500-72203-001 Rev. H Page 5 of 15
to the next. You should take enough readings to be statistically significant. The
more readings you take, the more accurate your “average” will be.
When taking measurements using a stack probe, you will be measuring the selected
locations in the stack as stated in your kiln drying operations procedure. This will be
the average moisture content of the scan area being measured directly under the
sensor plate.
Keep in mind that the scan area of the L722 stack probe is 2.5” square (6.35cm) by
1” deep (2.54cm). This varies greatly from an in-line system, which has a much
wider scan area (up to 12” square, 30.5cm), and deeper penetration. This will cause
a larger area to be “averaged” with an in-line system.
The L722 stack probe readings will read excessively high when measuring severely
knotted lumber due to higher density. Readings will be best when measured on a
clear, flat area on the wide face of the lumber that is free of defects. If there is
noticeable surface moisture, try not to measure at that spot until the moisture
dissipates, or skip the area completely. As the moisture disperses, the effect will be
minimized.
Kiln Drying, Moisture Gradients, and Temperature – Warm lumber tends
to lose moisture after it is removed from the kiln and it continues to dry. It can also
absorb moisture from the air in humid conditions during storage prior to planing. The
moisture content of kiln-dried lumber is typically higher in the center core and dryer
towards the ends and outer surfaces. Planing the dryer top and bottom sections of a
board will cause the MC value to be higher. Because it is difficult to control all of
these elements, it is best to try to maintain consistency in storage, and apply a
correction factor as discussed in the paragraph below.
Frozen lumber can affect the MC readings. Wagner Meters has found that if frozen
lumber has a moisture content value lower than 15%, the readings are accurate. If
the MC is higher than 15%, the actual reading obtained may be lower than the true
value.
In a good measurement process consistency, repeatability, and correlation are the
key factors. The goal is a better FINAL lumber product that meets your moisture
content and grade goals. The closer your final product is to your target MC, the more
cost efficient your drying process will be.
Here are some questions to ask yourself: Are all of your personnel taking the same
number of readings? Are the readings being taken at the same time in the kiln dry
schedule? Has my meter calibration been recently verified? Are the readings being
taken at the same location in the same packs every time? This is the key to
repeatability!
Applying Correction Factors - Due to variances beyond the control of the
user, the MC values obtained from kiln dried lumber as read from the Hand Meter
