Calibration curves for resistance-type moisture meters

Calibration Curves for

Resistance-type

Moisture Meters

Paper presented at Srd lUFRO International

Wood Drying Conference, Vienna,

August 18-211992

Trätek

Anders Samuelsson

CALIBRATION CURVES FOR RESISTANCE-TYPE MOISTURE METERS

Paper presented at 3rd lUFRO International Wood Drying Conference,

Vienna, August 18-21, 1992

Trätek, Rapport I 9302012

ISSN 1102- 1071

ISRN TRÄTEK - R - - 93/012 - - SE

Nyckelord

Stockholm februari 1993

CALIBRATIONCURVES FOR RESTISTANCE-TYPE

MOISTURE METERS

Anders Samuelsson,

Swedish institutet. Wood Technology Research, Drottning Kristinas väg

67, Box 5609,11486 Stockholm, SWEDEN

ABSTRACT F o r t h e i m p r o v a l o f t h e a c c u r a c y o f r e s i s t a n c e - t y p e m o i s t u r e m e t e r s f o r wood, t h e r e l a t i o n s h i p between m o i s t u r e c o n t e n t and e l e c t r i c a l r e s i s t a n c e i s i m p o r t a n t . I n Sweden, e x t e n d e d measurements have r e c e n t l y been made on S c o t c h p i n e and Norway s p r u c e . The r e s u l t s o f t h i s i n v e s t i g a t i o n a r e p r e s e n t e d i n t h i s p a p e r and some c o m p a r i s o n s a r e made w i t h a n o t h e r w o r k , ( P a a j a n e n 1 9 6 6 ) . The t e m p e r a t u r e c o r r e c t i o n f o r r e s i s t a n c e - t y p e m o i s t u r e m e t e r s h a s been s u b j e c t e d t o a l i t e r a t u r e s u r v e y , t h e r e s u l t of w h i c h i s a l s o p r e s e n t e d . 1 INTRODUCTION The r e s i s t a n c e of wood d e c r e a s e s w i t h i n c r e a s i n g m o i s t u r e c o n t e n t . T h i s have been know f o r l o n g t i m e . Many i n v e s t i g a t i o n s h a s been done o v e r t h e y e a r s t o e s t a b l i s h a r e l a t i o n s h i p between e l e c t r i c a l r e s i s t a n c e and m o i s t u r e c o n t e n t . Very few o f t h e s e were, however, made on S c o t c h p i n e

( P i n u s s i l v e t r i s ) and Norway s p r u c e ( P i c e a a b i e s ) . As a s t e p t o w a r d s t h e i m p r o v a l of t h e a c c u r a c y o f t h o s e r e s i s t a n c e - t y p e m o i s t u r e m e t e r s t h a t a r e u s e d i n t h e S c a n d i n a v i e n c o u n t r i e s , t h e r e l a t i o n s h i p between r e s i s t a n c e and m o i s t u r e c o n t e n t was d e t e r m i n e d f o r t h e two most common s p e c i e s above.

2 MATERIALS AND METHODS 2.1 M a t e r i a l

The m a t e r i a l s t e s t e d were S c o t c h p i n e ( P i n u s s i l v e t r i s ) and Norway s p r u c e

( P i c e a aJbies) . Both s p e c i e s were t a k e n from s i x s a w m i l l s i n Sweden, s i t u a t e d from North t o South.

The p i n e m a t e r i a l c o n s i s t e d o f 66 p i e c e s w i t h a mean d e n s i t y of 413 kg/m', and t h e s p r u c e m a t e r i a l 114 p i e c e s w i t h a mean d e n s i t y of 386 kg/m'. The d e n s i t y was d e f i n e d a s d r y w e i g h t o v e r raw volume. The d i m e n s i o n s o f t h e t e s t -p i e c e s were 15 t o 75 mm t h i c k and 75 t o 22 5 mm wide. 2.2 C o n d i t i o n i n g A l l t e s t p i e c e s were c o n d i t i o n e d a t f i v e d i f f e r e n t c l i m a t e l e v e l s a s f o l l o w s : L e v e l No. 1 T l = 50 "C, RH1= 97 % T2= 20 "C, RH2 = 96 % L e v e l No. 2 T l = 50 "C, RH1= 90 % T2= 20 °c. RH2= 89 % L e v e l No. 3 T l = 50 °c. RH1= 82 % T2= 20

_°c.

