28
Kiln Sample Boards
It is best to use sample boards to measure moisture
content on a daily basis. Moisture meters are not
accurate enough for most hardwood dry kiln
operations when the wood is above 30% MC and
there is a need to keep a close watch on the
drying rate. In Oak, for example, all checks and
honeycombing occur when the wood is drying
from green down to 40% MC, so that is when the
drying rate needs to be closely controlled.
Uses for Sample Boards
• To estimate the MC of the load in the chamber,
so that kiln conditions can be regulated according
to drying schedules.
• To measure the drying rate, which allows control
of drying quality.
• To check on any degrade development.
• To check on final MC and drying stresses.
• To develop a MC vs. time curve.
• To study variations in drying within the kiln.
• To monitor changes in MC after drying (during
storage and shipping) Note: It is a good idea to
keep sample boards with dried lumber so that
they can be used to track moisture content
changes in storage.
Taking Samples
1. Select sample boards. Keep in mind that they
need to represent a “sample” of the lumber in
the kiln. Do not select junk boards, and select
both the slowest and the fastest drying boards
of the load. Generally, you want six sample
boards.
2. Cut 30” samples, at least 12” from the ends of
each of the sample boards. Avoid having knots,
splinters or bark in the sample.
3. Number the samples you cut.
4. Cut two 1” sections off each end of the samples,
and mark them with the number of the board
they came from.
Example: the two sections that came from
sample board 3 would be labeled 3A and 3B.
5. Apply end coat to the 28” sample boards.
This assures that the sample board will dry as
though it were a larger piece of lumber.
End coatings are not usually required when
lumber is placed in the kiln directly after being
sawn. However when lumber is air dried prior
to kiln drying, end coating is often helpful in
preventing end checks
6. Weigh the 28” sample boards. An accuracy of
.025 pounds or 1 gram is recommended. Record
these values.
7. Put the 28” sample boards in the lumber stacks
in places where they can be reached and will
dry at the same rate as the lumber. Do not place
the sample boards where they will receive
more air ow than the rest of the lumber.
8. Take the “green weight” by weighing all the 1”
sections, to 0.1 gram accuracy is suggested.
Record these values.
9. Oven dry the 1” sections using one of the two
methods below.
• Microwave with carousel tray: Put sections
on a paper towel on the tray. Use low power
(to avoid smoking) for 20 minutes. Weigh
the section, and put back in the microwave
for 1 minute. If the section has not lost any
weight, it is oven dry. If still losing water,
continue drying in 5 minute increments
until a constant weight is achieved.
• Oven: Place the 1” section in an oven at
215ºF (101ºC) until the section stops losing
weight, usually takes 24 hours. Check hourly
until the section weighs the same in separate
weighings.
10. Calculate the moisture content of each 1” section
separately using the following formula:
%MC=(
Wet Weight
Oven Dry Weight
-1)X 100
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