United States 

Department of 

Agriculture Dielectric Properties

Forest Service

Products

Forest of Lumber Loads in

Laboratory

Paper

Research a Dry Kiln

FPL 436 

William L. James 
Abstract

The dielectric properties of wood were studied in a

laboratory configuration that simulated typical lumber loads 

in a dry kiln. The transient effect of kiln conditions on the 

lumber stickers was found to influence dielectric properties 

more than had been recognized previously. Other details of 

the dielectric properties apparently depended on interaction 

of transverse moisture gradients with other variables in the 

kiln and in the electrical instruments. 
United States 

Department of 

Agriculture

Forest Service 

Forest

Products

Laboratory1

Research

Paper

FPL 436 

August 1983 

Introduction

Dielectric Properties 

of Lumber Loads in 

a Dry Kiln 

William L. James, Physicist 

Electronic devices are commercially in use that infer the 

moisture conditions of lumber drying in a kiln from 

measurements of dielectric properties of the lumber load. 

These devices respond to the overall capacitive admittance 

(inverse of impedance) of the lumber load, but cannot 

indicate independently the capacitive or conductive 

components of the admittance. 

An empirical study of the dielectric properties of kiln loads 

was made previously, using typical commercial capacitive 

admittance monitors as the measuring instruments2

. The 

objective was to clarify some of the basic physical principles 

that relate to the operation of these devices. In that study, 

some important recurring observations could not be 

explained because the monitors could not discriminate 

between the capacitive and conductive components of the 

admittance.

Understanding these details of the monitor response is 

essential to correct interpretation of monitor data. 

Accordingly, the design of the present study is to measure 

the dielectric properties of wood in configurations and in 

environments that simulate a load of lumber in a dry kiln, 

using laboratory instruments that provide details of the 

dielectric response. 

1

 Maintained at Madison, Wis., in cooperation with the University of 

Wisconsin.

2

 James, William L. and R. S. Boone. 1982. Capacitive in-kiln wood moisture 

content monitors: principles of operation and use. Wood Sci. 14(4):146-164. 

The specific objective is to establish a realistic equivalent 

circuit for a kiln and its load of lumber, and to determine 

how the elements of this equivalent circuit vary with kiln 

conditions, moisture condition of the load (average MC and 

distribution), and measurement parameters (primarily 

frequency). This information should enable more reliable 

inference of the moisture status of lumber as it dries in a

kiln, using capacitive admittance monitors. 
Background Theory and Experimental Design 

The electric circuit traversed by the sensing signal from a

capacitive admittance kiln monitor is from the electrode into 

the load of lumber and back to the monitor. The return path 

includes random capacitance and leakage conductance 

between the lumber and grounded structure of the kiln, and 

then through the ground back to the instrument. The 

impedance of this return path introduces an unknown and 

uncontrolled quantity into the monitor readout. The 

impedance of the actual ground portion of this path would 

be expected to be negligible, so the significant unknown is 

the admittance between the load and ground. 

Clearly, the variable of primary interest is the impedance (or 

its inverse, admittance) of the load of lumber, because it is 

this quantity that reflects changes in moisture content (MC) 

of the lumber. Details of the variation of the dielectric 

properties of wood with MC under laboratory conditions are 

fairly well known3

. But again, in the environment of a dry 

kiln, conditions that influence the apparent dielectric 

behavior of the load are not as well controlled and cannot 

be recognized independently from readings of typical 

capacitive admittance monitors. 

For example, in the previous study,2

 at the beginning of a

typical kiln run, the monitors showed a rapid spike response 

that quickly went off scale, and remained for an hour or 

more before dropping down to on-scale readings again. This 

rapid increase in response was tentatively explained by the 

expected increase in the admittance of the lumber with 

increasing temperature, but both the rate and magnitude of 

the increase were larger than might be expected from a

pure temperature effect. The rate of the increase seemed to 

be appreciably greater than could be expected from the 

actual rate of increase of the temperature of the lumber, 

and the magnitude of the increase seemed excessive 

considering that the effective admittance of the lumber 

should have been limited to that of the fixed capacitance 

between the load of lumber and the overall earth-potential 

mass of the kiln. 

Further, the correspondingly rapid decrease in monitor 

response, 1 or 2 hours into the run, to a relatively constant 

or slowly changing value, could not be explained with 

certainty, and in particular could not be a temperature 

effect.

Another unexplained observation was the rapid response of 

the monitors to certain relatively large changes in kiln 

conditions, particularly to rapid increases in humidity. 

Specifically, the monitors responded faster than could be 

explained by changes in the actual MC of the lumber. 

Understanding these and other monitor characteristics is 

important for correct interpretation of monitor data, and 

improving this understanding is the objective of this study. 

3

 James, William L. 1975. Dielectric properties of wood and hardboard: 

variation with temperature, frequency, moisture content and grain orientation. 

USDA For. Serv. Res. Pap. FPL 245. For. Prod. Lab., Madison, Wis. 

The equivalent circuit of the kiln and its load of lumber 

could be hypothesized in various ways, depending on what 

kiln-load details are selected to be represented by circuit 

elements. I propose that a reasonable and potentially useful 

equivalent circuit would be as shown in figure 1. The first 

elements are a capacitor with a parallel conductor, 

representing the interface between the electrode and the 

kiln load. The parallel conductance could be reduced to near 

zero by electrode insulation. 