The minimum c o n d i t i o n i n g t i m e was f o r T l 11 d a y s and f o r T2 1 day. B e f o r e e n d i n g t h e c o n d i t i o n i n g p e r i o d t h e w e i g h t s of some t e s t p i e c e s were c h e c k e d so t h a t t h e y would not change w i t h t i m e . Even t h e m o i s t u r e g r a d i e n t was c h e c k e d i n a few t e s t p i e c e s . 2.3 Measurements A f t e r t h e c o n d i t i o n i n g o f t h e t e s t p i e c e s t o e a c h c l i m a t e l e v e l , t h e e l e c t r i c a l r e s i s t a n c e was measured w i t h a s p e c i a l l y d e s i g n e d ohmmeter. I n t h e p i n e m a t e r i a l , 45 measurements were made, but t h i s work o n l y p r e s e n t s 4 of t h e s e .

406

I n t h e s p r u c e m a t e r i a l , 4 measurements were made. T h i s amount o f measurements was made i n e v e r y p i e c e a t e a c h c l i m a t e l e v e l , w h i c h g i v e s a t o t a l amount o f m e a s u r i n g s o f 1320 f o r p i n e and o f 2280 f o r s p r u c e . The measurement s p e c i f i c a t i o n s were a s f o l l o w s : E l e c t r o d e t y p e : i n s u l a t e d s t e e l n e e d l e s D i s t a n c e between t h e two n e e d l e s : 32 mm Measurement v o l t a g e : 60 V o l t s Measurement t e m p e r a t u r e : 20 °C Measurement t i m e : 1+1 s e c o n d Measurement depth: 10 mm

The ohmmeter measured w i t h t h e c u r r e n t i n two d i r e c t i o n s . The measurement t i m e was 1 s e c o n d i n e a c h d i r e c t i o n and t h e ohmmeter t h e n c a l c u l a t e d t h e mean v a l u e w h i c h was shown on t h e d i s p l a y . A f t e r m e a s u r i n g t h e r e s i s t a n c e , t h e w e i g h t o f t h e t e s t p i e c e s were measured s o t h a t t h e m o i s t u r e c o n t e n t c o u l d be c a l c u l a t e d a f t e r w a r d s w i t h t h e d r y w e i g h t method. 2.4 C u r v e f i t t i n g I n o r d e r t o e s t a b l i s h a r e l a t i o n s h i p b e t w e e n e l e c t r i c a l r e s i s t a n c e and m o i s t u r e c o n t e n t , a c u r v e f i t t i n g h a s t o be made w i t h t h e m e a s u r e d v a l u e s . The f i t t i n g model t h a t h a s b e e n used i n t h i s work i s : l n ( R ) =A-exp(-B-u) -C ₍₁₎ where R i s t h e r e s i s t a n c e [Mohm] a n d u t h e m o i s t u r e c o n t e n t [ % ] . The c u r v e f i t t i n g was made w i t h t h e s o f t w a r e package S t a t g r a p h i c s , w h i c h c a l c u l a t e d t h e c o e f f i c i e n t s A,B and C.

