Series editors' preface - Wood Tools

0 and 25% moisture content. They have the distinct
advantage of being able to take readings
without damaging wood surfaces but the disadvantage
of operating at a fixed depth of
field.
To get the best out of either type, familiarity
with the nature of wood–moisture relations
and with the meter is necessary and it is a
good idea to experiment with different methods
and compare the results. Other instrumental
methods that have been used to measure
moisture content include the neutron moisture
meter and nuclear magnetic resonance (Skaar,
1984).
The curve in Figure 2.14 showing equilibrium
moisture content as a function of relative
humidity (or relative vapour pressure) at constant
temperature is called a moisture sorption
isotherm. After initial seasoning, a sample of
wood taken through repetitive cycles of RH
exposure between 0 and 100% tends to follow
the same adsorption and desorption curve
repetitively. Therefore, an indirect method of
estimating moisture content is to place the
wooden object inside a well-sealed container
with a hygrometer and to measure the RH produced.
2.4.3 Dimensional change
Of all the properties affected by moisture content,
dimensional stability commands the greatest
attention. Not only is wood hygroscopic, it
is also anisotropic. That is, it exhibits different
properties when tested along axes in different
directions. Because of this, dimensional change
in wood is usually considered separately in the
three principal linear directions: longitudinal,
radial and tangential. In previous sections the
general effects of bound water sorption were
reviewed relative to the cellulose structure
within the cell wall. Discussion here will concentrate
on the quantitative effects of moisture
content on the anisotropic dimensional behaviour
of wood tissue. In the initial drying of
wood, there is no dimensional response to the
loss of free water. Only when a portion of
wood tissue has reached the fibre saturation
point and begins to lose bound water does
shrinkage begin.
The common basis for indicating the relative
dimensional instability of a given wood is to
measure the total amount of linear shrinkage
Wood and wooden structures 79
that takes place in a given direction from the
green to the oven-dry condition, expressed as
a percentage of the green dimension. Thus, the
total shrinkage percentage is calculated as follows:
S = (Dg – Dod)/Dg 100
where S = total shrinkage, expressed as a percentage
(St = tangential shrinkage, Sr = radial
shrinkage, Sl = longitudinal shrinkage) and D =
change in dimension (Dg = green dimension,
Dod = oven-dry dimension). Figure 2.15 illustrates
the application of the formula in the
determination of tangential shrinkage based on
green and oven-dry measurements of a tangentially
sawn strip of wood.
Total of shrinkage of wood along the grain
(i.e. longitudinal shrinkage) is normally in the
range of 0.1–0.2%. In practical situations
involving typical moisture content changes
over a moderate range, only a portion of this
small quantity would be effected and the
resulting dimensional change becomes insignificant.
It is reasonable to assume that wood is
stable along its grain direction, and for most
purposes longitudinal shrinkage and swelling
are ignored. In fact, longitudinal shrinkage data
Figure 2.15 The calculation for percentage of
tangential shrinkage (S t) based on green and oven-dry
measurements of a tangentially sawn strip of wood. D
= the change in dimension, D g = the dimension of the
wood when green, D od = the dimension of the wood
when oven-dried. Note that this formula is only
applicable to shrinkage starting from the green
condition. For dimensional change of partly seasoned
wood this formula will introduce an average error of
about 5% of the calculated change in dimension.
Calculation of the dimensional change of partly
seasoned wood is described in section 2.4.4