The next elements are a capacitor with a series resistor, 

these elements shunted by a parallel conductor; these three 

elements represent the load of lumber. The conductances 

include actual leakage conductance through the load and 

resistive impedance to the current that polarizes the wood, 

as well as the dielectric loss components of the lumber 

capacitance. The capacitance simply represents the total 

polarizability of the lumber load. The load is actually a

complex array of conductors and capacitors composed of 

the individual pieces of lumber and stickers, but lumping 

these into the three basic elements seems to be a

potentially useful simplification. 

The last elements again are a parallel capacitor and 

conductor, representing the admittance between the lumber 

load and the ground-potential parts of the kiln. The 

conductance element would be predominantly leakage 

through the stickers into the truck and rails or kiln floor. 

The only capacitive element that varies substantially with 

frequency is that of the lumber load. The air dielectric 

capacitance between the load and ground would not be 

frequency-dependent because the dielectric constant of air 

is not appreciably frequency-dependent. 

Figure 1.—A proposed equivalent circuit of a dry 

kiln and its load. C1 and R1 are the capacitive 

and conductive components of the connection of 

the electrode to the load; C2 is the capacitance 

of the load; R2 the resistance to the charging 

current in the load, R3 is the leakage 

conductance around the load; C3 and R4 are the 

capacitance and leakage conductance from the 

load to grounded conductors in the kiln structure. 

(ML83 5135) 

2
Experimental Descriptions 

The capacitances of the electrode insulations were 

determined experimentally to vary only slightly with the 

frequencies covered here. It follows that the degree of 

variation of the total capacitance with frequency is a

qualitative measure of the magnitude of the load 

capacitance relative to the other capacitances. Accordingly, 

the frequency effect is a factor in the experimental design 

not only for empirical evaluation of monitor performance, 

but also for determining a realistic equivalent circuit for the 

kiln and its load. 

Similarly, the influence of electrode insulation can be used 

to deduce the relative importance of the capacitive and 

conductive components of the impedance of the woodelectrode interface, and different sticker designs can show 

the relative importance of conductance and capacitance 

components of the admittance between the load and 

grounded kiln structure. 

Measurements of the capacitance and dissipation factor of 

simulated kiln loads were made at frequencies of 0.1, 1, 10, 

and 100 kilohertz (kHz), using four types of electrode and 

three types of specimen support (sticker). Data were 

obtained principally on one species, Douglas-fir, but several 

electrode-sticker combinations were repeated using 

specimens of cottonwood, basswood, or beech. The 

species other than Douglas-fir served to indicate any 

qualitative species effects, and also served as replicates of 

the electrode-sticker combinations. Kiln conditions used 

simulated various phases of commercial kiln-drying practice. 

Specimens

The specimens were 18-inch-long segments of rough-sawn 

green 2 X 4’s, prepared from logs. The green specimens 

were stored wrapped in plastic in a room maintained at 

38°F to retard drying and molding. 

Apparatus

Chamber

The test chamber was a miniature dry kiln (fig. 2) 10 by 12 

by 30 inches inside dimensions. An internal fan and baffles 

provided airflow of 200 to 300 feet per minute over the 

central third of the specimen. Wet- and dry-bulb 

temperatures were controlled by thermistor controllers. Heat 

was supplied by a 500-watt open electric element. Humidity 

was obtained with a peristaltic pump, controlled by the wetbulb temperature, that forced distilled water through a

0.004-inch-diameter orifice. The resulting fine jet of water 

was projected into the heater, where it quickly vaporized. 

The chamber was lined with aluminum foil that was 

electrically grounded. Insulated feed-through connectors 

were provided for thermocouple- and conductance-type 

moisture-meter probes, which were used to monitor 

temperature and moisture gradients in the specimens. 

Dielectrometer

The dielectric properties of the miniature simulated kiln 

loads were measured using a Shering resistancecapacitance bridge, driven by a function generator (sine 

wave), with bridge balances determined using a tuned null 

detector. Connection to the electrodes in contact with the 

specimens was through a 30-inch length of low-capacitance 

coaxial cable. The center conductor of the cable ended in 

an ordinary copper alligator clip to clip onto the electrode. 

The end of the cable was sealed with a high-temperature 

elastomer so the chamber humidity would not condense 

water inside the cable. A decade resistance and 

capacitance box was made as an accessory for the bridge 

to extend its range to large values of capacitance and loss 

tangent (equivalent parallel conductance). 

3
Electrodes

Electrodes were made in four forms (fig. 3) each 3 by 5

inches. This covered the central 3 inches of each specimen, 

in similitude of actual dry kiln practice, with sufficient 

overhang to permit clipping the cable to the electrode. 

Three electrodes were solid aluminum sheet; one was bare, 

one was coated with a layer of rubber adhesive about 0.003 

inch thick, and one was clad with l/Cinch-thick rubber 

foam. The surface of the foam was sealed to prevent 

moisture absorption. The insulating coatings eliminated the 

conductance component from the electrode-load 

admittance, and added two different levels of capacitive 

reactance to the electrode-load inter-face. Theoretically, this 

should facilitate estimation of the effective capacitance of 

the load itself, especially as modified by the influence of 

moisture gradients. 

The other 3- by 5-inch electrode was of aluminum window 

screen which provided electric contact while not interfering 

with drying under the electrode. 

The electrodes were held firmly on the specimen by a small, 

weighted table made from a piece of 1/4- by 3- by 4-inch 

tempered hardboard, with 1/16-inch-diameter by 1 -inch-long 

wooden legs fitted perpendicular to the hardboard near 

each corner. The thin legs held the electrodes firmly in place 

without serious interference with airflow (fig. 4). 

