ELECTRICAL CHARACTERISTICS OF CORN, WHEAT,

AND SOYA IN THE 1 - 200 MHz RANGE

R. N. Jones

H. E. Bussey

W. E. Little

R. F. Metzker

Electromagnetic Fields Division

Center for Electronics and Electrical Engineering

National Bureau of Standards

Boulder, Colorado 80303

1978

C. 2
1
NBSIR 78-897

ELECTRICAL CHARACTERISTICS OF CORN, WHEAT,

AND SOYA IN THE 1 - 200 MHz RANGE

R. N. Jones

H. E. Bussey

W. E. Little

R. F. Metzker

Electromagnetic Fields Division

Center for Electronics and Electrical Engineering

National Bureau of Standards

Boulder, Colorado 80303

October 1978

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

NATIONAL BUREAU

OF STANDARDS

LIBRARY

OCT 5 1981 r c J o.cc- &a

7*

CONTENTS

Page

1. INTRODUCTION AND BACKGROUND 1 1.1 U.S.D.A. Approved Method 1 1.2 Dielectric Parameters as Related to Moisture Content 2

2. SOURCES OF VARIATION IN MOISTURE MEASUREMENT 2

3. SOURCES OF ERROR IN CAPACITANCE METHODS 2 3.1 Correlation of Percent Moisture Content with Dielectric Constant 2 3.2 The Effect of Other Parameters on Dielectric Constant 2 3.3 Difficulty in Measuring the Dielectric Constant of Particulate Material 2

4. SCOPE 3

5. TECHNIQUE AND APPARATUS 3 6. MEASUREMENT THEORY 8

7. HIGHER FREQUENCY MEASUREMENT THEORY AND COMPUTATION PROCEDURE 8

8. ACCURACY OF e AND TAN <5 MEASUREMENTS 10

9. UNSETTLED AND SETTLED VALUES 10

10. DATA AND DISCUSSION 15 10.1 e' and Tan 5 as a Function of Percent Moisture Content 15 10.2 Density Versus Dielectric Constant 41 10.3 Temperature Coefficient of e' and Tan 6 42 10.4 Effect of Sample Holder Size and Kernel Dimensions 43

10.5 Sample Size (Fill Level in Holder) — 44

10. 6 Contact Impedance 45

10.7 Drop Test of Corn, Wheat, and Soya (Standard Deviation) 45

10.8 Overburden Effect on e' and Tan 6 45

10.9 Glass Bead Experiments 45

11. CONCLUSIONS ^6

12. ACKNOWLEDGMENT 49

13. BIBLIOGRAPHY 50

iii

ELECTRICAL CHARACTERISTICS OF CORN, WHEAT, AND SOYA

IN THE 1 - 200 MHz RANGE

R. N. Jones, H. E. Bussey, W. E. Little, R. F. Metzker

A set of coaxial sample holders together with a measurement and data

reduction technique has been developed and applied to the study of the

dielectric properties (e*=e'-je") of wheat, corn, and soya over the 1 to 200 MHz range. Particular attention was given to the behavior of the

dielectric properties as a function of percent moisture content, frequency, and packing density. Data were also taken to evaluate the

dependence of dielectric properties on temperature and sample holder

configuration. Some study was also devoted to the correlation between

dielectric constant (e 1 ) , loss factor (e"), loss tangent (e"/e')» and

percent moisture content. Particular emphasis was devoted to a study

of high moisture corn (up to 40%)

.

Key words: Dielectric properties; grain; loss tangent; moisture.

1. INTRODUCTION AND BACKGROUND

One of the most important parameters in the handling, storing, marketing, and use of

grain is the moisture content. At every phase, including seeding and harvest, through

marketing and transport to its final use as human and animal food, the moisture content

needs to be repeatedly and accurately monitored. There are many methods available for

measuring moisture content. In reference [1]* is a compilation of ten methods, together

with the advantages and disadvantages of each. Even if the same grain sample were tested

by each method and the measurement procedure were carried out perfectly, it is probable

that there would be a range of disagreement among the results. This is caused by the fact

that the various methods do not measure precisely the same phenomena and that the moisture

characteristics are sometimes altered by the measurement processes. There is general agreement that moisture in grain is present in several forms, and complete understanding of the

processes of moisture gain and moisture loss is lacking [1]. Thus, the measurement of

moisture content is sometimes difficult, and no single ideal method has been developed.

Furthermore, there is considerable disagreement as to which methods should be used for a specific purpose, and a variety of methods and instruments are in use in the United States

and throughout the world

.

Present methods of grain moisture measurement are unsatisfactory because they are inaccurate, inconvenient, or slow. A desirable method would be a quick one capable of

achieving accuracies of ± 0.5 percent or better without the necessity for special preparation of the grain.

1 . 1 U.S.D.A. Approved Method

A system widely used in the United States and the one adopted by the U.S. Department

of Agriculture utilizes an electric capacitance type of moisture meter which is referred,

through a series of calibration charts, to specific grain samples whose percentage moisture

content has been determined by the oven-dry method [2] . The electric meters are used because

they provide a rapid method of measurement that is easily adapted to field use. In essence,

the electrical method uses a capacitance cell to provide an indication of percent moisture

content based upon the dielectric constant of grain. The electric meter reading, which is some function of the dielectric constant, is converted to moisture content by means of

U.S.D.A. calibration charts.

^Figures in brackets indicate the literature references at the end of this paper.
1.2 Dielectric Parameters as Related to Moisture Content

It should be clear that the present work is a straight-forward measurement of dielectric

constant and loss versus moisture content. The packing density of the grain samples was measured at the same time as the dielectric constant. A literature search did not reveal a previous extensive effort of this type.

Most commercial electrical moisture meters give a reading approximately proportional to dielectric constant with some allowance for density. This reading may be combined with calibration tables (or more correctly, correlation tables) which convert the reading into a percentage moisture content. This being so, tables can also be made that convert the fundamental physical quantities, dielectric constant and/or loss, into moisture content. The

present research is a start in examining the usefulness of absolute dielectric measurements

per se for agricultural moisture measurements.

