ / kg
• thermally dried wood pellets or wood briquettes 17.5–18 MJ / kg.

In the combustion of wood adjacent to the wood moisture content is also water vapor released, which consists of the oxidation of the hydrogen originated atoms in the ingredients (mainly cellulose , hemicellulose and lignin ) chemically bonded are. This “combustion water” ensures a difference between the gross calorific value and the calorific value : a certain specific energy is required for its evaporation (or also for the evaporation of the volatile organic compounds ) . This is precisely the amount of energy that can be used in condensing boilers through recondensation . The wood to be burned is therefore dried before burning, for example using a wood chip drying system.

## literature

• Thomas Trübswetter: Holztrocknung: Process for drying sawn timber - planning of drying systems , Hanser Verlag, 2006, ISBN 978-3-446-40477-9 (definitions in particular Chapter 3.3 Wood moisture , pp. 23-38)

## Individual evidence

1. ↑ Wood moisture and water content of logs (on waldwissen.net) , accessed on January 11, 2019
2. In organic wood processing, excessively dried wood is referred to as “dead wood”: In this process, so much water is withdrawn from the inside of the cell that the lignin structure changes. Technically, this is an advantage because natural work is restricted: Extremely dry wood is primarily used for wood-based materials.
3. a b Roland Ulmer: [Possibilities for defining tolerable short-term fluctuations in the relative air humidity for cultural goods made of wood], section “3 physical parameters” p. 21; Diploma thesis, 2004, Technical University of Munich, study course restoration, art technology and conservation science; accessed in May 2019.
4. a b solid cubic meters or atro-bin?  ( Page no longer available , search in web archivesInfo: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.   , in the Styrian Chamber of Agriculture: Agrarnet .
5. Wood fairs . Presentation, Federal Research and Training Center for Forests, Natural Hazards and Landscape, weight measurement table - conversion factors according to FPP , p. 11 (pdf; 1.01 MB); FFP ... cooperation agreement for forest-board-paper.
6. a b Standard DIN 52183: 1977-11 , beuth.de.
7. of Baobabs , species Adansonia digitata , storage amounts of up to 130,000 liters of water (over 100 tons) are reported at a height of up to 20 m. With a weight of a few tons of wood, this is a water storage capacity of the order of 1000% of the wood mass. Specification of the amount of water according to plants - baobab tree , in Madagascar Lexicon ( pdf , dilag-tours.ch)
8. See wood drying , topic on holzwurm-page.de; Firewood storage , dezentrale-energieversorgung.com; Table quoted according to the water content of freshly felled beech , forum post NetSeeker, Fri Mar 31, 2006.
9. DIN 68101: 2012-02 Basic dimensions and tolerance fields for woodworking and processing ; Specification sheets according to Grosser and Teetz; quoted from reference Trübswetter: Holztrocknung , 2006, Tab. 3-1 Wood properties , p. 31.
10. a b c Robinia: Worth knowing ( Memento of June 14, 2013 in the Internet Archive ) , holz-pur.ch, accessed November 8, 2012
11. a b c raw density ( memento of November 10, 2013 in the Internet Archive ) , Storch Industrie-Anlagen GmbH (storch-ind.com), accessed November 13, 2012.
12. a b M. Schardt: The problem with the "wood moisture" and the "water content". In: LWF aktuell 54, 2006, pp. 50–51. (Online version: wood moisture and water content of logs , waldwissen.net, February 2, 2012, accessed November 10, 2012).
13. Lit. Trübswetter: wood drying , 2006, ch. 3.3 Wood moisture and climate , p. 26, column 2.
14. Vera Steckel: Influence of drying and test conditions on the emissions of volatile organic compounds from pine (Pinus ylvestris L.) and spruce (Picea abies (L.) H. Karst.). , University of Hamburg 2011, ( PDF file ).
15. Michael Golser, Wilfried Pichler, Florian Hader: Energy wood drying. Summary of the final report. On behalf of FFP. HFA no. F1887 / 04, Holzforschung Austria, Vienna, March 2005 ( pdf  ( page no longer available , search in web archivesInfo: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. , Netzwerk-land. at).
16. Leopold Lasselsberger: Fundamentals of Combustion Technology and Technical Implementation , Federal Agency for Agricultural Engineering ( PDF file , bosy-online.de).