Figure Z.-The model kiln with associated controls and 

electric instruments. (M 150884) 

Figure 3.—Electrodes used in this study were (left to right) 

bare aluminum, aluminum insulated with 0.003-inch rubber 

adhesive, aluminum insulated with 1/4-inch foam rubber, 

and aluminum screen. (M 150887) 

The solid electrodes were distorted slightly to simulate the 

typically imperfect contact between the electrode and 

specimens in actual practice; this permitted some drying 

under the electrode. 

Specimen Supports 

Data were obtained with the specimens supported on 

typical stickers, and also when resting on special supports 

designed either to be electrically conductive or to eliminate 

electric leakage that could occur from moisture condensing 

on the stickers (fig. 5). 

The typical stickers were 4-inch lengths either of a heavily 

resin-impregnated paper laminate with a cross section of 

11/16 by 1-1/4 inches, or of solid natural oak, about 3/4 by 

1-1/4 inches. The oak stickers were kiln dry (less than 8 pct 

MC) at the beginning of each run in which they were used. 

The special low-leakage stickers consisted of a pair of 

1/4- by 1-3/4-inch dowels mounted vertically about 3 inches 

apart in a hardboard base. Each dowel was covered by a

small plastic cup inverted over the dowel and secured using 

a cellulose glue. The specimen rested on two such 

supports, one near each end of the specimen. These 

supports provided a long leakage path (the inverted cups), 

with a small probability of condensation on the cups 

because of their small heat capacity. 

The other special stickers were electrically conducting to 

eliminate the load-ground impedance; the conductance (R4,

in fig. 1) then would be effectively infinite. These were made 

simply by wrapping the solid wood stickers in aluminum foil. 

4
Other Instrumentation 

The specimens were fitted with probes to indicate both the 

temperature and MC profiles across the thickness of the 

specimen. Two copper-constantan thermocouples of No. 30 

wire were located in the middle of a 4-inch face, one about 

1/8 inch below the specimen surface and the other about 

1/2 inch below the specimen surface. Holes for the 

thermocouples were drilled just big enough to accept them; 

the near-surface hole was drilled at an angle of about 5° 

from the specimen surface to provide a fairly long wire 

length from the thermojunction to the surface. 

Thermocouple potentials were read using a portable 

temperature-compensated potentiometer calibrated in 

degrees Fahrenheit. 

Moisture gradients were inferred from a measurement of 

MC, using a conductance-type moisture meter, at the 

specimen surface and about 1/2 inch below the surface of 

the specimen. To do this, three probes were implanted in 

the specimen: one essentially at the center, one to a depth 

of 1/2 inch, and one at the surface. The probes were 16-

gage bare copper wire inserted into snug-fitting holes drilled 

to the appropriate depth. Electric contact was stabilized by 

inserting a small drop of silver-loaded lacquer at the bottom 

of each hole, with a hypodermic syringe, just before pushing 

the wire in place. The silver paint was thinned as needed to 

enable it to be injected. 

The surface probe was mechanically stabilized by shaping it 

to lie on the surface of the specimen with about 1/8 inch of 

the end bent at a right angle so as to bear against the 

surface. The probe was secured to the specimen by staples 

insulated from the specimen and probe by strips of electric 

insulating tape. The contact point was electrically stabilized 

through a small drop of the silver-loaded conductive paint. 

Procedures

The basic procedure was to measure the capacitance and 

effective loss tangent of the simulated kiln loads under 

various kiln conditions and combinations of electrode and 

sticker type. These data were then related to temperature 

and moisture gradients in the specimen and to other 

recognizable variables of the kiln environment. Past 

experience (2) has shown that the most important electric 

variable regarding capacitance kiln monitors is frequency; 

accordingly, data were taken at frequencies of 0.1, 1, 10, 

and 100 kHz at each combination of the other variables 

(such as electrode type, kiln conditions, and elapsed time). 

Two categories of data were obtained: (1) When kiln 

conditions were changing rapidly, such as at startup or at 

step changes in kiln settings, and (2) when kiln conditions 

were constant and moisture and temperature profiles in the 

specimen were changing with time. Typically, both 

categories of data were obtained in each experimental run 

by setting the kiln controls to various conditions and 

observing the specimen response to the transient effects as 

the kiln approached these settings, and then observing the 

specimen response as a function of time after the kiln 

reached constant conditions. 

Figure 4.—A typical specimen, showing moisture 

and temperature probes and means of securing 

the electrodes on the specimen surface. (M150886) 

Figure 5.—Stickers used in this study were (left to right) 

special low-leakage stickers, synthetic stickers, and 

solid-wood stickers. (M 150885) 

When the kiln conditions were changing rapidly, data were 

taken at 5-minute intervals. Even though the readings were 

made very quickly, it still took typically 2 minutes to record 

all the data. Therefore, the relationships between the four 

frequencies, temperature, and moisture are only 

approximate with respect to time. 

5
Results and Discussion 

The results of this study are essentially qualitative, because 

it is the nature of the relationships between the variables, 

not the magnitudes of the data, that is important. The 

magnitudes of the data depend on arbitrary details such as 

the size of the load and kiln structure. The primary results 

identify some factors that relate dielectric response of the 

load to its MC and moisture distribution. 

For simplicity, the capacitance and conductance data 

presented are not corrected for the capacitance and loss 

tangent of the measuring instruments except when the 

corrected data are specifically needed, as these are 

constants that do not affect the form of the response data. 

