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P u b l i s h e d In Forest Research I n s t i t u t e M a l a y s i a . 5*210'.) Kepong. Selaugoi Darul Ehsan___No. ' M > , 2005 ISSN:

Measuring the moisture content of wood during processingK . S. Can, S. C. Lim & C. E. Ong

Introduction

Sap, mainly water in a tree stem/trunk, i s an important element to s u s t a i n t h e l i fe of thetree. When a tree is felled an d the trunk is converted into s aw n timber a n d subsequently t of i n a l product, such a s f u r n i t u r e , a large amount of t h i s water is removed. There a re manyreasons for such removals t a b i l i z i n g of timber before usage, reduction of weight,enhancement of durab i l i t y and strength, etc. Informationon the amount of moisture presentin timber a t various stages of conversions or processing is crucial in maintainingth e materialq u a l i t y an d i t s e f f e c t i ve u t i l i z a t i on .

The amount of moisture in timber, or better known as moisture content ( m e ) , is normallyexpressed as the percentage of water present over the oven-dry weight of timber. The processof determining the moisture content is destructiverequiring the limber sample to bedried in an oven maintained a t 10 3 + 2 "C until a constant weight is reached (Figure la &1)). Other means of determining t h e moisture content of timber is by estimation using a ne l e c t r i c a l meter that measures either the electrical resistance or dielectric properties oftimber (Figure 2a & b). These electrical meters are called resistance type or dielectric t ypemoisture meters, respectively.

Figure la Oven-dry method of moisture content determination. A) Electronic balance, B)Oven equipment required

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Figure Ib Oven-dry method o f " moisture content determination. A ) Band-scroll saw, B ) Rad i narm-sawlor sample preparation

Figure 2a Dielectric type moisture meters

Figure 2b R e s i s t a n c e type moisture meters

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Table 1 Methods of m o i s t u r e c o n t e n t d e t e r m i n a t i o n

Oven-drv method Resistance type moisture meter Di el ec t r i c t y pe moisture meterDesc r ip t i o ns D e s t r u c t i v e t e s t method

Species

Woodtemperature

Standard method for measuring woodmoisture content Based on w e i g h t loss of sample

placed in an oven set at 103 2 "C for24 3 6 hours or u n t i l c o n s t a n t wei gh t Expressed as a percentage of oven

d r y wei gh t

Indepe ndent of species. However, t i m b e rspecies with very high volatile extractive( r e s i n ) m a y i n d i c a t e higher m o i s t u r ec o n t e n t t h a n a c t u a l .

Wood temperature has no influence onmoisture content determination.

N o n - d e s t r u c t i v e test methodMeasures the r e s i s t anc e of wood betweentw o electrode p i n s t h a t a r e d r i ven i n to i t .Leave two sm a l l holes on the t imbe r .The meter measures the wettest wood incontacts w i t h both pins.Reading o f m o i s t u r e c o n t e n t i n percentage ofoven dr y w e i g h t .Species depende nt. E a c h timber species hasit s characteristic curve and the meter readingsh a v e to be corrected, either m a n u a l l y basedon correction t a b l e or by se l e c t i ng theappropriatespecies correction s e t t i n g inmodern i n s t r u m e n t .Wood temperature i n f l u e n c e s meter reading.Temperature correction has to be appliedeither m a n u a l l y based on chart provided byi n s t r u m e n t suppl i e r o r by selecting th eappropriate temperature setting in modern

i n s t r u m e n t .

No n-des t ruc t i ve test methodM e a s u r e m e n t based on either the power loss f a c t o r orin c o n j u n c t i o n w i t h t h e d i e l e c t r i c c o n s t a n t o f t h e wood.Use the sur f ac e c o n t a c t electrodes. A good c o n t a c tb e t w e e n he electrode and wood s u r f a c e m u s t beestablished to obtain good measurement.R e a d i n g o f moisture content in percentage of oven dry-weig h t.

Species depende nt. Ea c l i t i m b er species has i t scharacteristic curve and the meter readings have to becorrected, either m a n u a l l y based on correction tableorb y s e l e c t i n g the appropriate species correction s e t t i n gin modern i n s t r u m e n t .

Wood t em per a tu r e i n f l u e n c e s m ete r reading. Unlessthe temperature correction data are supplied by theequipment manufacturer, the use of t h i s meter isr e s t r i c t e d t o a m b i e n t c o n d i t i o n .

M o i s t u r econtent ( M C )d i s t r i b u t i o n

MC distributions could be determinedby s l i c i n g the timber ac c o rd ing ly .

W i t h u n - i n s u l a t e d pins t he meter providesthe va lue corresponding to the wet t e s t wood.W i t h i n su l a t ed p i n s , t h e M C reading a t t h erespective depth of p i n s could be measured.

