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Björn Esping
Moisture content
the misunderstood drying quality
What can we do about it?
Paper presented at the European COST Action E15
Workshop, Helsinki, Finland, June 11-13,2001
Trätek
Björn Esping
MOISTURE CONTENT - THE MISUNDERSTOOD DRYING QUALITY WHAT CAN WE DO ABOUT IT?
Trätek, Rapport 10112034 ISSN 1102-1071 ISRN TRÄTEK - R — 01/034 — SE Nyckelord drying quality moisture content wood Stockholm december 2001
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Trätek - Institutet för träteknisk forskning - betjänar sågverk, trämanufaktur (snickeri-, trähus-, möbel- och övrig träförädlande industri), skivtiJlverkare och bygg-industri.
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The Swedish Institute for Wood Technology Research serves sawmills, manufacturing (joinery, wooden houses, furniture and other woodworking plants), board manufacturers and building industry. The institute is a non-profit company with industrial and institutional customers. R&D projekcts are performed as contract work for individual indust-rial customers as well as joint ventures on an
industrial branch level. The Institute utilises its own resources as well as those of its collaborators and outside bodies. Our research units are located in Stockholm, Växjö and Skellefteå.
MOISTURE CONTENT
THE MISUNDERSTOOD DRYING QUALITY.
WHAT CAN WE DO ABOUT IT?
Fuktkvot - den missförstådda torkningskvaliteten
Björn Esping bjom.esping@tratek.se AB Trätek, Swedish Institute for
Wood Technology Research P.O.B. 5609, SE-114 86 Stockholm
Sweden ABSTRACT
The Moisture Content (MC) has an effect on many quality parameters. It means we have to understand as much as possible about MC to get the right drying quality. We must avoid the most common mistakes regarding the effects of MC on wooden products such as shrinkage, swelling, deformations, checks/shakes, opening closed checks, mould, bluestain, decay, etc. There are many effects - new and old - which are not so well known in the industry and the research area. Some of them are marginal and some are very important due to various conditions. In this paper we point out four of them.
SAMMANFATTNING
Fuktkvoten inverkar på många kvalitetsparametrar. Om vi vill ta fram en bra torkningskvalitet bör vi därför känna till så mycket som möjligt om fuktkvoten. Vi måste undvika de mest vanliga misstagen som förorsakas av fuktkvoten och som medför oväntade krympningar, svällningar, deformationer, sprickor, öppnande av stängda sprickor, mögel, blånad, röta, etc på färdiga träprodukter. Det fmns många effekter - nya och gamla - som inte är så välkända i industrin och inom träforskningen. I denna skrift beskriver vi fyra av dem.
FOUR E F F E C T S ON M C 1. Different definitions of M C
The definition of MC coi in the European standard EN 844-4 (4.1) is: The weight of the water content expressed as % of the weight of absolute dry wood:
Another definition of MC (0;^ we can find in ISO 4470 and in the new European standard "Round and sawn timber - Method of measurement of moisture content - Part 1: Method for determining moisture content of a piece of sawn timber". These standards explain how to measure coi. It is a practical method to measure the MC and is called "oven dry method". The standards explain the oven dry method:
« , = ^ ^ ! ^ x i o o [%]
wherem\ is the mass of the test slice before drying, in grams;
mo is the mass of the oven dry test slice, in grams;
0)2 is the moisture content, in percent.
But C02>C0i
These two MCs are not the same. The oven dry method not only evaporates water, but some volatile organic compounds (VOC) such as terpenes also evaporate during the oven dry
method. The MC {(1)2) will always be higher than coj - and especially high for resinous
species.
Due to higher VOC content in pine {Pinus silvestris) the MC variation is higher for pine than for spruce {Picea abies karst), figure I below. The MC 0)i can be several MC percents too high. Also hardwoods such as beech and birch have been measured. They have a natural MC variation of between pine and spruce.