Original data in the form of loss tangent or dissipation 

factor are converted to equivalent shunt (or parallel) 

conductance and expressed in microsiemens (μS).

Preliminary experiments showed that the response of 

dielectric properties to the variables studied was essentially 

the same with either one or two specimens simulating the 

kiln load, so data were taken using a single specimen for 

each load. 

The data plots (figs. 6 through 16) are each from a single 

kiln run. The general form of each plot and the order of 

magnitude of the data were verified by replications using 

the other species; these replications also suggested that 

species effects would be minor. Data from these other 

species are not presented here when they offer no 

important additional information. 

Apparatus Constants and General Results 

The fixed capacitances and loss tangents (or equivalent 

parallel conductances) of the measuring apparatus with 

various electrodes and configurations were measured 

(table 1). 

Theoretically, specimens wrapped in aluminum foil should 

show the maximum possible capacitance to ground of any 

specimen of that size and shape resting on the same 

supports (stickers). Specimens of lesser conductivity should 

show smaller effective capacitance, so the foil-wrapped 

specimen defines an upper limit to the load-ground 

capacitance in this study. 

It is obvious (table 1) from the substantial parallel 

conductances and the variation of capacitance with 

frequency that data obtained when the foilwrapped 

specimen was supported on the laminated paper stickers 

were distorted by leakage conductance of the stickers, even 

though they had been baked for several hours. 

Measurements of actual direct current resistance of the 

stickers at the end of several kiln runs revealed that the 

laminated paper stickers were affected to a greater degree 

and more persistently (as to leakage conductance) by the 

hot, humid kiln conditions than were the solid oak stickers. 

This apparently is a permanent characteristic of these 

synthetic stickers, and could be a significant source of 

variability in the data from capacitive kiln monitors where 

such stickers are used. 

Table 1 .—Some constant capacitance and loss tangent values1

of the experimental apparatus 

Loss tangent 

Configuration Frequency Capacitance (or equivalent 

parallel (kHz) (pF) conductance)

(μS)

Measuring system, 

no electrode 213 < 0.001 

Specimen wrapped 

in aluminum foil 

on dry, laminated 

paper stickers 0.1 311 (0.008)

1 287 (0.083)

10 262 (0.36)

100 256 (1.0)

on dry, solid 

oak stickers 256 < 0.001 

on special lowleak stickers 251 < 0.001 

Any 3- by 5-inch 

electrode on 

1-inch-thick

Styrofoam 221 < 0.001 

3- by 5-inch electrode, 

thin insulation, 

on grounded 

metal plate 0.1 390 (0.005)

100 380 (0.008)

Peak values 

in hot, 

humid kiln 0.1 680 (0.62)

1 580 (0.84)

100 500 (12)

3- by 5-inch electrode, 

thick insulation, 

on grounded 

metal plate 240 < 0.001 

1

 At room conditions except where noted. Where no reference to 

frequency is made, data did not vary with frequency. 

Corresponding data using the ordinary solid wood or the 

special low-leakage stickers appear valid, however. The 

maximum load-to-ground capacitances in the present 

configuration for solid wood stickers are 43 picofarads (pF) 

(256 pF minus the system capacitance of 213 pF) and 38 

pF (251-213) for the low-leakage stickers. 

Similarly, the net series capacitances of the two electrode 

insulations are about 170 pF for the thin and 27 pF for the 

thick insulation. 

6
These data are useful for interpreting the results of this 

study and inferring details of the equivalent circuit of the kiln 

and its load. For example, in kiln runs in this study, the 

differences in total capacitances observed using the two 

different thicknesses of electrode insulation did not reflect 

the 143-pF difference in series capacitance of the two 

insulations. Differences in total capacitance were much less, 

which establishes the limiting effect of other series 

capacitances, particularly the load-ground capacitance. The 

form of the response curves was essentially the same for 

either type of insulation, so only data obtained using the 

thin insulation are shown in this report. 

On the other hand, these data in conjunction with data from 

the kiln runs reveal some of the complexity of the 

composite dielectric properties of the kiln and its load. For 

example, it was common, especially when using insulated 

electrodes, for the total measured capacitance of the 

electrode-load-kiln to exceed the capacitance calculated for 

the two- or three-series capacitances in the equivalent 

circuit, even in later stages of drying. This could be 

explained only by significant conductances shunting one or 

two of the capacitances, but I did not expect the 

persistence of these conductances as the kiln went to drier 

conditions. In commercial practice, the existence of these 

conductances even in the later stages of drying, where 

capacitance naturally is the principal mode of admittance, 

could be a source of variability in readings of an 

admittance-type monitor. This is consistent with the 

somewhat erratic progression of conductances illustrated in 

figure 16, which simulates late stages in a drying program 

typical of commercial practice. 

Temperature and MC Gradients 

Figure 6 shows the typical moisture and temperature 

distributions for green Douglas-fir specimens exposed to the 

kiln conditions plotted. The response of all Douglas-fir 

specimens to these conditions was similar; the differences 

observed were not sufficient to change the characteristics 

of the dielectric responses. 

The apparent small decrease and momentary recovery in 

surface MC in the first 20 minutes of a typical run were 

deduced from readings of a conductance-type moisture 

meter. These readings were made when the specimen 

temperature was changing rapidly, so accurate temperature 

corrections were difficult. For this reason, this momentary 

variation in surface MC may be spurious. 