2. SOURCES OF VARIATION IN MOISTURE MEASUREMENT

Some of the difficulty in achieving accurate measurements of moisture content is

attributable to the grain itself and therefore is not controllable by the measurement method.

Included in this category is the fact that moisture content is not a stable condition, but one which will vary with other conditions, such as temperature and humidity. There is

evidence that different hybrid types of grain may give varying results both on the electric

meters and by the oven-dry process, and it is well known that grain samples from the same

stand taken at the same time will show variations in moisture content. Thus the complexity

of an unstable material is added to the variations existing between different measurement

methods.

3. SOURCES OF ERROR IN CAPACITANCE METHODS

When relying on the capacitance method to measure percent moisture content, several sources of error are encountered. They are mentioned here because the work to be described

is directed toward their evaluation and possible elimination or reduction.

3 . 1 Correlation of Percent Moisture Content with Dielectric Constant

The principle of capacitance-type meters is predicated on the assumption that dielectric

constant is a reliable indicator of moisture content. This assumption is generally correct,

but other factors such as packing density, temperature, and measurement frequency affect the

relationship. This report will provide data resulting from experiments designed to study the

influence of these parameters. In addition to the influencing factors considered here, there are probably others such as protein and starch content, hybrid variety, growing conditions

and others; however, they are not included in the work described.

3 . 2 The Effect of Other Parameters on Dielectric Constant

The scope of the work described in this report includes experiments which show the

effect of several other parameters upon the observed dielectric constant of various grain

samples. These other parameters include measurement frequency, density of the grain being

measured, temperature, sample holder configuration, and overburden.

3 . 3 Difficulty in Measuring the Dielectric Constant of Particulate Material

The measurement of a homogeneous material, such as a liquid or a solid, is relatively

simple compared to the problems encountered in measuring the dielectric constant of an inhomogeneous sample. In measuring the dielectric constant of grain we are dealing with a mixture of grain and air with the proportions of these two constituents being unknown. As this proportion changes, the observed dielectric constant will also change. This is not only true for a grain and air mixture, but it is true for any other mixture, such as air

and glass beads or for a liquid-solid mixture. This is a serious problem which is detrimental to any dielectric measurement approach and one which will impose a limit on the

(attainable) accuracy. This problem is of course related to the variation of dielectric

constant with density as mentioned in section 3.2 above, but is probably more complex when

the solid matter is itself inhomogeneous as is the case with grain or other organic materials.

2
4. SCOPE

The work described in this report has been done by the Electromagnetic Fields Division

of the National Bureau of Standards in cooperation with the Grain Standardization Laboratory

of the U.S. Department of Agriculture in Beltsville, Maryland. The thrust of the work has

been the study of the dielectric properties of grain as a function of several variables including moisture content, frequency, and temperature. Because the electric capacitance type

of moisture meter is a fast, inexpensive and convenient means for percent moisture content

determination and because it is widely used, it is desirable to study the factors which

contribute to measurement inaccuracy in the hope of bringing about improvement. Of particular concern is the measurement of high moisture corn above about 26 percent. This is

brought about by changes in farming methods and the use of pickersheller equipment to harvest the grain in the very early stages of maturity when moisture percentages are large.

A clear cut solution to the moisture measurement problem was not the expected result of

this work; indeed, that would be far too ambitious. But, recognizing the need for a data

base which will provide quantitative information related to the problem of measuring the

moisture content of grain by electrical or electromagnetic methods, we have developed measurement techniques and accumulated a quantity of data which may prove to be of value as work continues. Presented here are only some initial findings, and it is recognized that

there is more to be done and improvements which could be made.

The data presented is for wheat, corn, and soya over the frequency range from 1 to 200

MHz, and the specific parameter investigated was the apparent or measured complex dielectric

constant as a function of several influencing parameters including frequency, percentage

moisture content, packing density, and temperature. Capacitance variation with sample

holder configuration is also examined briefly. An attempt has been made to correlate percent moisture content with dielectric characteristics, and it is apparent that this relationship is affected by several other variables. Data are needed to show quantitatively what

effect the other variables have. This work is a beginning in the development of such a data

base.

5. TECHNIQUE AND APPARATUS

Three coaxial sample holders were constructed for use in the study of grain as well as other materials; one each for 5.04 cm(2"), 7.62 cm(3"), and 10.16 cm(4") diameters of the

outer cylinder. To provide adequate space for large kernel grains, the sample holding

portions of the holders have a 75 ohm characteristic impedance (see table 1) instead of the

usual 50 ohms where the space between center and outer electrodes would have been smaller.

Figure 1 is a photograph of the 10.16 cm (4") holder along with a sample dropping mechanism

and a depth gauge for measuring the depth to which the holder was filled with grain. Construction details of the sample holders are shown in figure 2. At low frequencies (less than 1 MHz) where an electrostatic approximation is adequate,

the sample holder may be regarded as a coaxial capacitor with capacitive sections 1 through

4 (see figure 2) connected in parallel. In addition, changes in diameter of the tubing

and/or the center conductor must be represented by shunt capacitors at the changes [3]. At higher frequencies the holder must be treated as a coaxial transmission line terminating in

an open circuit. The exact location of the open circuit is somewhat beyond the end of the

center conductor [3,4] as indicated in figure 2. Alternatively, a shunt capacitor may be

used exactly at the end of the center conductor. We used the former representation.

The open circuited holder is convenient because inserting the sample consists simply in

pouring the sample into the open end. Also, such a holder works at any low frequency and at higher frequencies if the criterion of equation (12) is obeyed. The sample space, section 1 in figure 2, extends beyond the end of the center conductor, as will be discussed.

