500-72203-001 Rev. EPage 7of 15 L722 Stack Probe Manual
one kiln charge 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 Electronics 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 goodmeasurement 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!