Figure 6.—Typical gradients of temperature and 

MC that formed in Douglas-fir heartwood 

specimens under the kiln conditions in this study, 

in which the wet-bulb depression was increased 

after about 2 h of drying. The apparent minimum 

in surface MC at about 5 min may be due to 

imperfect temperature corrections, and therefore 

may not be real. (ML83 5136) 

Bare Electrodes 

Bare Electrode and Solid Stickers 

Figures 7 and 8 illustrate the transient capacitance and 

conductance of a green kiln load for which the electrode 

was bare metal and the stickers were typical solid wood or 

resin-impregnated paper. The rapid increase in capacitance 

at the lower frequencies, as the kiln temperature and 

humidity increase, indicates a large increase in the electric 

polarization within the wood. The influence of frequency is 

in agreement with data on the variation of the dielectric 

constant of wood with frequency,3

 which is strong evidence 

that it is indeed the capacitance (polarizability) of the wood 

that causes this large total capacitance. Also, it is very 

likely that this is the same phenomenon as observed at 

startup of actual kiln runs using commercial-type monitors.2

If the equivalent circuit (fig. 1) is valid, it is clear that for the 

total capacitance to have values in the thousands of pF, 

arising from the very large dielectric constant of the wet 

lumber load, both the electrode-wood admittance and the 

load-ground admittance would have to be very large. With a

bare metal electrode on green lumber, the electrode-wood 

admittance would indeed be large because of the large 

conductance, but the load-ground admittance would be 

limited to the vector sum of sticker conductance and the 

susceptance of the load-to-ground capacitance. This latter 

capacitance was shown earlier to be about 43 and 38 pF 

for the solid wood and special stickers, respectively. Kiln 

conditions could increase these capacitances appreciably 

only by increasing the effective dielectric constant of the 

stickers, which, because of the relatively small area of the 

stickers, would remain a limited contribution to the loadground admittance. 

7
Figure 7.—Capacitance of a load of Douglas-fir 

heartwood at various frequencies, using a bare 

metal electrode and solid wood stickers, as 

conditions changed according to fig. 6. (ML83 

5138)

On the other hand, large load-ground admittance could be 

provided by leakage conductance across the stickers if 

moisture condenses on them. The data in figure 8 are 

consistent with this hypotheses: The initial values of 

conductance plotted in figure 8 (shown at 1 min elapsed 

time-this is actually zero elapsed time, but the log plot 

cannot show zero) are the equivalent parallel conductances 

associated with the loss tangents of the kiln and its load at 

the four frequencies, and when the stickers are still cool 

and dry. These conductances are roughly proportional to 

the frequencies, because the loss tangents vary only from 

about 0.2 at 0.1 kHz to 0.04 at 100 kHz, a factor of 5.3

 But 

as the kiln heats up, with both the dry- and wet-bulb 

temperatures increasing rapidly, the conductances increase 

greatly and, more important, approach peak values that are 

nearly independent of frequency. This frequency invariance 

identifies these conductances as real, not equivalent 

dielectric loss. This requires that both the stickers and the 

load of lumber be moist and therefore relatively highly 

conductive.

In the run represented in figures 7 and 8, and in subsequent 

runs, the capacitance and conductance reach peak values 

at about the same time as the surface MC also shows a

small peak. As noted earlier, this peak in surface MC is not 

absolutely established, but the simultaneity of the three 

peaks is consistent with the assumption that they result 

from condensation of kiln humidification on the still-cool kiln 

load.

Bare Electrode and Low-Leakage Stickers 

The role of the stickers in the previous situation is indicated 

further by the data plotted in figure 9, where both 

capacitance and conductance data are plotted on the same 

graph. These data were obtained with the specimen 

supported on special stickers designed to minimize 

condensation and leakage conductance. With these 

stickers, the capacitance increase is small and, in fact, at 

1 kHz or greater is essentially zero. The corresponding 

increase in total conductance is an order of magnitude less 

than that for solid stickers. It follows then that the large 

increase in total admittance typically observed with 

capacitive monitors as the kiln starts is due to moisture 

condensation on the stickers and other elements in the path 

from the load to ground. This condensation occurs early in 

the kiln schedule or at any other time that kiln 

humidification increases so rapidly that parts of the kiln or 

load are momentarily cooler than the dewpoint inside the 

kiln.

Figure B.-Conductance of a load of Douglas-fir 

heartwood at various frequencies, using a bare 

metal electrode and solid wood stickers, as 

conditions changed according to fig. 6. (ML83 

5139)

It was hypothesized earlier2 that the spike increase in 

admittance at startup of the kiln resulted from the effect of 

temperature on dielectric properties, in conjunction with a

surface phenomenon at the electrode-wood interface. The 

surface hypothesis was that the conductance between 

electrode and wood would decrease quickly as the surface 

dried, with the capacitance decreasing later as the dry 

surface layer deepened. The fact that the conductance and 

capacitance peak at about the same time shows that this 

hypothesis is not valid, and confirms the involvement of the 

stickers to a degree not recognized earlier. 

8
Figure 9.—Capacitance and conductance of a

Douglas-fir heartwood load at various 

frequencies, with a bare metal electrode and 

special lowleakage stickers, as conditions 

changed according to fig. 6. (ML83 5140) 

Bare Electrode, Metal Stickers 

The marked difference in data obtained with solid stickers 

and with low-leakage stickers confirms the significant 

influence of sticker design. It would seem that the opposite 

extreme in sticker design, electrically conducting stickers, 

should result in data that maintain, from time zero, values 

at least as large as the peak values observed with solid, 

nonconducting stickers. To prove this, runs were made on 

Douglas-fir and basswood, with the bare electrode and solid 

stickers wrapped in aluminum foil to make them electrically 

conductive (figs. 10 and 11, respectively). 