Table 1. Sample holder characteristics

Holder

Diameter

Overall

Length

*

Z of Grain

Sample Section

Length of outer

Tube, Section 1. Center

Length

Conductor

(Section 1)

5.08 cm(2")

7.62 cm(3")

10.16 cm(4")

17.0 cm (6. 16.8 cm (6, 15.6 cm(6,

.68")

.60")

.13")

7 5 ohms

75 ohms

75 ohms

13.3 cm(5.25")

11.4 cm(4.495")

12.6 cm(4.962")

8.176

4.214

3.386

cm(3.219")

cm(1.659")

cm(1.333")

*Z represents

o

r the characteristic impedance of Section 1 when empty.

3
4
FIGURE 2. CROSS SECTION OF DIELECTRIC SAMPLE HOLDER

SHOWING COAXIAL SECTIONS 1, 2, 3, AND 4.

5
6
7
6. MEASUREMENT THEORY

At low frequencies, 1 MHz or below, we may use electrostatics; then the admittance

of a capacitive circuit such as a sample holder containing a dielectric material can be

expressed as

Y = G + io}£*C (1)

s where G = the effective conductance between the electrodes including the series resistance of

the structure,

oo = angular frequency,

C = capacitance in vacuum between electrodes of the sample holder,

and e* = e' - je", the complex dielectric constant of the dielectric material in the sample

holder. Using the complex form of the dielectric constant, expression (1) becomes

Y = G + jioC(e' - je") (2)

which reduces to

Y

g = (G + coe"C) + jwe'C . (3)

To determine the complex dielectric constant of the material in the sample holder, the

admittance of the empty (air filled) sample holder is subtracted. Assuming the structure

losses are unchanged from the filled condition and the complex dielectric constant of air

is 1, the empty admittance is

Y = G + jwC . (4)

e

J

This gives, for the admittance difference between the full and empty conditions

AY = Y

g - Yg = AG + jwAC = e"ojC + jo)C(e'-l) . (5)

Equating real and imaginary parts from each side of the equation gives

AC

e

1 = 1 + — for dielectric constant, (6)

and e" = for loss factor. (7) wC

e" By definition, the loss tangent of the material in the sample holder is tan 6 = —, . (8)

Thus the dielectric properties of a material, such as grain, can be determined from the

difference between the admittances of the full and empty sample holder.

At low frequencies where the dimensions of the sample holder are sufficiently small

with respect to a quarter wavelength, the computation procedure is simple, and the dielectric

properties are easily obtained by simply substituting measured admittance values for the

empty and full sample holder into eqs (6) and (7). However, this assumption leads

to large errors as the measurement frequency is increased and a modified computation

procedure is used in the 1 to 200 MHz range. This procedure is described in the following

section.

7. HIGHER FREQUENCY MEASUREMENT THEORY AND COMPUTATION PROCEDURE

At higher frequencies the concept of simple coaxial capacitors does not apply. The

measurable quantity at the interface with the measuring instrument (bridge), or at any

transverse plane, is the admittance, reference [5], defined as the ratio of longitudinal

current to transverse voltage for the TEM mode of a coaxial line. This definition reduces

to the admittance used in equations (1) and (5) as the frequency approaches zero.

In order to calculate the dielectric constant and loss of the sample from the measured

admittance, Y , we must obtain the theoretical admittance as a function of the propagation m r constant, the distance, d, and the TEM characteristic admittance,

8
Y = 2it (e e*/y ^/fcnCb/a) , (9)

c o o where b and a are radii of the tube and the center conductor respectively, u and e are the

permeability and permittivity of vacuum, and e was defined for (1).

The theoretical admittance at the plane of measurement is predicted by transforming

the admittance from the plane of the open circuit of section 1 to the bottom of section 4. The relative admittance, y , at the top (output) end of any section of line is transformed

to the input (generator) end of the same section by the equation

y in

= (1 + y coth Yd)/(cothyd + y ) , (10)

where y^ is the relative input admittance, y is the complex propagation constant in the

section, and d is the length of the section. The term relative admittance means that the

absolute admittance has been divided by the characteristic admittance, eq (9). Both Y , o * h * c and y = (-u> zu e^e ) , depend upon the complex permittivity e of the section, w is the

radian frequency.

Equation (10) is applied four times to transform the admittance from the open circuit

to the measurement plane. The absolute admittance at a dielectric interface is continuous

except when changes in radii a and b occur. Then the shunt admittance of the step

* Y = io) C e must be added. C is given in reference [31. With air in section 1 step J step step

the measured admittance Y is used to check that all quantities, e.g., section lengths,

dielectric constants, Y , Y , are correct, and that the known e =1 results for air.

c step

To evaluate e of an unknown dielectric in section 1 we use eq (10) with yg = 0 at the

plane of the effective open circuit, d = d^ + d', where d' is the added distance [4]

beyond the end of the center conductor to the effective open circuit, and y is obtained

from measured Y^ of section 4 by transforming backward from the bridge measurement plane to the input of section 1, using eq (10) in the known invariant sections, 4, 3, 2. The transcendental equation to be solved using (10) again is

y, = Y' /Y = tanh Y (d, + d'), (11) m m L I where the complex unknown e appears both in Y„ and in y. Y ' is the admittance that would

L ' m

have been measured at the input of section 1. To find a solution we start with an estimated

*

e in (11) and iterate by Newton's method until the equation is satisfied. The equation has

multiple roots but at low frequencies, with grain as the dielectric, only the lowest root is

reasonable

.

A correction is required in d^ of eq (11) for the spherically tapered end. Only the

flatly truncated center conductor is treated in references [3] and [4] . We assume that

the terminating capacitance [3] and the equivalent extension [4] are still just the same as

for the flat ended conductor, and then we subtract a small quantity from the true physical

length of section 1. The amount subtracted is approximately 37% of the radius of the center

conductor. This method was found to be valid experimentally (over the required frequency

range), i.e., dielectric measurements of known materials, e.g., air and carbon tetrachloride, were correct and were the same as when a flat ended termination was used.