Natural MC variation:
is the MC variation of the boards in a lot of timber after a long MC equalisation and when the MC is measured with the standardised oven dry method;
- appears when not only water evaporate but also VOC and other substances during the standardised oven dry process;
- does not influence shrinkage, deformations, etc as the MC does not change in a constant climate;
must be taken into consideration when forming a product standard - especially for high quality products;
is only actual when using oven dry method - not electric MC meters; - is not enough investigated - other parameters besides VOC can be involved
The natural MC variation axpreaaad aa atandankl«viatk>n a I ' 5 a 0,8 0,7 0,6 0,5 OA 0,3 0,2 0.1 1^ = 0 . 9 ? ^ 0.74 10 15 Equlibrium MC, % 20 25 30
Figure 1. The natural MC variation at different MC levels. MC measured by oven dry method at 103 °C The natural MC variation is about twice as high for pine as for spruce due to higher volatile organic compounds Figur 1. Den s.k naturliga fuktkvotsspridningen vid olika fuktkvotsnivåer. Fuktkvoten mätt med torrviktsmetoden vid 103 f. Den naturliga fuktkvotsspridningen är ungefär dubbelt så stor för furu (Pinus silvestris) än för gran (Picea abies karst) beroende på ungefär dubbelt så hög andel flyktiga ämnen i furu
Normally all standards ought to use the practical 0)2 as the definition. But when producing product standards we must be aware of the effect of natural MC variation. Especially when high drying quality classes will be used.
Using the coi as a definition of the MC the MC must be measured with Karl-Fischer and distillation method at low temperatures, which is too complicated in practice.
Summary 1: MC expressed as "the mass of water in wood divided by the mass of dry wood substance multiplied by 100" is normally never used.
In all industrial applications we use the MC expressed as "mass of the wood before drying minus mass of the oven dried wood divided by the oven dried wood multiplied by 100". Sometimes we must be aware of the "the natural MC variation" - especially for high drying quality products.
Sammanfattning 1: Definitionen på fitktkvoten uttryckt som "Massan vatten i träet
-divider at med massan av torr träsubstans - multiplicerad med 100" används normalt inte! Iså gott som alla industriella applikationer använder vifiiktkvoten med definitionen
"Massan trä före torkning i ugn - minus massan av det ugnstorkade träet - dividerat med det ugnstorkade träet - multiplicerat med 100". Dessa två definitioner ger olika
fuktkvoter. Ibland måste man ta hänsyn till "den naturliga fuktkvotsspridningen - särskilt för produkter med hös torknings kvalité t
2. M C variation is influenced by measuring method
When checking the MC of a lot of timber, all indirect measurement methods of MC result in higher MC variation than when using the oven dry method. On average the standard deviation will be 1,3-1,5 higher with a hand held resistance meter. This divergence depends mostly on - the precision of penetrating the hammer electrodes to the right measuring depth in the
boards;
- the variation of the MC gradient in boards (Different MC gradients depend mostly on different densities of the boards).
The MC variation is also higher when measured with capacitive hand meters and in-line MC meters. A test we carried out showed that the standard deviation of MC measurements for a capacitive in-line MC meter is about 2,5 times higher than the true standard deviation. In this case the true standard deviation was low.
Summary 2: Indirect measurements (electric) result in a much higher MC variation than the true MC variation. It is necessary to take this into account during the quality control procedure.
Sammanfattning 2: Indirekta mätningar (elektriska mätningar) resulterar i en mycket större fuktkvotsspridning än den verkliga fuktkvotsspridningen vid mätning av ett virkesparti. Det är nödvändigt att ta detta i beräkning vid kvalitetskontrollen. (Resistansfuktkvotsmätningar ger 1,3-1,5 gånger högre fuktkvotsspridningar än torrviktsmetoden)
3. M C limits and fungus (mould, blue stain and decay)
If wood has too high a moisture content, mould will easily grow on the surface. If the time with high MC is long enough, the blue stain fungi or decay start growing deeper and deeper in the wood.
The expression "shipping dry" in the Nordic countries means: "The boards shall have a maximum MC so that they will not have discoloration due to fungus, during transport to the customer and his normal storage conditions".
The Nordic standard INSTA 141 created in 1989 translated this expression to MC figures by using a huge test material of "shipping dry" boards from different sawmills. We calculated the MC limits for a lot of timber and we changed the expression "shipping dry" to "delivery dry" and found the maximum MC limit to be 24 %. The definition of the MC formulated: "97,7 % of test slices of the board cross sections must have a MC < 24 %."
Shipping dry boards have a steep MC gradient. After kiln drying, the boards are very dry on the surface (down to 7 %) and humid in the middle (up to 28 % MC). Normally the boards still have an MC gradient left when the kiln packages are unstickered and banded to "delivery packages". Even if the MC in the middle of the board was high and the average MC of the
cross section was high, there was no mould growth on the dry board surface. The MC gradient with the dry surface and moist centre cord was the reason for the mould free wood surface -even if the average MC of the board cross section was as high as 24 %.