The data for Douglas-fir (fig. 10) are essentially the same as 

corresponding data obtained with solid nonconducting 

stickers (figs. 7 and 8). In particular, the total conductance 

even when using electrically conducting stickers increases 

rapidly in response to the rapid kiln humidification at 

startup. This indicates that the kiln humidity not only 

induces leakage paths around the stickers but also 

simultaneously greatly increases the admittance of the 

specimen material. The specimen material itself reacts to 

the kiln humidity in this case because it is Douglas-fir 

heartwood and has therefore a moderately low initial MC— 

somewhat less than 30 percent. By contrast, the data for 

green basswood (fig. 11), show very large values of 

admittance without any kiln humidification, because of the 

much greater initial MC of the basswood. For this material, 

kiln humidification would have little effect because the 

specimen material is already nearly saturated with water. 

The small increase in conductance and capacitance in this 

run is probably a temperature effect. 

These runs, using conducting stickers, emphasize that 

superficial moisture condensing on the specimens has a

strong effect on the total capacitive admittance of the kiln 

load, at least when the initial MC of the load is not 

appreciably greater than fiber saturation. The superficial 

moisture resulting from high kiln humidities would be 

expected to increase strongly the conductance denoted by 

R3 (fig. 1), but the data in figure 10 show that the 

capacitance C2 may be strongly increased as well. 

Insulated Electrodes 

There was no important difference between results from 

using thin or thick electrode insulation, so only data for thin 

insulation are shown. 

Figure 12 shows the startup transient peak of capacitance 

when using an electrode with thin insulation and with the 

load supported on solid stickers. Comparing this with the 

corresponding data for a bare electrode (fig. 7) reveals 

some interesting relationships. At the highest frequency, 

100 kHz, the electrode insulation has no effect on the total 

capacitance. This shows that the reactance of the insulating 

film is negligible compared to the impedance of the kiln and 

its load, so the insulated electrode is equivalent to the bare 

electrode. The impedance of the kiln plus load is a complex 

combination of reactances and resistances, including 

leakage across the stickers. At 100 kHz, however, the total 

capacitance of the electrode, load, and kiln is nearly 

independent of the leakage conductance across the stickers 

(as indicated by the constant value with time in figs. 7 and 

12) and, in fact, is roughly equal to the capacitance of a

specimen wrapped in metal foil resting on the same stickers 

and located in the same place in the kiln (table 1). This 

latter capacitance was measured at 256 pF (including 

apparatus capacitance). It follows from this that at 100 kHz 

the green specimen experiences negligible polarization and 

acts essentially as a conductor. For a bare electrode this is 

true not only at 100 kHz but also at 10 kHz (fig. 7). With an 

electrode with thin insulation, however, there is at 10 kHz 

an increase in total capacitance when leakage conductance 

across the stickers increases (fig. 12). The electrode 

insulation effectively introduces a series capacitance, which 

from first principles cannot increase the total capacitance of 

a network. This apparent contradiction is resolved when the 

associated conductances are considered (figs. 8 and 13); 

the electrode insulation reduces the total admittance of the 

network by sharply reducing the conductance, and the 

apparent increase in capacitance is probably due only to the 

increase in the fraction of the total impedance that is due to 

capacitive reactance. 

As the frequency is reduced further, the relative contribution 

of capacitive reactance to the total impedance continues to 

increase, enhanced by the strongly increasing polarizability 

of moist wood as the frequency decreases.3

 The total 

admittance of the electrode-load-kiln circuit of course 

continually decreases as the frequency decreases when 

using an insulated electrode, and the total admittance 

cannot be as large as that observed when using a bare 

electrode.

9
Figure 10.—Capacitance and conductance of a load of Douglas-fir heartwood 

at various frequencies, with a bare metal electrode and metal stickers, as 

conditions changed essentially according to fig. 6, but with the slightly 

different history of surface MC shown. (ML83 5141) 

The conductances observed with use of an insulated 

electrode (fig. 13) depend strongly on frequency, because 

they are largely the equivalent parallel conductances of the 

dielectric losses in the system, which are frequency 

dependent. In addition, the electrode insulation itself 

displayed equivalent leakage conductances in the hot, moist 

kiln environment of about 1 μS at 0.1 kHz to about 10 μS

at 100 kHz. These conductances would add vectorially to 

the susceptance of the insulation and thereby increase the 

admittance of the electrode. 

Insulated Electrode and Low-Leakage 

Stickers

The dielectric properties of the kiln and load when using an 

insulated electrode and special low-leakage conductance 

stickers are shown in figure 14 where both conductance 

and capacitance are plotted. 

Before the kiln is started, the solid stickers are dry and act 

the same as the special stickers. This is reflected in the 

rough similarity in initial values of conductance when the 

two types of stickers are used (see figs. 13 and 14). When 

the near-saturation conditions in the kiln are established at 

startup, even the low-leakage stickers show some increase 

in conductance, except at the highest frequency. The overall 

response of the conductance to frequency and time 

(essentially condition of leakage conductance) is similar in 

form to that when using an insulated electrode and solid 

stickers, but is an order of magnitude smaller. The 

corresponding similarity between conductances as a

function of frequency and time when using a bare electrode 

with either solid or special stickers, again with a magnitude 

difference between sticker types of about a factor of 10 

(compare figs. 8 and 9) shows that the leakage 

conductances in either case are real, essentially frequency 

invariant. The frequency effect on the overall response 

when using the insulated electrode results because the 

component of the conductance that is associated with 

dielectric loss becomes relatively more prominent when the 

direct electrode-specimen conductance is removed by the 

electrode insulation. 