The electric field which extends beyond the end of section 1 interacts with the

dielectric sample and contributes to the measured admittance Y . This is correctly taken m

into account in the above method. Not only does y contain e' but also the extended distance d

1 in d depends on e ' . d' was adjusted for the value of e' during the Newton

iterations. A correction for the loss e" was not used in d'. More work would be required

to find the effect of e", and this remains to be done.

9
Finally, since the field extends beyond the center termination, we need to calculate

how far the grain sample should extend. Theoretically it should extend to infinity.

Practically, if the hollow tube is well below cutoff, i.e.,

e'« [.387c/(bf)] 2

, (12)

where f is the frequency in hertz, b is the radius of the hollow tubing, and c is the velocity of light in the same length units, then the sample needs to extend beyond the center

conductor only a distance of 1.5 b. The evanescent field in the tube decays as exp (-2 . 405z/b)

where z is distance beyond the end of the center conductor. To a good approximation, the

material in the region beyond z = 1 . 5 b is unimportant if eq (12) is true.

Table 1 gives some dimensions of the three holders. The impedance Z may be obtained

from the inverse of in eq (9) .

8. ACCURACY OF e' AND TAN 6 MEASUREMENTS

To verify the measurement data from the sample holders, standard liquids were measured.

These included carbon tetrachloride (CCl^) and chlorobenzene (C^H^Cl) . These liquids have

for their e' values 2.232 and 5.656 respectively at 23°C according to Buckley and Maryott

[8] . Measurements from 1 kHz to 200 MHz yielded results which agreed with accepted values

to within ± 0.3 percent.

An assessment of the error in e' due to the error in bridge admittance was carried out

by computing the changes in e' resulting from inputting known admittance increments into

the computer program. This yielded very nearly a one-to-one correspondence between the

percentage capacitance error and the resulting percentage change in the value arrived at for e

' -1 (see eq 6)

.

A characteristic of the high frequency Twin-T bridge [6] is that the sensitivity is

somewhat reduced near the low end of its frequency range and circuit residual immittances

introduce uncertainties at the upper end of its frequency range. As a result, the capacitance uncertainty is considered to be ± 0.2 percent at 5 MHz and also in the region above

100 MHz, with uncertainties of ± 0.1 percent in the intermediate region. The Twin-T

bridge used for the 1 MHz measurements [7] has an uncertainty of ± 0.1 percent.

Tan 6 values are generally not measurable to the same accuracy, especially as the

value of tan 6 becomes smaller. In general the tan 5 values of the samples ranged from 0.1

to 1. Over this region, for the bridges used, the uncertainties for tan 6 would vary from

± 0.15 percent to 1 percent in the frequency range where the bridge uncertainty was ± 0.1

percent, and from ± 0.3 percent to 2 percent in the frequency range where the bridge

uncertainty was ± 0.2 percent. In the foregoing statement the larger uncertainties apply

to the smaller values of tan 6 and vice versa.

As a generalized conclusion to the foregoing uncertainty discussion, it is believed

that the absolute uncertainty of the values for e ' and tan 6 given in the data are approxipercent for tan 6 from 10 to 100 MHz, and ± 2 percent for

The uncertainty of the tan 6 values at 1 MHz is approximately ± 1 percent. These error bounds of course do not include the nonrepeatability due

to packing density variations.

mately ± 0.5 percent for e', ± 1 tan 6 at 5 MHz and above 100 MHz

9. UNSETTLED AND SETTLED VALUES

To examine the effect of density (or settling) upon the values for dielectric constant

and loss tangent, a procedure was adopted for adding the grain to the sample holder and

also for settling the grain in a consistent manner. The center object in figure 1 is a dropping container arranged to set atop the sample holder (shown on the left) . Inside the

dropping container is a collapsible metal diaphragm which, upon release, allows the grain

sample to drop into the sample holder. All values in this report that are designated

"unsettled" were taken immediately after the sample was dropped into the holder in this manner

.

10
After the grain was dropped into the holder and an admittance measurement was taken,

the grain-filled holder was removed from the bridge and subjected to a vibration procedure

which was performed in two stages. The first stage consisted of bumping the side of the

sample holder with the heel of the operator's hand one hundred times in quick succession.

This was followed by the application of a small vibrator to the side of the sample holder

for a period of 30 seconds. Following this procedure the grain-filled sample holder was reconnected to the bridge, and a second admittance measurement was taken. Values in the

report designated "settled" were obtained after this procedure was performed.

The wooden plug with attached scale (shown on the right) was used to measure the level

of the grain in the holder, both before and after the settling procedure. All admittance

values were taken with the wooden plug removed from the holder. It is noted here that the

presence of the plug in the holder made no significant difference in the bridge admittance

reading because the grain samples were made large enough so that the electric field region

inside the holder was entirely filled by grain. In a later portion of this report the

effect of adding weight to the grain surface is discussed.

The settling procedure just described was repeated several times on various samples of corn, wheat, and soya, and after the first settling procedure, repeated vibrating did not

bring about significant additional changes in the observed values of e' and tan 6.

A logarithmic mixing rule by Lichtenecker , which is discussed in Von Hippel [9], is

used to determine the permittivity e ' of a mixture from e{ and e£ and the volume fractions

Ql and &2 of the components:

log e' = 0! log e] + e 2 log e£ . (13)

In this instance one of the components is air which essentially has a relative permittivity

of 1. Unfortunately, there is no convenient or accurate method of determining the volume

ratios of the air and the grain so this rule is of no direct benefit, in determining the

permittivity of the grain. However, it is assumed that the dielectric properties of an average kernel of grain, at a given time are constant. Therefore, from equation (9) we can write for the relationship between dropped and settled conditions

log log c'

s

d s (14)

where P^ and P

g are the dropped and settled densities, respectively. This being true, it

would be possible to arrive at a consistent value for the permittivity provided the packing

density of the mixture were measured, and in this way errors due to density variations could

be removed

.

