Maximum rate of MC loss per day
Beech-4.5
Birch,Yellow-6.1
Cherry-5.8
Elm,American-10.4
Maple(soft)-13.8
Maple,Hard-6.5
Oak,Red-3.8
Oak,White-2.5
Oak,Southern-Var 1.0-3.0
Walnut-8.2
It is important to adhere to this rate when the lumber is first ut in the kiln as the first half of the cycle is most critical in preventing degrade. As the lumber dries below 22% moisture, the risk of creating new degrade is nil.
In the solar dryer, the drying rate is slowed by covering up part of the collectoor, and on very hot, sunny days by shutting off the fans. Theis safe rate refers to the loss on one day, not the average loss over several days. An 8% loss on
e day and 2% the next is not equivalent to a "5% per day" safe rate.
In order to measure the moisture loss rate, it is impractical to weigh every board in the dryer and electric meters are not accurate enough. Therefore, we use the sample board method to represent the moisture of the lumber in the kiln. This method uses short,carefully chosen sample boards sawn from larger pieces of lumber and these sample boards are wirghed periodically and estimated moistures are calculated,
Generally, because the wettest lumber in the kiln has the highest risk of degrade development, the sample boards should represent the wettest lumber in the dryer; this means the lumber that is the most recently cut,is the widest and thickest, contains the heartwood, and is quartersawn.
The precise procedures in preparing sample boards are:
1. Select lumber to be used.
2. Cut a 30" sample board and then two 1" sections from the sample board, avoiding areas near knots and areas closer than 12" to the ends of the lumber. (Fig 8)
3. Number the sections and sample board.
4. Immediately weight the sections (accuracy of about 1 gram required). It is important that they not lose weight after cutting and before weighing. Record weight on the section with a marker pen.
5. Weigh the sample board and record weight on sample (accuracy of 0.1 pounda).
6. End coat the sample board.
7. Place sample board in the lumber stack in a location where it will dry at the same rate as the rest of the lumber in the dryer.
8. Place sections in an oven 215-218^ F.
9. After the samples dry-usually 18-36 hrs depending on the oven-reweigh and obtain the oven dry wieght.
10. Calculate the moisture content of each section and average the MC of two sections to obtain the MC of the sample board.
%MC=(wet weight/oven dry weight)-1)x100
11. Estimate oven dry weight of sample board using average MC above and weight
from step 5.
Est O.D. weight=wet weight,(step 5)/(100+%MC))x100
12. Write
the est.O.D. weight on sample board when it is next weighed so that it is readily available.
13.Periodically reweigh the sample board to obtain a new,current moisture content.
Current%MC=(current weight/Est O.D. weight) -1x100
14. Occasionally it is necessary to cut new sample boards when the lumber is at 20% MC to obtain more accurate values. To do this, cut one new moisture section 6" from the end of a sample board. Then follow steps 3-11, except that only one section is used. The wet weights in steps 10 and 11 are the weights determined after the sample and new section are sawn.
Figure 8
Design Fundamentals
Construction Details
Drying Lumber
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