Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
2002-04-30
2004-03-09
Deb, Anjan K. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S521000, C324S683000
Reexamination Certificate
active
06703847
ABSTRACT:
This invention relates to a method of determining the dielectric properties of wood, in particular for purposes of deriving therefrom a measure of the moisture content of the wood, and to means for use in such method.
Definition of Terms
Moisture Content
Defined as the following percentage,
M
.
C
.
=
100
M
w
M
d
M
d
,
where M
w
is the mass off wet sample and M
d
is the mass of the bone-dry sample.
Free water
Water present in the trachea of the wood sample.
Bounded Water
Water bounded to the cell walls of the wood sample.
Fiber Saturation Point (f.s.p.)
The maximum moisture content where all water is absent from the trachea and all remaining water is bounded to the cell-walls.
The typical f.s.p. for softwoods such as Pinus is 30%. For hardwoods, this is typically 30-40%. This however varies from species to species.
BACKGROUND OF THE INVENTION
When drying wood in a wood drying kiln, an end-point moisture content of 5% to 20% is normally required. Traditional methods of measuring the moisture content of wood, whilst reasonably accurate towards the end-point moisture content, become less accurate at higher values of the moisture content. At a moisture content of above 30%, the traditional methods become completely unreliable.
For the proper control of the environment in which wood is dried, for example, in a wood-drying kiln, it is important for the moisture content of the wood to be known accurately while the moisture content is still relatively high, e.g. above 30%. If the moisture content is accurately known at these relatively high values it becomes possible to accelerate the drying process considerably, without causing undue stresses in the wood.
The complexity of wood is easily under estimated. Wood is highly an-isotropic both in anatomy and by its electrical and dielectric properties. It is a complex composition of air, water cell-wall structure, organic materials such as cellulose, lignin and resins, inorganic salts and ion concentrations. The anatomy is comprised of solid cell-wall structures combined with trachea (tangential hollow tubes) which is either filled with water or air depending on the moisture content (m.c.) of the sample. Furthermore, variations within species is remarkably striking regarding ion-content which translates in conductivity and resistive variations. Species-to-species variations in ion content is even more influential and wide species to species changes in conductivity is experienced. In order to measure the dielectrics of such a complex medium, the influences of each of these components need be addressed before valuable and usable measurements and methods can be devised. The measurement of the dielectric properties of wood is particularly and unexpectedly troublesome as reported in detail in the book by Torgovnikov[C]. (The citations herein identified by upper case letters are o the bibliography at the end of the specification.) Not only is the dielectric highly an-isotropic and grain direction dependent, the unexpected temperature behavior of the conductivity of wood is worth mentioning. It would be expected that wood would have similar characteristics as usual carbon based resistors, which displays a decrease in conductivity with increased temperature (increase in resistance respectively). The conductivity of wood in fact does not follow this trend at all, but rather displays the temperature dependence strikingly similar to a semi-conductor i.e. the conductance increases with increasing temperature. It is clear that if this is not taken into account measuring methods of e.g. capacitance of the wood-dielectric will fail at elevated temperatures as large errors will be introduced. This particular fact resulted in several measurement systems to fail in industry for obvious reasons James[R]. To make matters even more troublesome, extremely non-linear anomalies occurs regarding the other relevant dielectric component namely the relative permittivity &egr;
r
also known in layman's terms as the dielectric constant. Since the relative permittivity gives rise to capacitance via the probe geometry and since capacitance will be what is measured, this influence will be discussed in terms of the capacitance but is equally valid for the relative permittivity. Dielectric constant and as a consequence capacitance increases substantially with increase in temperature compared to more homogeneous dielectrics. However, Torgovnikov [C] cites James's results to display the following anomalies. Not only is the relative permittivity and therefore capacitance wildly frequency dependent, it does so in an unexpected manner. Completely dry (bone-dry) wood has a relative permittivity of 4, while water has a relative permittivity of 80. The relative permittivity of water and bone-dry wood is for all purposes frequency independent except for the normal dispersion variations not of relevance here. However, when water and wood is combined i.e. wet wood is measured, we do not obtain the intuitive combined relative permittivity of 84, but instead values are reported by Torgovnikov and James [C] of &egr;
r
=650000 at certain lower frequencies. This is most certainly an anomaly and to date still unexplained and seemingly not challenged however unlikely it seems. Furthermore the relative permittivity and therefore the capacitance increases dramatically with decrease in frequency compared to minimal change in &egr;
r
detected for pure water and bone-dry wood when not in combination over the same frequency range. In addition the loss-tangent tan &dgr;, which is an indication of how lossy a material is in an applied electromagnetic field, also displays curious anomalies generally not expected from dielectric media. Even the most complex composites usually has a loss-tangent, for which each value of loss-tangent only one value of element of composition can be obtained. With wood as dielectric the loss-tangent generally becomes a relation i.e. the loss-tangent plotted against moisture content is that of a bell-curve Torgocvnikov [C] resulting in two moisture contents giving the same loss-tangent reading. This clearly cancels loss-tangent for measurement above f.s.p. in most cases as it results in ambiguity. These complications dwarfs the already significant an-isotropic behavior of &egr;
r
which has different values when the applied electromagnetic field is applied tangential and radially to the wood respectively. The remaining significant behavior of the wood-water relationship is at f.s.p, where free water starts to assemble in the hollow trachea and dissolves salts. These ions then drastically increase the conductivity above f.s.p. to enormous proportions and in effect making any correlation of moisture content above f.s.p. difficult if not impossible. The conductivity of wood therefore becomes an almost constant high value above f.s.p. literally independent of higher moisture contents. The reason for the sudden conductivity increase above f.s.p. is due to the minerals K, P, Al, Fe, Zn, Ca, Mn, Cl, Na and Mg, to name a few which are naturally encountered in wood. The majority of these minerals are dissolved and present in the free water as ions and therefore has a phenomenal influence on conductivity above f.s.p. Below f.s.p. no free water exists and these minerals are then deposited on the cell walls with less influence.
The bounded water (adsorbed water on cell walls) is also changed fundamentally in that the water which is now adsorbed by the cell-walls clearly cannot be rotated easily as a dipole in the applied field. As the wood dries the adsorption to the cell-walls increases giving even more resistance to rotation in the applied electromagnetic field. This results in a curved relationship between &egr;
r
at moisture contents below f.s.p. Above f.s.p. the free water in the hollow trachea are the dominant influence on &egr;
r
and &egr;
r
versus moisture content and the H
2
O molecules as dipole can easily and unrestrictedly be oriented in the applied electromagnetic field. This is the reason
Venter Liebrecht
Viljoen Jacob
Aitken Andrew C.
Deb Anjan K.
Venable LLP
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