10
Figure 11.—Capacitance and conductance of a load of green basswood at 

various frequencies, with a bare metal electrode and metal stickers, as the 

dry bulb temperature changed according to fig. 6, but with no kiln 

humidification. (ML83 5142) 

Screen Electrode, Solid Wood Stickers 

Figure 15 shows dielectric data from use of a screen 

electrode and solid stickers. These results are similar to 

those observed for a bare sheet metal electrode, except 

that the response to changing the kiln conditions seems to 

be somewhat more pronounced. At 300 minutes into the 

run, the wet-bulb depression was increased from 10° to 

25° C. The resulting accelerated drying is apparent from 

most data, although plotting limitations prevent showing this 

clearly for the conductances at the two lower frequencies. 

Also, the capacitance change at the two higher frequencies 

is not clearly definable. 

At 1,200 minutes, the wet-bulb depression was decreased 

to 5°C and the resulting increase in surface moisture is 

apparent again in all data except the capacitance at the two 

higher frequencies. 

Figure 12.—Capacitance of a load of Douglas-fir heartwood 

at various frequencies, using an electrode with thin 

insulation and solid wood stickers, as conditions changed 

according to fig. 6. (ML83 5137) 

11
Figure 13.—Conductance of a load of Douglas-fir 

heartwood at various frequencies, using an 

electrode with thin insulation and solid wood 

stickers, as conditions changed according to fig. 

6. (ML83 5143) 

These data indicate the strong influence of surface 

conditions on the dielectric properties that relate to kiln 

monitors. In this run, as in other runs where kiln 

humidification was used to minimize drying gradients, the 

conductance or dielectric loss appeared to be more 

responsive to drying progress than was the capacitance 

when the rate of moisture change was relatively large. 

No Kiln Humidification 

Figure 16 shows data on capacitance and conductance of a

load, using a bare metal electrode and dry, solid wood 

stickers, as it dried in a kiln at 70°C and no humidification. 

The wet-bulb depression was about 25°C after the kiln 

temperature was reached (about 10 min after the run 

began). The specimen was Douglas-fir heartwood, with an 

initial MC slightly less than 30 percent as indicated by a

conductance-type moisture meter. 

The capacitance data were corrected for the fixed 

capacitance of the measuring apparatus to show more 

clearly the relatively small changes in capacitance of the kiln 

load as it dried. The plotted values are the total capacitance 

of the kiln and load corresponding to the equivalent circuit 

(fig. 1) and therefore include the series load-ground 

capacitance. The total capacitance is limited to the value of 

this fixed, relatively small series capacitance, so the 

changes in the actual capacitance of the load alone would 

be larger than the capacitance data plotted in figure 16. The 

variation in capacitance indicated by a capacitive 

admittance monitor as the load dries theoretically would be 

substantially greater if the effective load-ground capacitance 

could be increased or eliminated. Achieving this in practice 

apparently would not be a trivial exercise, because the 

obvious solution of using electrically conducting stickers to 

shunt the load-ground capacitance produced conflicting 

results. The data for metal stickers (fig. 10) are not greatly 

different from those for wood stickers (fig. 15) but, where 

metal stickers were used (fig. 11) and drying was quite 

rapid because of low kiln humidity, the response of 

electrical properties to drying progress was particularly 

strong. Apparently, the use of conducting stickers in 

conjunction with capacitive kiln monitors needs further 

study.

Figure 14.—Conductance and capacitance of a

load of Douglas-fir heartwood, using an electrode 

with thin insulation and special low-leakage 

stickers, as conditions changed according to fig. 

6. Note that the conductance scale is one-tenth 

as large as in fig. 13. (ML83 5144) 

12
Figure 15.—Conductance and capacitance of a load of Douglas-fir heartwood 

at various frequencies, with a bare aluminum screen electrode and solid 

wood stickers, as conditions changed essentially according to fig. 6, but the 

wet- bulb depression was increased from 10° to 25°C at 300 min and 

decreased to 5°C at 1,200 min. (ML83 5145) 

Conductance was erratic and unresponsive to the drying 

process after the surface MC had been reduced to about 

18 percent (fig. 16). In this case, where no kiln 

humidification was used, a surface MC of 15 percent was 

achieved within 10 minutes. Only the conductance at 100 

kHz fell monotonically as the drying continued to the end 

conditions. The early stages of rapid drying are confounded 

by corresponding rapid changes in temperature. The 

increase in conductance that occurred at the three lower 

frequencies between 10 and 100 minutes may be explained 

by the increasing temperature of the specimen material. 

Temperature equilibrium was reached about 200 minutes 

into the run, so temperature changes cannot explain the 

continual increase in conductance at 10 kHz as the load 

dried. In short, these conductance data are not consistent 

with basic dielectric properties of wood, nor are they 

correlated with any clearly defined factor in the drying 

process. The conductance values are very small, so the 

variations observed are due most likely to random effects, 

rounding errors, or similar sources of variation. Again, the 

measurable electric quantity best correlated with MC in the 

later stages of drying is capacitance. 