To test the foregoing hypothesis, data from measurements at 1 and 30 MHz on wheat,

corn, and soya at various percentage moisture levels were examined. These data are shown

in tables 2 through 7. In the tables the values shown under the "%" heading are the percentage moisture levels as determined by the oven-dry process. In all cases, in tables 2 through 7, the settled values were derived after one execution of the settling procedure.

As previously mentioned the values P^ and P2 were the density values determined before and

after settling. The percentages shown in the right-hand column of tables 2 through 7 are uncertain by an estimated ± 1.5. This uncertainty includes a measurement uncertainty of

± 0.5 percent in the measured values for e{ and and an uncertainty of ± 0.25 cm (0.1 inch)

in the depth gauge measurement of the fill level in the sample holder. The depth measurement

was hampered by the uneven grain surface, but to reduce error the gauge plug was rotated

through 360° while setting on the grain to smooth and level the surface. If the relationship of eq (10) were perfectly valid, the numbers in the right-hand columns of tables 2 through 7 would all have been zero. The fact that the variations from zero are fairly

evenly distributed in both magnitude and direction shows no definite departure from eq (10)

.

However, it does not present a reliable method for density corrections when measuring an individual sample. The reason for the one very large value in table 4 may be due to an erroneous data point.

11
Another aspect of the question of settling is whether a settling procedure, such as the one described, improves the ability to reproduce a given sample density. The data in

tables 8, 9, and 10 pertain to this question. Here the average density values and their

standard deviations are tabulated for all of the settling operations performed. The three

tables give data for corn, wheat, and soya. Overall, the standard deviations are smaller

for the density of the dropped grain than for the settled grain, and the dropping procedure,

in addition to being more convenient, is probably superior in obtaining repeatable measure- ments of dielectric constant.

log ej log e'

2 Table 2.

1 p

l

' 2 P

2

2 -C

Grain y

f S h

C

l - x 100

Corn 17. 1 1. 121 1. 164 3. 8 ± 1.6

Corn 23. 3 1. 409 1. 391 -1. 3 Corn 26. 0 1. 378 1. 395 1. 2 All Data

Corn 29. 0 1. 820 1. 745 -4. 1 taken at 1 MHz Corn 30. 9 1. 902 1. 850 -2. 7 Corn 34. 0 2. 140 2. 130 -0. 47

Corn 8. 5 Q. 762 0. 769 0. 92

Corn 10. 4 0. 804 0. 834 3. 7 Corn 12. 9 0. 91Q 0. 919 0. 9 All Data

Corn 15. 8 1. 032 1. 028 -0. 3 taken at 1 MHz

Corn 19. 1 1. 213 1. 216 0. 25 Corn 22. 4 1. 409 1. 397 -0. 85

Corn 25. 7 1. 604 1. 549 -3. 4 Data taken in

Corn 28. 6 1. 727 1. 740 0. 7 reverse order

Corn 30. 9 1. 943 1. 909 -1. 7 (bottom to top)

Corn 33. 7 2. 126 2. 086 -1. 9 as grain dried

Corn 39. 0 2. 481 2. 469 -0. 4 from 39% to 8.5%

Table 3. log e'

Grain

log e

i2

1 C

2 P

2

% C_2

c -c -jr-^ x 100

°1

Wheat 10.7 0.769 0.768 -0.13

Wheat 13.4 0.827 0.849 2.6

Wheat 14.3 0.866 0.859 -0.81

Wheat 16.0 Q.939 0.955 1.7 All Data

Wheat 16.6 0.973 0.970 -0.31 taken at Wheat 18.4 1.012 0.980 -3.3

Wheat 18.5 1.089 1.096 0.64

Wheat 21.2 1.194 1.202 0.67

12
Table 4. loe el log

2 C

l

" P

Grain

C

2

"

7,

P

2

f c,

1 Co

2 Vc

i

c n x

1 100

Soya 10. 4 0. 842 0.835 -0..84 Soya 10. 4 0. 755 0.831 10

Soya 11. 4 0. 908 0.937 3.,1 Soya 11. 4 0. 917 0.948 3..3 Soya 13. 1 0. 951 0.992 4..3 All Data

Soya 13. 9 1. 098 1.104 0..55 taken at 1 MHz

Soya 14.-5 1. 032 1.078 4,.5 Soya 14. 8 i. 116 1.149 3,.0 Soya 16. 1 l. 200 1.250 4,.2 Soya 16. 9 l. 278 1.310 2,.5

log ej

Table 5. log el

Grain

-

; ^ =

°A

P

2 C_2

C 2" C

1

c x 100

Corn 17. 7 0. 983 0.974 -0. 91

Corn 23. 3 1. 082 1.100 1. 7 Corn 26. 0 1. 113 1.155 3..8 All Data

Corn 29. 0 1. 203 1.250 3..9 taken at Corn 30. 9 1. 380 1.332 -3. 5 Corn 34. 0 1. 356 1.377 1..5

Corn 8. 5 0. 679 0.677 .29 Corn 10. 4 0. 728 0.741 1..8 Corn 12. 9 0. 814 0.834 2..5 Corn 15. 8 0. 908 0.899 -0..99 Corn 19. 1 0. 999 1.004 0.,50

Corn 22..4 1. 071 1.077 0..56 All Data

Corn 25. 7 1. 164 1.154 -0..86 taken at Corn 28. 6 1. 221 1.231 0..82 Corn 30. 9 1. 302 1.303 0..08 Corn 33..7 1. 355 1.343 -0,.89 Corn 39..0 1. 524 1.528 0..26

Table 6.

x 100

Wheat 10.,7 0. 684 0.,687 0..44 Wheat 13..4 0..757 0..764 0.,92

Wheat 14..3 0..765 0. 775 1..3 Wheat 16,.0 0..834 0..843 1..1 All Data

Wheat 16..6 0..868 0..857 -1..3 taken at Wheat 18,.4 0..890 0.,891 0..11 Wheat 18 .5 0..889 0..891 0..22 Wheat 21..2 0..991 0..985 -0..61

13
IdUlc / . log

C

l

" P,

1

log £

' °2 " P.