3 RESULTS AND DISCUSSION 3.1 C a l i b r a t i o n c u r v e s The f i t t e d e q u a t i o n s i s a s f o l l o w s f o r S c o t c h p i n e : and f o r Norway s p r u c e : l n ( R ) =26,00-exp(-0.0845-u) -2,79 ( 3 ) E q u a t i o n s (2) and (3) a r e p l o t t e d i n F i g u r e 1. R e s i s t a n c e ln(R)(Mohm] 16 12 e H — Scotch pine - - Norway spruce 6 l e 15 20 26 Moisture content [%) 3e F i g u r e 1: C a l i b r a t i o n c u r v e s f o r S c o t c h p i n e and Norway s p r u c e . An e a r l i e r work by P a a j a n e n (1966) a l s o i n c l u d e s measurements on S c o t c h p i n e and Norway s p r u c e . T h e s e measurements have been u s e d h e r e t o g e t two o t h e r c a l i b r a t i o n c u r v e s t o compare w i t h . The f i t t e d e q u a t i o n s a c c o r d i n g t o P a a j a n e n (1966) and h i s measurements a r e a s f o l l o w s f o r S c o t c h p i n e : l n ( R ) =28, 66-exp(-0,0967 01) -2,30 ^4) And f o r Norway s p r u c e : l n ( R ) =27,18-exp(-0,085211) -2,30 ( 5 ) T h e s e c a l i b r a t i o n c u r v e s , ( 4 ) and (5) , a r e compared w i t h t h o s e w h i c h a r e d e s c r i b e d i n E q u a t i o n s ( 2 ) a n d ( 3 ) . F i g u r e 2. shows t h e two c u r v e s f o r S c o t c h p i n e and F i g u r e 3. shows t h e two c u r v e s f o r Norway s p r u c e .

R e s i s t a n c e ln(R)(Mohm] 15 12 8 h — T h i s work - - P a a j a n e n (1966) e 6 l e 15 20 26 38 Moisture content [%] F i g u r e 2; C a l i b r a t i o n c u r v e s f o r S c o t c h p i n e a c c o r d i n g t o E q u a t i o n s (2) and ( 4 ) . R e s i s t a n c e ln(R)[Mohni) T h i s work Paajanen (1966) 5 18 IB 28 Moisture content [%) 25 38 F i g u r e 3; C a l i b r a t i o n c u r v e s f o r Norway s p r u c e a c c o r d i n g t o E q u a t i o n s (3) and (5) . I n both F i g u r e s 1. and 2., t h e d i f f e r e n c e i s a p p r o x i m a t e l y i n t h e same r a n g e a s t h e measurement e r r o r . F i g u r e 3. shows t h a t i t i s a c l e a r d i f f e r e n c e between t h e two Norway s p r u c e c u r v e s , w h i c h g i v e s a maximum m o i s t u r e c o n t e n t d i f f e r e n c e o f 2 % a t a c e r t a i n r e s i s t a n c e . A p o s s i b l e r e a s o n f o r t h e d i f f e r e n c e i n t h e Norway s p r u c e c a s e c o u l d be t h e i n s t r u m e n t s t h a t had been u s e d f o r m e a s u r i n g t h e r e s i s t a n c e . A s p e c i a l l y d e s i g n e d ohmmeter by t h e K . L i d s t r o m company was u s e d w i t h a measurement v o l t a g e o f 60 V o l t s . P a a j a n e n (1966) u s e d a D e l m h o r s t RC-1 m o i s t u r e m e t e r w i t h a measurement v o l t a g e o f 10 V o l t s . The K . L i d s t r o m i n s t r u m e n t measured w i t h t h e c u r r e n t i n two d i r e c t i o n s , 1 s e c o n d i n e a c h d i r e c t i o n , and s u b s e q u e n t l y i t c a l c u l a t e d a mean v a l u e w h i c h was u s e d h e r e . 3.2 T e m p e r a t u r e c o r r e c t i o n The e l e c t r i c a l r e s i s t a n c e i n wood d e c r e a s e s when t h e wood t e m p e r a t u r e i n c r e a s e s . T h e r e f o r e a c o r r e c t i o n i s n e c e s s a r y . S e v e r a l i n v e s t i g a t i o n s h a v e been made o v e r t h e y e a r s . I n t h i s work t h e r e s u l t from P f a f f and G a r r a h a n (1986) was u s e d and r e w o r k e d so t h a t t h e c a l i b r a t i o n p o i n t was s e t t o + 20 °C. The c o r r e c t i o n above + 50 °C i s e x t r a p o l a t e d so t h a t t h e c o r r e c t i o n i s from - 40 t o +70 °C, w h i c h i s a u s e f u l r a n g e i n S c a n d i n a v i a . _ U + 0, 567-0. 0260-t4^0, 000051-t 0,881-(1, 0056)^ (6) E q u a t i o n (6) i s t h e o r i g i n a l t e m p e r a t u r e c o r r e c t i o n f o r B l a c k s p r u c e a c c o r d i n g t o P f a f f and G a r r a h a n ( 1 9 8 6 ) . The r e w o r k e d e q u a t i o n (7) i s u s e d f o r S c o t c h p i n e and Norway s p r u c e , and i t i s p r e s e n t e d below. _ u + 0, 567-0, 026 0-x-t-O, 000051-X' 0, 881-(1, 0056) (7) T h e r e i s a s m a l l d i f f e r e n c e between t h e Norway s p r u c e and S c o t c h p i n e c u r v e s i n F i g u r e 1., but i t o n l y g i v e s a maximum m o i s t u r e c o n t e n t d i f f e r e n c e o f 0,5 % a t a c e r t a i n r e s i s t a n c e . The two S c o t c h p i n e c u r v e s i n F i g u r e 2. h a v e a l s o a maximum m o i s t u r e c o n t e n t d i f f e r e n c e o f 0,5 % a t a c e r t a i n r e s i s t a n c e . u= m o i s t u r e meter r e a d i n g [%] u = t e m p e r a t u r e c o r r e c t e d m o i s t u r e c o n t e n t [%] t = wood t e m p e r a t u r e ["C] x= t+2,8 ["C] F i g u r e 4. shows t h e t e m p e r a t u r e c o r r e c t i o n f o r S c o t c h p i n e and Norway s p r u c e a c c o r d i n g t o e q u a t i o n ( 7 ) .