To day we know that some mould species can grow on a board surface which has an MC of 18 % or higher. Therefore the building industry states max 18 % as a limit on the board cross section (AMA-nvtt Beskrivningsdel HUS 1/1999, punkt HSD.l 11. Stockholm. Sweden). But they mix the definitions. The max 18 % in the cross section normally means about 7-15 % at the surface and about 20-24 % in the middle of he boards. Of cause sometimes during humid autumns there are boards, which have been waiting too long time outside in front of the sawmill trimmer department. These stickered boards can come up to an average MC of 21 %.
Summary 3: When stating MC limits for fungus growing on board surfaces, we must differ between maximum MC on board surface and the MC of the board cross section.
Sammanfattning 3: Vid utformning av regler för maximal fuktkvot för att undvika svamptillväxt på virkesytor, måste man skilja mellan maximal fuktkvot på virkesytan och virkets maximala tvärsnittmedelfuktkvot
4. Using resistance M C meters - what is the right measurement depth in the board to meet the average M C ?
During drying of timber in kilns with air as a heat and moisture transport medium we always build up an MC gradient in the board cross section. The gradient can be calculated. It means we can also calculate the average MC of the board cross section at the end of the drying. Such a theoretic calculation results in a measurement depth of 20 % of the board thickness.
During the work in standardisation groups there have been lots of discussions regarding the right measurement depth to meet the average MC. The problem was that we all have different information and experiences of the right measurement depth. In the USA "Dry Kiln
Operator's Manual" they have stated that the measurement depth shall be Va of the board thickness. Most suppliers of the resistance hand MC meters in Europe state that it shall be 1/3 of the thickness, etc.
We found that there is no exact figure. If the boards have been dried and well
conditioned/equalised in the kiln, the MC gradient in the cross section is nearly evened out and the measurement depth does not matter. Otherwise, if the boards are newly dried, the right measurement depth is very close to 20 % (1/5) of the thickness - the same as the theoretic calculation. But what happens in the practice? Directly after kiln drying without conditioning/equalisation treatment the boards are very dry on the surface (MC 3-8 %) and pick up moisture from the surrounding air. In the Nordic countries the summer is the driest season (EMC 13-15 %) but in autumn (EMC 15-20 %) and in winter much more humid (EMC 19-21 %). In winter (at low temperatures) the diffusion/MC pick up goes very slowly, but in autumn it can be warm and humid and the boards pick up the moisture very quickly, especially if the boards are still stickered (kiln packages).
As we can now see, in all seasons the outer part of the boards will more or less pick up moisture from the surrounding air and the inner part of the boards, and consequently the average MC increases. When using resistance MC meters, the electrodes must be hammered
deeper into the wood to compensate for the higher MC. How much deeper depends on several factors (mostly RH, temperature and time), but something between 20 % (1/5) and 33 % (1/3) of the board thickness. The choice in USA was Va and in the new European standard 0,3 times the board thickness - perhaps due to different average climate. But probably the most
important point in this discussion is that the buyer and seller of wood now have a standard which can be used in contracts, as well as when measurements in the case of claims on MC. The buyer and seller will avoid many discussions if they are aware of the background to the MC measurement technique.
Summary 4: When using a hand held resistance MC meter, the measurement depth is 0,2 times the board thickness for newly dried boards. After the kiln drying these boards will pick up moisture from the surrounding air. That means the electrodes must be hammered deeper into the wood. The new European standard states that the average MC for a board cross section (and for the whole board) is in the depth of 0.3 times the board thickness. Further, the measurement shall be done from the flat side of the board, 0,3 times of the width from the edge and 0,3 meters from the end of the board. It is now recommended that the 4 new European standards for dried wood in new contracts and product standards be used. This is a good step into universal "right and well
definitioned material" and gives wooden material a higher status.
Sammanfattning 4: När en elektrisk resistansfuktkvotsmätare används för mätning av medelfuktkvoten av nytorkat virkes, ska mätdjupet vara 0,2 gånger virkestjockleken. Efter torkningen kommer virket att successivt ta upp fukt från omgivande luft. (Även virke med en relativt hög medelfuktkvot kommer att ta upp fukt på grund av
fuktkvotsgradienten). Detta innebär att fuktkvotsmätarens elektroder måste slås in djupare i virket. Den nya Europeiska standarden anger att virkets medelfuktkvoten för virkets tvärsnitt (och för hela virkes stycket) fås vid inslags djupet 0,3 gånger
virkestjockleken. Vidare ska mätningen göras från virkets flatsida, 0,3 gånger virkesbredden och 0,3 meter från virkesänden. Det rekommenderas nu att de jyra kommande nya standarderna för torkat virke ska användas i försäljningskontrakten för
undvikande av felmätning. Detta innebär ett stort steg mot "väl definierat material" och ger trämaterialet en högre status.
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