/

12 Vc

i 1UU

Grain X C

2 cl x

Soya XUin a . H nu . 7m/ Ul n 7? ^ oJ . /.

. H Soya 1Uin / . 4 nU ah 9 . Do

z

un 7 1 t . / 1

J

A c; So ya ii / n 7/1 ^ n 77

H ., u Soya ii / n 7/, ^ n 7 Rn nu

,

Soya XJit i. X nU . 7QA / y 0 U . OUo i

j.

.

Q

. J AIT n a f- a Soya IT Q n a^T n 87T 9

"3

. J haVon at- "30 MT4'7 Soya 14.5 0.832 0.858 3,.1 Soya 14.8 0.886 0.919 3..7 Soya 16.1 0.918 0.955 4..0 Soya 16.9 0.956 1.003 4.,9 •

Table 8. The effect of settling on the density of corn.

% No. of Drops Average Density in g/cc ± Standard Deviation

Grain Moisture & Settlings Dropped Settled

Corn 17 .7 8 0 .668 + 0 .0075 0 .715 + 0 .010

Corn 23 .3 8 0 .608 + 0 .0028 0 .678 + 0 .0028

Corn 26 .0 8 0 .638 + 0 .0053 0 .690 + 0 .010

Corn 29 .0 8 0 .522 + 0 .0072 0 .578 + 0 .010

Corn 30 .9 8 0 .510 + 0 .0065 0 .574 + 0 .011

Corn 34 .0 8 0 .510 + 0 .0059 0 .570 + 0 .0057

Corn 8 .5 7 0 .611 + 0 .0032 0 .637 + 0 .0088

Corn 10 .4 8 0 .616 + 0 .0077 0 .640 + 0 .0058

Corn 12 .9 8 0 .619 + 0 .0057 0 .648 + 0 .0097

Corn 15 .8 7 0 .608 + 0 .0081 0 .651 + 0 .0054

Corn 19 .1 7 0 .587 + 0 .0051 0 .639 + 0 .0039

Corn 22 .4 7 0 .556 + 0 .0049 0 .617 + 0 .0037

Corn 25 .7 7 0 .534 + 0 .0035 0 .599 + 0 .0047

Corn 28 .6 8 0 .517 + 0 .0058 0 .580 + 0 .0037

Corn 30 .9 8 0 .510 + 0 .0063 0 .570 + 0 .0061

Corn 33 .7 8 0 .503 + 0 .0064 0 .564 + 0 .0087

Corn 39 .0 8 0 .504 + 0 .0036 0 .569 + 0 .0038

Table 9. The effect of settling; on the packirig density of wheat.

% No . of Drops Averag;e Density in g/cc + Standard Deviat:

Grain Moisture & Settlings Dropped Settled

Wheat 10. 7 6 Q. 828 + Q. 0015 Q. 889 + 0. 0044

Wheat 13. 4 5 0, 809, + 0. 0035 0. 862 + 0. 0093

Wheat 14. 3 6 0. 798 0. 0090 0. 842 + 0. 0018

Wheat 16. 0 5 0. 777 + 0. 0046 0. 832 + 0. 0018

Wheat 16. 6 6 0. 721 + 0. 0032 0. 789 + 0. 0035

Wheat 18. 4 5 0. 745 + 0. 0068 0. 818 + 0. 0036

Wheat 18. 5 6 0. 731 + 0. 0134 0. 781 + 0. 0122

Wheat 21. 2 5 0. 692 + 0. 0043 0. 762 + 0. 0078

14
Table 10. The effect of settling on the packing density of soya.

No. of Drops Average Density in g/cc ± Standard Deviation

Grain MoT Qf~11T~<^ tit OCUI—1 Lll&O Dropped Settled

Soya 10.4 8 Q. 710 + Q, 0104 0.755 ± 0..0081

S o ya 10.4 8 0.723 + Q..0038 0.756 ± 0

.

.0047

Soya 11.4 8 0. 716 + 0..0060 0.750 ± 0,.0086

Soya 11.4 8 0.721 + 0,.0043 0.763 ± 0..0078

Soya 13.1 8 0, 700 + 0,.0025 0.740 ± 0..0104

Soya 13.9 8 Q.703 + Q,.0049 0.746 ± 0..0094

Soya 14.5 8 0.714 + 0 .0057 0.754 ± 0..0056

Soya 14.8 8 0.681 + 0,.0072 0.728 ± 0,.0104

Soya 16.1 8 0.677 + 0 .0049 0.720 ± 0,.0052

Soya 16.9 8 0.673 + 0 .0035 0.720 ± 0..0081

10. DATA AND DISCUSSION

To provide an accessible quantity of data for future use, the tables of Appendix 1 containing essentially all of the data accumulated during this phase of the project are

provided. The data are for corn, wheat, and soya at various moisture levels, and values of

e', tan 6 and e" are given for both the dropped and settled condition at various frequencies,

including 1, 5, 10, 30, 50, 100, 150, and 200 MHz.

It should be noted that the grain samples were cycled through the dropping and settling

procedure at each frequency at which data are given. Had an automated measurement system

been available covering this frequency range and providing comparable accuracy and resolution, rehandling of the grain between each measurement frequency would not have been

necessary and much time could have been saved. Although this would have been beneficial in one respect, it would likely have shown a different set of results than appears here because

the data from all of the measurement frequencies would have been taken after one dropping

and settling procedure so that the plots of a single run would have shown less inconsistency.

The data tabulated here do not provide smooth curves because of the nonrepeatability of

the dielectric parameters from one time the grain is put into the holder to another.

Nevertheless, the value of an automated impedance measurement system would have been

appreciable because much more data could have been accumulated in the same time period.

An estimate of the uncertainty in the moisture content (by oven drying) of the various

grain samples is not given. Upon examination of the various plots included on the succeeding

pages, it is evident that errors in measurement of moisture content may be present. This

is discussed in greater detail in 10.1.

