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.