Infrared Thermography for Monitoring Surface Checking for Wood During Drying

Joo Hoon SONG, Satoshi SHIDAGraduate School of Agriculture and Life Sciences, The University of Tokyo, Japan

11th International IUFRO Wood Drying ConferenceSkellefteå, Sweden, January 18-22, 2010

Table of Contents

1. Introduction

2. Experiment

3. Results and Discussions

4. Conclusions

Reduction in checking

• Finding the small end checking (formation of small cracks in surface) in the early drying stage, before they actually develop.

Fig. Various types of dry checks.

Introduction

• It is very difficult to prevent surface checking.There is a need for drying schedule for Japanese

cedar(sugi) boxed-heart square lumber(most commonly used for column in Japanese wooden house).

• We focused on temperature which is the most usable factor to control drying.

The relationship between surface temperature change and surface checking in the early drying stage.

• We tried to use thermography for monitoring and controlling drying.

This technique may provide an essential tool for the development of more precise and effective control of kiln drying.

Occurrence of surface checking

In commercial kiln drying of sugi boxed-heart square timbers, it is verydifficult to prevent the occurrence of surface checking.

Fig. Check development from cross section of sugi boxed-heartsquare timbers (120×120×400mm used for column).

Infrared camera

• Recently, infrared camera becomes much smaller, lighter and cheaper.

Fig. Prices of infrared camera (NEC Avio.Co.).

Objectives

• The objective of this work is to examine the relationship between the surface temperature and checking.

• To control the drying condition using the surface temperature in order to reduce checking.

Materials

Species Sugi（Cryptomeria japonica D.Don）boxed-heart square lumber（Initial MC 47～80%)

Size 120mm×120mm×250mm(L)

Condition 105ºC constant in oven

Coated inblack

Measurement interval：every 1 hour.(every 10 min. until 2 hours.)

Infrared camera

Experiment

Fig. Thermal imaging with infrared camera (TVS-100, NEC Avio Co.).

Styrofoam Vinyl wrap

Specimen

Infrared camera

Door

Dry oven

1 2 3

4 5 6

7 8 9

After 11h.After 2h.

3×3 division B area

A area

Division in cross section

C area

Classification Degree of the drying checkingA area Area where the checking did not occur. B area Area where the checking occurs in the early

drying stage.C area Area where the checking is more severe in the

final drying stage.

B area

C area

Length of surface checking

F.S.P.

Fig. (A) The relationship between surface temperatureand total length of surface checking.

Fig. (B) The relationship between moisture content and total length of surface checking.

Schematic change of surface temperature

Period Ⅰ: In the early drying stage (until 5 hours), Appearance of end/surface checking.

Period Ⅱ: In the second drying stage(from 5 hours), Temperature rise, Large difference of temperatures between A and B, B and C.

Period Ⅲ: In the final drying stages(after 20 hours), Temperature rate decreases.

Time(h)

Tem

pera

ture

(℃)

CA

Ⅰ

Ⅱ Ⅲ

Fig. Schematic diagram of change in the surface temperature versus drying time.

B

Cross section

Sections for comparison Ⅰ

24mm

Checkedarea

Uncheckedarea

Surface

5×5 division in cross section

120mm

Checking

Center Surface

5×5 division

120mm

24mm

Fig. Sections for comparison of the surface temperature between unchecked area and checked area.

X axis01234 160

01

120

Y axis

2

Pixel

120mm

120mm

MC vs. surface temperature

55

60

65

70

75

80

10 20 30 40 50 60

Unchecked areaChecked area

Surfa

ce te

mpe

ratu

re(℃

)

Moisture content(%)Fig. Comparison of the surface temperature between unchecked areas and checked areas.

• Surface temperature of checked areas is lower than unchecked areas.

Unchecked area vs. checked area

Fig. Comparison of the surface temperature between unchecked area and checked area.

• Average temperature of the checked area was lower, but the standard deviation was higher.

Sections for comparison Ⅱ

120mm

120m

m

②Outer surface①All-surface ③Middle surface

5×5 division in cross section

CV＝（standard deviation/average)×100(%)

Fig. Sections for comparison of the coefficient of variation(CV) of the surface temperature among the all-surface, the outer surface and the middle surface.

Comparison of CV

Fig. The relationship between drying time and coefficient of variation(CV) of surface temperature.

Checking occurred the CV reached more than 8%

Drying control system in lab.

Portable infrared camera

Detection element

48×47

Wave length 10μm

Angle of visibility

60°×60˚

Temperature range

-20ºC to 300ºC

Display resolution

0.5ºC

Emissivity 1.0(default)0.1 to1.0 adjustable by internal switch (0.01 increments)

Weight 150g

Size W62×H62×D59 (mm)

Fig. Specifications of portable infrared camera TP-L series (Chino. Co.)

An example in drying control method

Start

Input data: Wo, DBTi, DBT1,DBT2, RHi, CVⅠ,CVⅡ, Final MC,ΔRH, Complete time

Calculate CV1 from the surface temperature

DBT≧DBTiRH≧ RHi

CV1≧CVⅠ(5%)

RH1=RHi+ΔRHDBT1

Calculate CV2 from the surface temperature

CV2≦CVⅡ(3%)

RH2=RH1-ΔRHDBT2

MC≦Final MCTime≧Complete time

Stop

YesNo

No

Yes

Yes

No

No

Wo: estimated oven-dry weightDBTi: dry bulb temperatureDBT1,2: DBT in step1,2

MC: moisture contentRH: Relative humidityCVⅠ,Ⅱ: CV for surface checkand internal check

Fig. Flow chart of controlling a dry kiln using the CV of the surface temperature.

Result of drying control

10

20

30

40

50

60

70

80

90

0

1

2

3

4

5

6

0 5 10 15 20 25

Initial_DBT80℃,RH85%

DBT,

RH(

%), M

C(%)

CV(%)

Drying t ime(h)

DBT

RH

CV

MC

CVⅠ

CVⅡ

Conclusions

1) Determination of the occurrence of the surface checking from the surface temperature of the cross section was made possible using thermography.

2) By using the coefficient of variation of the surface temperature is used as index, kiln-drying control became possible.

Application in dry kiln

Fig. An example of back and front view of a conventional dry kiln.

• The most popular dry kiln in Japan (about 80% of all kilns) usessteam produced by burning oil as fuel.

• It is possible to take a photograph of the lumber through the small window of a conventional dry kiln using an infrared camera.

Thank you for your attention.