10 . 1 e' and Tan 6 as a Function of Percent Moisture Content

A particular effort was made with respect to high moisture corn, and these results

will be discussed first. Experiments were carried out in two phases. In one phase a sizeable sample of corn was gathered shortly before harvest from a field in central Colorado. When gathered, this corn was at approximately 40 percent. Measurements from this corn sample were made every few days at a number of frequencies as the corn was allowed to air dry down to a moisture level of 8.5 percent. Figures 5 and 6 show e ' for eleven moisture levels between 39 percent and 8.5 percent for frequencies of 1, 5, 10, 30, and 200 MHz

for both dropped and settled samples. Although the plots are neither linear nor log normal,

they do appear to yield a fairly smooth curve over this moisture range.

As mentioned earlier, the standard for grain moisture measurement is an oven-dry

process which has been developed to its present state by the U.S.D.A. Grain Standardization

Laboratory in Beltsville, Maryland. All of the percentage moisture levels given in this

report were determined by the Beltsville laboratory prior to shipment to the NBS labs in

Boulder, Colorado, where the dielectric measurements were performed. For shipment the

grain samples were sealed in 0.015 cm (0.006") thick plastic bags to retard moisture gain

or loss. Once received at Boulder, the grain was weighed out in appropriate sample sizes

15
DIELECTRIC CONSTANT OF DROPPED CORN

VS

PERCENT MOISTURE CONTENT AT VARIOUS FREQUENCIES

ALL DATA ON THIS GRAPH TAKEN FROM THE SAME CORN SAMPLE

FROM JOE SMITH FARM, NIWOT, COLO., NOV. 1976

PERCENT MOISTURE CONTENT

Figure 5.

16
DIELECTRIC CONSTANT OF SETTLED CORN

VS

PERCENT MOISTURE CONTENT AT VARIOUS FREQUENCIES

ALL DATA ON THIS GRAPH TAKEN FROM THE SAME CORN SAMPLE

FROM JOE SMITH FARM, NIWOT, COLO., NOV. 1976

PERCENT MOISTURE CONTENT

Figure 6.

17
and the dielectric characteristics measured as quickly as possible. The dielectric measuring

procedure often required a day or more to complete, and during this time the grain samples were kept in sealed glass jars and under refrigeration when not actually being subjected to the measurement process. In spite of these precautions against change in sample moisture

content following the oven-dry determination, some changes may have occurred during shipment

of the samples from one laboratory to the other. The shipping time duration was generally

from two to five days; and, while in transit through the mails, they could have been subjected

to some fairly wide temperature excursions. Future experiments should include precautionary

procedures which would preclude errors in the values finally used for percent moisture

content. The advisability of having the oven-dry facility located at the same place as the

electrical measurement facilities seems obvious. This would avoid the need for shipping,

packing, extra handling, and time lapse between the two experiments which may have resulted

in some errors in the final data.

A similar experiment was carried out using six corn samples supplied by the U.S.D.A.

These samples came from random locations around the country and were of various hybrid

varieties. Figures 7 and 8 show e

' as a function of moisture content for the same five

frequencies as before, but there is much less tendency for the results to follow a predictable pattern or behavior law. There is probably very little that can be definitely con- cluded from these relatively brief experiments, except that different hybrid characteristics

or growing conditions add an extra dimension of uncertainty to the dielectric behavior of

the grain. Figures 9 through 12 show similar plots for randomly gathered samples of wheat

and soya. These also exhibited some inconsistent behavior of e' as a function of percent

moisture content.

Figures 13 through 28 are plots of loss tangent and loss factor which correspond to the dielectric constant data in figures 5 through 12. In general this data supports previous observations by other researchers indicating that grain coming from different locations does not demonstrate the consistent dielectric constant versus percent moisture

characteristics. This brings about the need for local control and is currently being done

by the U.S.D.A.

A notable phenomenon in the loss tangent data is the crossover of the frequency plots

in the 10 to 20 percent moisture region, which is noticeable in the corn and wheat data.

This would make it imperative that all moisture testing be done at the same frequency if

loss tangents were to be employed as the indicating parameter.

18
DIELECTRIC CONSTANT OF DROPPED CORN

VS

PERCENT MOISTURE CONTENT AT VARIOUS FREQUENCIES

CORN TAKEN FROM MISCELLANEOUS LOCATIONS IN U.S.A.

20

15 20 25 30 35

PERCENT MOISTURE CONTENT

Figure 7.

19
DIELECTRIC CONSTANT OF SETTLED CORN

VS

PERCENT MOISTURE CONTENT AT VARIOUS FREQUENCIES

CORN TAKEN FROM MISCELLANEOUS LOCATIONS IN U.S.A.

40

V

A

O

20

10

8

1 MHz

10 MHz

30 MHz

100 MHz

200 MHz

15 25 30

PERCENT MOISTURE CONTENT

Figure 8,

20
DIELECTRIC CONSTANT OF DROPPED WHEAT

VS

PERCENT MOISTURE CONTENT AT VARIOUS FREQUENCIES

WHEAT SAMPLES FROM

VARIOUS PARTS OF U.S.A.

MHz

V - 10 MHz

30 MHz

G 100 MHz

200 MHz

PERCENT MOISTURE CONTENT

Figure 9.

21
DIELECTRIC CONSTANT OF SETTLED WHEAT

VS

PERCENT MOISTURE CONTENT AT VARIOUS FREQUENCIES

WHEAT SAMPLES FROM

VARIOUS LOCATIONS

e' 6

4

—

i

1 r-

- 1 1

: -4

—_ -

1 —

—— - — —

—_ P

-

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i 4

j<7

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fc \t

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@ 1 MHz

10 MHz

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100

200

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{ —

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PERCENT MOISTURE CONTENT

Figure 10.

22
v —

o

1 MHz

10 MHz

30 MHz

100 MHz

200 MHz

10 12 14 16 13 20

PERCENT MOISTURE CONTENT

Figure 11.