13
Figure 16.—Conductance and capacitance of a load of Douglas-fir heartwood 

at various frequencies, with a bare metal electrode and solid wood stickers, 

as the dry-bulb temperature changed according to fig. 6, but with no kiln 

humidification. The resulting MC profiles are also plotted. Note that the 

conductance and capacitance scales are one-tenth as large as in fig. 15. 

(ML83 5146) 

14
Summary and Conclusions 

A study was made of the dielectric properties of wood in 

the configuration and environment of a lumber dry kiln to 

obtain information for improving the design and application 

of capacitance dry kiln monitors. Variation in apparent 

capacitance and loss tangent (equivalent parallel 

conductance) of the kiln loads was measured in response to 

both changing and steady kiln conditions, and as a function 

of time, sensing frequency, MC of the load, and design of 

the electrodes and stickers. From observed relationships 

between these variables, some details of the equivalent 

circuit of the kiln load were inferred. 

When the kiln is started, dewpoints are established that 

may be higher than the temperature of the load and 

stickers, so moisture condenses on these surfaces. This 

results in a large temporary increase in the admittance of 

the load. Under these conditions, capacitance and 

conductance reach maximum values at the same time, 

which indicates that this phenomenon is due to properties 

of the load as a whole, not just an electrode-wood contact 

effect. The peak conductances when a bare metal electrode 

is used are nearly independent of frequency, so are real as 

opposed to equivalent dielectric loss. The peaks are greatly 

reduced when using special stickers designed to minimize 

condensation and electric leakage, confirming that the effect 

is dependent upon leakage conductance across the 

stickers. Experiments using metal stickers demonstrated 

that condensation on the load itself also can be important 

when the initial MC of the load is not much greater than 

fiber saturation. 

As drying continues and the leakage conductances become 

small, the load-to-ground admittance becomes 

predominantly capacitive, although sometimes a surprising 

degree of overall conductances may persist. The loadground capacitance can be substantially smaller than the 

effective capacitance of the load, and as these capacitances 

are in series, relatively large changes in the load 

capacitance may appear small because of the limiting effect 

of the small load-ground capacitance. If the load-ground 

admittance could be increased, the overall sensitivity of the 

system possibly could be increased, but it appears that this 

would complicate application of the method to kiln 

monitoring. For example, the simple expedient of using 

electrically conducting stickers to increase the load-ground 

admittance was found to produce inconsistent results. 

When the electrode was insulated from the load, so the 

electrode-load admittance was essentially pure capacitive 

susceptance, the observed total conductances were strong 

functions of frequency under all kiln conditions. This 

confirms that these effective conductances were due mostly 

to dielectric loss, not real conductances as observed under 

high-humidity kiln conditions with bare metal electrodes. 

When drying had progressed to where leakage conductance 

across the stickers was small, the observed conductances 

were strongly frequency dependent for all electrode types. 

The conductance seemed to be a somewhat more sensitive 

indicator of drying progress in the first stages of drying, 

with the capacitance being somewhat more sensitive near 

the end of drying. 

The results of this study are consistent with a simple 

assumed equivalent circuit (fig. 1), but do not preclude other 

forms. In particular, representing the load by three elements 

(two resistors and a capacitor) is a recognized 

oversimplification because of the complex structure of the 

load, but even this simple approximation provides adequate 

decription of the observed dielectric properties of the 

electrode-load-kiln assembly. 

Most variability in the response of a capacitive admittance 

kiln monitor seems to be associated with the conductances 

that shunt the entire load and that shunt the load-ground 

capacitance. These conductances may be affected by 

changing kiln conditions to a degree that confounds 

somewhat the effect of changing MC of the load. Physically, 

these conductances involve the stickers used to assemble 

the load. Specially designed stickers could possibly reduce 

this source of variability, but probably only with 

considerable cost and complication. 

15
Glossary

Admittance — A property of a circuit that permits current to 

flow through it when a voltage is applied; numerically 

equal to the current flow with 1 volt* applied. It is the 

reciprocal of impedance and the vector sum of 

conductance and susceptance. 

Capacitance — The property of a circuit element that 

permits it to store electric energy in the form of 

polarization.

Conductance — The property of a circuit element that 

permits it to carry or transport electric charge. 

Numerically equal to the current flow with 1 volt* applied 

across the conductor. 

Equivalent parallel conductance — The conductance in 

parallel with a pure capacitor that results in the same 

energy dissipation as a given practical capacitor of the 

same capacitance. 

Impedance — The reciprocal of admittance; the vector sum 

of resistance and reactance. 

Reactance — The property of a capacitor or inductor that 

impedes current flow through it. Numerically equal to the 

voltage* required to produce unit current flow. 

Resistance — The reciprocal of conductance. 

Susceptance — The reciprocal of reactance. 

*The voltage here is the effective or RMS alternating 

current voltage. 

2.0-16-8/83

16
U.S. Forest Products Laboratory 

Dielectric Properties of Lumber Loads in a Dry Kiln, by William L. James, 

Madison, Wis., FPL, 1983. 

16 p. (USDA For. Serv. Research Paper FPL 436). 

The dielectric properties of wood were studied in a laboratory configuration that 

simulated typical lumber loads in a dry kiln. The transient effect of kiln conditions 

on the lumber stickers was found to influence dielectric properties more than had 

been recognized previously. Other details of the dielectric properties apparently 

depended on interaction of transverse moisture gradients with other variables in 

the kiln and in the electric equipment. 

Keywords: Dielectric properties, kiln control, kiln drying, sticker design, moisture 

gradients, moisture measurement.