The surface electrode of dielectric meter does notpenetrate the wood. The e l ec t ro -mag ne t i c field penetratesup to a depth of 25 mm. H o w e v e r , the s u r f a c e m o i s t u r ehas a predominant ef f ec t o n the reading. A s s u c h , t h emeter could not be used to trace m o i s t u r e d i s t r i b u t i o nover the cross-section.

(continued)

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Table 1 ( c o n t i n u e d )Oven-dry method Resistance type moisture meter Dielectric type moisture meter

Timbert h i c k n e s s

M e a s u r i n gdirection

MeasuringrangeSamplingpoint

Requirements

Cal ib r a t i o n

No restriction. The whole strip ofcross-sectional specimen is taken intoc o n s i d e r a t i o n lor ave r ag e me.G r a i n d i r e c t i o n has no e f f e c t onmeasurement.

N o l i m i t a t i on

Specimen of 25 mm l e n g t h (along theg r a i n ) sh a l l be t a k e n at least 300 mm f r omcither end of a sample board. For shortlength timber ( < 6 0 0 m m ) , t h e specimensha l l be taken at the middle of the board.A forced convection oven w i thtemperature controlled at 1()32"C, ab a l a n c e w i t h a c c u r a c y 0 .1 g and a su i t a b l esaw.

Check and/or c a l i b r a t e the oven andb a l a n c e .

Limited by the depth of the driven pins. Tomeasure the average m e, the p i n s sh a l l bedr i ven to i-thc t h i c kness of t imbe r .Sl i gh t d i f f e r en ce on meter readings betweenpa r a l l e l and perpendicular o r i e n t a t i o n of p i n sr e l a t i ve to g ra in direction. Below 15% MC thee f f e c t of grain could be neglected. Above 15 %M C , th e readings perpendicular t o g r a in maybe lower than that of parallel to the grain. Takenote of the measuring di r ec t i o n i nd i c a t ed w h e nusing the equipment or c a l i b r a t i o n chart.7 to 27%

Reading sh a l l be t a k e n at least 300 mm f r omeither end and at the center of the wide f a ceof a board. M e a s u r e m e n t sh a l l not be t a k e nover checks, k n o t s , r es i n pockets, etc.

Meter should be easy to operate and handle.Come w i t h i n s t r u c t i o n manual and relevantspecies and temperature corrections for thevarious timber species. A calibration standardto check t h e i n s t r u m e n t a c c u ra c y .Check for accuracy of meter by us ing 'check-box'supplied b y e q u i p m e n t m a n u f a c t u r e r r egu l a r l y .M e t e r could b e c a l i b r a t e d w i t h r e l eva n t t i m b e rspecies should the need arise.

Good for t i m b e r b e t w e e n 30 and 50 mm t h i c k .

Gra in d i r e c t i o n has no i n f l u e n c e on m e a s u r e m e n t .

6 to 30%

Reading sh a l l be t a k e n at least 300 mm f r om either endand i n t he center o f t he f a ce of a board ( away and i n t hef r om the edge). Readings should be made in areas t h a ta re f r ee o f de f ec t s ( c hec ks , kno t s , resin pockets, e t c . )

M e t e r should be easy to operate and handle. Come w i t hi n s t r u c t i o n manual and relevant species corrections fo rhe various timber species. A calibration block to check hei n s t r u m e n t ac c ur ac y .

Check f o r a c c ur ac y o f meter r egu l a r l y by using ' s t a n d a r d -pad s u p p l i e d by equipment m a n u f a c t u r e r . Meter couldbe c a l i b r a t e d w i t h r e l e v a n t t i m b e r species should theneed arise.

(cont inued)

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Table 1 (continued)

Other (actorsi n f l u e n c i n gt h e iniethod/i n s t r u m e n t

Oven-drv method Resi s t anc e t y pe m o i s t u r e m e t e r Die l e c t r i c t y pe m o i s t u r e meterWood preservatives normally do noti n f l u e n c e t h i s method o f M Cd e t e r m i n a t i o n .

Influence by wood preservatives, adhesives andother v a r i a b l e s ( s u c h as a i r - s p a c e , hand, w o r k - b e n c h ,etc. near li e surface or below the t i m b e r ) on reading( e s p e c i a l l y w i t h t h i n n e r , < 25 m m t h i ckn ess , m a t e r i a l ) .

Wood preservatives, adhesive and resin wil li n f l u e n c e th e meter reading. Gen er a l l y , ch em i ca lwil l increase th e e l ec t r i ca l conductance (o r reduceth e r e s i s t anc e ) a n d r e su l t ed in higher wi th va l u e .At very low 8% MC the d i f f e r e n c e may be negligible,but above 10% MC the errors grow r a p i d l y ander r a t i ca l l y . A r e l i a b l e c o r r e c t i o n f o r p r eser va t i vetreated wood is impossible because of v a r i a b l econcentration and depth of chemical penetration.

