by Gene Wengert
email@example.com WOOD DOCTOR’S Rx
QIt looks like the humidity sensors in our plant are not accurate anymore. How is
the interior plant humidity
related to outside humidity values?
AFirst, it is common to find relative humidity (RH) sen- sors lose calibration in a few years, especially in a dusty
environment. So, I do suggest evaluating their readings every six months. A
local heating/air conditioning person
should be able to do this quickly.
In a plant, especially with dust collectors, open doors or windows, and so
on, will draw outside air into the plant.
Oftentimes, this air will be cooler than
the plant’s desired temperature, so
heat is added. When heat is added to
air, its humidity drops.
For example, start with foggy air at
100 percent RH and 50F. This condition will result in about 28 percent MC
within wood, with the surface and ends
changing within a few hours, while the
core may take a month.
To give us woodworkers a good feel
for the air’s condition, we use the term
EMC (Equilibrium Moisture Content);
at 50F and 100 percent RH, the air is at
28 percent EMC.
If this air is heated to 60F, the
humidity will drop to 70 percent RH,
which is 13 percent EMC. When heated
to 70F, 49 percent RH and 9 percent
EMC. If we have some finishing ovens
that heat the air to 110F, the humidity
is 14 percent RH and 3 percent EMC.
Another example: Assume the
outside is sleeting with 99 percent
RH and 30F. Heating this air to 70F
results in 24 percent RH and 5 percent
EMC. Note that if we have some oak
parts that came from lumber that was
at 8 percent MC and are now exposed
to this 5 percent EMC condition, the
wood will lose rather quickly (days) because parts have a lot of surface area,
With a 3 percent MC loss in oak in
this example comes roughly 1 percent
shrinkage in width and thickness and
Obviously, it is important to match
the lumber’s MC and the plant’s EMC,
Of course, when the plant is too
dry, to avoid shrinkage defects, we add
moisture to the air. Ideally, we want to
match exactly the EMC in the cus-
tomer’s home or office…or at least be
within 2 percent EMC, as most of the
time a 2 percent EMC difference is not
So, how can you determine when
you will likely have to adjust your plant
humidity? You can use a humidity sensor, but they are not always accurate.
Plus, the sensor tells you that you are
out of range after the fact. It might be
more helpful to know when a moisture
situation is developing. So, here is a
DI Y technique.
See more at the Wood
Dr. Knowledge Center
See more columns at
Sponsored by Northwest Hardwoods.
Measuring plant humidity the right way
How can you speed up the drying process?
; Want more? To search a full list of
Wood Doctor’s Rx question and answers, go to
Gene Wengert, “The Wood Doctor,” has been training
people in efficient use of wood for 35 years. He is
extension specialist emeritus at the University of Wisconsin-Madison.