408

4 8 12 16 20 24 28

Du, Q.P., A . G e i s s e n and D.Noack: W i d e r s t a n d s k e n n l i n i e n e i n i g e r Handelshölzer und i h r e M e s s b a r k e i t b e i d e r e l e k t r i s c h e n H o l z f e u c h t e m e s s u n g . ( I n German.) H o l z a l s Roh- und W e r k s t o f f 49:1-6 (1986) James, W.L.: E l e c t i c m o i s t u r e m e t e r s f o r wood. R e s . n o t e FPL-08. Madison, U.S. Department o f A g r i c u l t u r e (1963) . P a a j a n e n , T.: M e a s u r i n g t h e m o i s t u r e c o n t e n t o f wood w i t h e l e c t r i a l m o i s t u r e m e t e r s . ( I n F i n n i s h . ) T h e s t a t e i n s t i t u t e f o r t e c h n i c a l r e s e a r c h , F i n l a n d . ( 1 9 6 6 ) . P f a f f , F . , P . G a r r a h a n : New t e m p e r a t u r e c o r r e c t i o n f a c t o r s f o r t h e p o r t a b l e r e s i s t a n c e - t y p e m o i s t u r e meter. F o r e s t P r o d u c t J o u r n a l 3 6 ( 3 ) : 2 8 - 3 0 ( 1 9 8 6 ) . Samuelsson, A.: R e s i s t a n s k u r v o r för e l e k t r i s k a fuktkvotsmätare. ( I n Swedish) Swedish I n s t i t u t e f o r Wood T e c h n o l o g y R e s e a r c h , R e p o r t L 9006029 ( 1 9 9 0 ) . S k a a r , C.: Wood-Water R e l a t i o n s . S p r i n g e r - V e r l a g 1988, 283 p. Vermaas, H.F.: D.C. r e s i s t a n c e m o i s t u r e m e t e r s f o r wood. P a r t I . S o u t h A f r i c a n F o r e s t r y J o u r n a l 121:88-92 (1982) .

Noggrannheten hos en elektrisk fuktkvotsmätare av resistivtyp är beroende av relationen

mellan elektrisk resistans och fuktkvot. Omfattande mätningar har utförts i Sverige för att

beskriva relationen resistans - fuktkvot för furu och gran. Resultatet från dessa mätningar

redovisas i detta arbete, även några jämförelser görs med tidigare mätningar. Korrektion

för träets temperatur har varit föremål för en litteraturundersökning som även den redovisas

i detta arbete.

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