Skill of operator Operators should be pro pe r l y t r a ined i n each method t o o b t a in good M C! va lue s . Sel ec t i on of wood samples fo r M C ! d e t e r m i n a t i o n s is e q u a l l y i m p o r t a n t . Cares ha l l be exercised at a l l t i m e w h e n u s i n g M C ! mete r s o r ba l anc e to ensure good m e a s u r e m e n t .

Recommendations Use the oven-dry method on green timber (MC > 27%) to establish a more precise moisture content for controlled drying at the early stages

of kilning. Resistance type meters are useful when the MCs of sawn timber are in the region 7 to 27%. Both the temperature and species correction

f a c tor s should be applied to obtain good measurements. May be used at later stages of k i l n i n g and sorti ng & check ing before further processingsuch as planing/moulding/turning, etc.

Die lec t r i c type meters arc useful dur ing tertian- processing, especially for timbers a f t e r planing/moulding or under-going f i n i sh ing processes,a nd fo r c h e c k i n g M C u n i f o r m i t y w i t h i n a n d b e t w e e n pieces of dried t i m b e r before f i ng e r - j o in t i ng o r l a m i n a t i o n .

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One can either use the oven-dry method or electrical meters, which va r y in terms ofaccuracy and ease of measurement, to measure the moisture content of a piece of timber.Of the three, the most accurate measurement method is the oven dry method. However,such destructive method may not be applied all the times. A comparison on thecharacteristics of these methods is presented in Table 1.

Water in woodTo measure the moisture content con f iden t l y , it may be use fu l to understand how w a t e r ormoisture exists in wood.

As with density variation w i t h i n a tree, the amount of water in a stem/trunk varies fromthe cambium to the pith as well as along the stem/trunk. At the same t i m e , there arevariations between trees. As such, the moisture content in a length of f r eshly sawn timbermay not be uniform throughout.

Water exists in timber in two forms: free water and bound water . Free water are watermolecules located in the vessel while bound water are associated to the cell wa l l . Thetransfer or movements of free water are through the pits and capillary actions that do nota f f e c t the cell structures. However, the transfer/movement of bound water through thecell structure is associated with changing cell wal l thicknesses, thus timber dimension.

Timber dries from the outer surfaces towards the core while the water molecules movefrom inside out. At any stage in the drying process, there is a lways a moisture gradientbetween the core and shell or outer surface layer of a piece of timber. In other words, themoisture content is not constant throughout its cross section (Figure 3). Towards the f i n a lstages of k i l n drying, after equalizing and conditioning treatments, the moisture gradientwould be reduced to a minimal level compare to early stages of drying. However, fortimber that has reached equilibrium with the environment after a long period of exposure,there is prac t ica l ly no moisture gradient. Such an important phenomenon should be takennote of when measuring the moisture content of timber.

70,

. 60 -

50 -

30 -

20 -

-10-

In i t i a l moisture content

Fina l moisture content

-tFigure 3 Moisture content d i s t r i b u t i o n over the cross-section of a piece of rubberwood board

( a f t e r pressure treatment to about 10% f i na l m e ) at v a r i o u s stages of drying

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Measuring or checking of moisture contentMoisture content of timber needs to be monitored during processing, and measurementsare required to be made at several stages. The frequency and accuracy required dependsvery much on the process requirements and the service environment the end products aresubjected to.

Three typical measurement scenarios are Air-dried timbers Kiln-drying control Production controlTraditional timber app l i c a t ions require the m o i s t u r e c o n t e n t to be in e q u i l i b r i u m wi th

the outdoor/ambient environment, i.e. moisture con t en t be low 19%. Typica l app l i c a t ionsof timber in the c o n s t r u c t i o n of non air-conditioned i n s t a l l a t ion s are in roof ing, s ta i r case ,flooring, walling, ceiling frames, door and window frames, and furniture (Table 1 inAnonymous 2 0 0 1 ) . This is n o r m a l l y achieved by air-drying. T he moisture content of air-dried timbers can be checked very quickly by using the resistance type meter during thef ina l stage of ai r -drying, processing and in s t a l l a t ion .

In k i ln drying process, the accuracy of moisture content measurements is very crucialfor process control. Without good moisture content measurement, the f i n a l drying qualityof timbers may be severely affected. The only method t h a t could be used for moisturecontent above the Fiber Saturation Point (FSP, ca. 28%) is the oven-dry method. Below theFSP, the resistance type moisture meter may be used. When the moisture content is abovethe FSP, electrical meter mayproduce erroneous readings that would lead to wrong decisionsin process control.

