Electrical impedance spectroscopy device for measurement of moisture
gradients in wood
M. Tiitta
a)
and H. Olkkonen
Department of Applied Physics, University of Kuopio, P.O. Box 1627, 70211 Kuopio, Finland
͑Received 27 August 2001; accepted for publication 22 April 2002͒
A prototype of the electrical impedance spectroscopy ͑EIS͒ device for the measurement of internal
moisture gradients in wood was developed. The EIS device consists of a hand-held probe connected
to a control unit interfaced with a portable personal computer and a power unit. In the measurement,
parallel flat electrodes of the measuring probe are laid against the wood specimen and the sine wave
excitation is applied in the frequency range 1–100 kHz. The measured amplitude and phase spectral
data were analyzed using the model based on constant phase elements. A spectral analysis software
package was designed for measurement of subsurface transverse moisture gradients. The EIS device
was tested with many types of uniform, desorption, and absorption gradients in lumber, pulpwood,
and log specimens from spruce, pine, and birch. The EIS device can be easily transferred in a small
case allowing field measurements. © 2002 American Institute of Physics.
͓DOI: 10.1063/1.1485783͔
I. INTRODUCTION
Wood is a renewable natural resource and thus it would
be important to optimize its use. The moisture content ͑MC͒
is an important factor affecting the properties of wood.
1,2
MC
influences, e.g., strength properties, stiffness, hardness, abra-
sion resistance, machinability, heat value, thermal conductiv-
ity, yield, and quality of pulp and resistance of wood against
biodeterioration. The determination of MC and the knowl-
edge of relations between the MC and wood are therefore of
the utmost importance. MC is defined as the weight of water
contained in the wood, expressed as a percentage ratio to the
weight of the oven-dry wood:
MCϭ100
͓͑
w
m
Ϫw
d
͒
/w
d
͔
, ͑1͒
where w
m
is the weight of a specimen after moisture condi-
tioning and w
d
is the oven-dry weight after drying at (103
Ϯ2)°C.
In addition to the gravimetric method, there are numer-
ous techniques, which can be used for monitoring the MC of
wood.
2
Relative humidity ͑RH͒ measurement based indirect
methods have been quite widely used in wood industry. The
methods include hygrometers, dew-point sensors, and psy-
chrometers. Humidity probes have also been used to monitor
the RH in the interior of wood. As MC affects the physical
properties of wood, almost the whole electromagnetic spec-
trum has been applied to the direct MC measurement of
wood products though the electrical methods have been most
frequently used.
2–7
Time domain reflectometry based meth-
ods have been used for the measurement of moisture in a
living tree, in which the MC is above the fiber saturation
point ͑FSP͒.
8,9
The MC of the FSP varies between
20%–40%.
2
Measuring the MC of wood by electrical methods is
based on the significant effect of moisture on electrical char-
acteristics, which changes rapidly as a function of moisture if
the MC is below the FSP.
2,10,11
Two frequently applied meth-
ods for measuring the MC of wood are electrical resistivity
and dielectricity measurement. Existing resistance moisture
meters are essentially dc megaohm meters, which are not
fully nondestructive because the pin electrodes must be
driven into the specimen. Surface electrodes can be used in
dielectric moisture meters, which are completely nondestruc-
tive, quick, and easy to use. The lower practical limit for
most electrical meters is of the order of 6–8 wt %, and the
highest limit is about 25%–30%. The measuring range is
adequate for most applications in the MC measurements of
wood products. Moisture meters are usually planned to mea-
sure the average moisture content of a wood specimen
though it is obvious that the moisture gradient ͑MG͒ also has
a huge influence on meter readings. It has been shown that
dielectric moisture meters are predominantly affected by the
surface MC.
12,13
As a hygroscopic material, wood is affected by the at-
mospheric conditions.
2
Thus, the MG is usually involved in
wood, the variation of the MG in wood is dependent on, e.g.,
wood species, porosity, MC, and the humidity and tempera-
ture changes in atmosphere. In natural state, a large MG is
present in wood, the MC difference between heartwood and
sapwood may be over 50% in a living tree. MG is also highly
dependent on the tree species. Accurate MG measurement
would be very valuable in many applications including the
drying of wood, examination of weather exposed timber con-
structions, and in pulpwood freshness ͑surface MG͒ mea-
surement before the mechanical pulp production in the paper
industry.
Today, MG can be measured nondestructively by using
laboratory equipment based on x rays, gamma rays, or
nuclear magnetic resonance ͑NMR͒.
5,14,15
The effect of den-
a͒
Electronic mail: markku.tiitta@uku.fi
REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 73, NUMBER 8 AUGUST 2002
30930034-6748/2002/73(8)/3093/8/$19.00 © 2002 American Institute of Physics