Kiln dried timber is typical ly close-stacked, plastic-wrapped and stored under shed beforef i n a l processing. However, the shed condition is often not controlled. In most cases, themoisture content of t i m b e r may rise. Therefore the moisture content would need to bechecked again before the next stage of processing. Res i s t ance type meter may be used formoisture content checking and sorting of dried timber before cross-cutting and ripping tosize, finger-jointing and planing. In some instances, when dealing w i t h planed stock, thedielectric type meters may be used to check moisture content u n i f o r m i t y before f inger -jointing or laminating. It could also be used in the sanding and f i n i s h i n g sections of aproduction line. At these f i n a l stages of production, the oven-dry method or the resistancetype meters may not be suitable; the oven-dry method is destructive while resistance typemoisture meter leaves behind two unsightly holes. However, the oven-dry method may beused to ver i fy the readings when necessary.

A typica l process flow of a laminating plant and the stages where moisture c o n t e n tmeasurements are taken for process and quality control purposes is presented in Figure 4.The appropriate measurement methods are also indicated while it should be noted thatthe oven-dry method is used when there is suspicion on the meter measurements.

Besides accuracy of moisture content values required and ease of use, thickness of timberalso determines the appropriateness of a measuring method. Figure 5 shows the influenceof thickness on the moisture content measurement and the selection of the appropriatemethod.

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F O R M

Log

Sawn t imber (green)

Sawn t imber(kiln dried)

Finger jointed str ips

Lamina ted board

Finished laminatedboard/tabletop

PROCESS

Sawmill

Chemica l t r ea tment

Kiln drying

Storage

Sor t ing /g rad ing

Ripping & blanking

Finger-jointing:Composing and assembling

Planing

L a mi n a t i n g (edgewise):Glue spreading, c l ampinga nd curing

Planing (after cur ing)

Sanding

Finishing

Final Product

M C Measurementspossible method(s)

Oven-drv method

Oven-dry me th o d /Resistance type meter

Resistance type meter

Resistance type meter/Dielectric type mete r

Dielect r ic type meter /Resistance type mete r

Figure 4 Mois tu re content check s in a typical l a m i n a t i n g p l a n t

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Figure 5 Ef fe c t of timber t h i c k n e s s on moisture content measurements: a) Shel l and coresamplesare cut to determine the moisture contents at d i f f e r e n t depths in the oven-drymethod, h) Insu la t ed pins of resistance type meter are dr iven to d i f f e r e n t depths tomeasure moisture contents over the t h i ckness , and c) A b a l lpa r k measurement up tothe penetration depth of electromagnetic f ield (ca . 25 mm) in d ie lec t r i c type meter.

Accuracy of a moisture meterThe usefulness of moisture meter to a great extent depends on the accuracy of its readings.This may be looked upon at two levels: (a) accuracy of the meter and ( b ) accuracy of thereadings for specif ic timber species.

Accuracy of the meter should be checked regularly against a "standard checkbox (orblock)" supplied by the meter manufacturers. This will ensure t h a t the meter is a lways ingood working order.

Accuracy of measurements relates to the correction f a c t o r s to be applied on meterreference-curve readings for spec i f i c timber species. This requires the meter readings tobe calibrated against the actual moisture content of timber species concerned. Modernmoisture meters may be supplied with correction factors for some M a l a y s i a n timber species.However, if correction factors for a particular timber species are not provided by metermanufacturer, the meter could be calibrated for that timber in the laboratory.

It should be noted that not all moisture meters would give the same reading for a pieceof timber. When big differences occur in meter readings and dispute arises, then the oven-dry method shall be used. A neutral third party l ike FRIM is of ten referred to.

ReferencesAnonymous. 2001.Mal ays i an Standard. Code Of Practice For Structural Use of Timber:Part 1: General (First Rev i s i on ) . MS 544:Part 1: 2001. Department of Standards Malays ia .

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Back IssuesTTB 2 Dust Extraction System for a Sa wm i l l RM 3.00TTB 3 Powder-post Beet le Minthea spp. (Lyc t idae ) RM 3.00TTB 5 Tension Wood in Rubber-wood R M 5 . 0 0TTB 6 Fans RM 5.00TTB 7 Some Defec t s in Veneer and Plywood RM 5.00TTB 8 Timber N o t es H e a v y Hardwoods I RM 5.00TTB 9 Timber NotesLight Hardwoods I RM 5.00TTB 10 Timber N o t e s L i g h t Hardwoods II RM 5.00TTB 11 Timber N o t e s M e d iu m Hardwoods I (Kapur, Kasai , Ke la t , R M 5.00

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Back IssuesTTB 24 Observations of Termite-Fungus Interactions of Potential RM 5.00

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Forest Research Ins t i t u t e Malaysia 2005Series Editor: Y. E. TanTypesetter : Roshafiza Mohamad

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Printed bv Publications Branch, Forest Research I n s t i t u t e M a l a y s i a