23
DIELECTRIC CONSTANT OF SETTLED SOYA

VS

PERCENT MOISTURE CONTENT AT VARIOUS FREQUENCIES

SOYA SAMPLES FROM VARIOUS LOCATIONS

6

5

4

-Mill

— -|—

-1— • l MHz — 7

k

in mm-?

t 30 MHz

O

1 1 ?nn MH7

- } —

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3 . U I I LI 1 I I I I i I I I

,

I I II I M I U I I 1 -J. I -l I M M M I, 10 12 14 16 18

PERCENT MOISTURE CONTENT

Figure 12.

24
LOSS TANGENT OF DROPPED CORN

VS PERCENT MOISTURE CONTENT AT VARIOUS FREQUENCIES

CORN SAMPLES FROM JOE SMITH FARM NIWOT, COLO. NOV. 1976

5 10 15 20 25 30 35 40

PERCENT MOISTURE CONTENT

Figure 13.

25
LOSS TANGENT OF SETTLED CORN

PERCEN1 MOISTURE CONTENT AT VARIOUS FREQUENCIES

CORN SAMPLES FROM JOE SMITH FARM, NlWUf, COLO. NUV. 1976

PERCENT MOISTURE CONTENT

Figure 14.

2 b
LOSS TANGENT OF DROPPED COKN

VS

PERCENT MOISTURE CUiMihNi AT VARIOUS KREQUENU'ES

COKN SAMPLES EROM MISCELLANEOUS LOCATIONS IN U.S.A.

1 =»i

I AN 6

.b

.3

.01

.07

.06

EBSEEfi3EEE£E&±

trr 33 we

lb 20 25 30

PERCENT MOiSlUKE CUNItNT

35

Figure 15,

11
LOSS TANGENT OF SETTLED CORN

VS

PERCENT MOISTURE CONTENT AT VARIOUS FREQUENCIES

CORN SAMPLES FROM MISCELLANEOUS LOCATIONS IN U.S.A.

TAN 6

20 25 30

PERCENT MOISTURE CONTENT

Figure 16,

28
LOSS TANGENT OF DROPPED WHEAT

VS

PERCENT MOISTURE CONTENT AT VARIOUS FREQUENCIES

WHEAT SAMPLES FROM VARIOUS

LOCATIONS THROUGHOUT U.S.A.

.6

.4

.06

— ;;::!— i : : : 1 • • 1

•A

o

1 MHz

10 MHz

30 MHz

100 MHz

cnn Mu-r

-

1 4 ..

1 V

I / m V

t'"H—i—

-

- ...

:

'

1

V

'

; ;

[* Ii 1—-J 1

'

1 1 1 1 10 15 20 25 30

PERCENT MOISTURE CONTENT

Figure 17.

29
LOSS TflNGFNT OF SFTTI FO WHEAT

VS

PERCENT MOT STIJRF CONTFNT AT VARIOUS FREQUENCIES

WHEAT SAMPLES FROM VARIOUS

I OCATTONS THROUGHOUT U.S.A.

1*=

.!____. .J. JLL

Lj_i_U J_.

A

G

n

1 MM 7 ~

1 — 10 MH7 ___J_

— - 30 MH7 — 100 MH?

JLi_iJ_U

TAN <S

10 IS 20

PERCENT MOISTURE CONTFNT

Figure 18.

30
Figure 19.

31
LOSS TANGENT OF SETTLED SOYA

VS

PERCENT MOISTURE CONTENT AT VARIOUS FREQUENCIES

SOYA SAMPLES FROM VARIOUS LOCATIONS THROUGHOUT U.S.A.

10 11 12 13 14 .15 16 17 PERCENT MOISTURE CONTENT

Figure 20.

32
LOSS FACTOR OF DROPPED CORN

VS

PERCENT MOISTURE CONTENT AT VARIOUS FREQUENCIES

CORN SAMPLE FROM JOE SMITH FARM, NIWOT, COLO. NOV. 1976

PERCENT MOISTURE CONTENT
LOSS FACTOR f)F SETTLED CORN

VS

PFRf.FNT MOT.STIIRF CONTFNT AT VARIOUS FREQUENCIES

CORN SAMP) F FROM ,10F SMTTH FARM , NT WOT, COLO, NOV. 1976

10

Figure 22,

15 20 25

PERCENT MOISTURE CONTENT

30 35 40

34
Figure 23.

35
LOSS FACTOR OF SETTLED CORN

VS

PERCENT MOISTURE AT VARIOUS FREQUENCIES

CORN SAMPLES FROM VARIOUS

LOCATIONS THROUGHOUT U.S.A.

20 25 30

PERCENT MOISTURE CONTENT

Figure 24.

36
LOSS FACTOR OF DROPPED WHEAT

VS

PERCENT MOISTURE CONTENT AT VARIOUS FREQUENCIES

WHEAT SAMPLES FROM VARIOUS LOCATIONS THROUGHOUT U.S.

A

PERCENT MOISTURE CONTENT

Figure 25.

37
LOSS FACTOR OF SETTLED WHEAT

VS

PERCENT MOISTURE CONTENT AT VARIOUS FREQUENCIES

WHEAT SAMPLES FROM VARIOUS LOCATIONS THRUUtiHUUl' U.S.

A

4 t i i i i i i i i ~r

10 14 16

PERCENT MOISTURE CONTENT

22

Figure 26,

38
LOSS FACTOR OF DROPPED SOYA

VS

PERCENT MOISTURE CONTENT AT VARIOUS FREQUENCIES

SOYA SAMPLE FROM VARIOUS LOCATIONS THROUbHOUT U.S.A.

e

PERCENT MOISTURE CUlNI'ElU

Fxgure 27.

39
LOSS FACTOR OF SETTLED SOYA

VS

PERCENT MOISTURE CONTENT AT VARIOUS FREQUENCIES

SOYA SAMPLES FROM VARIOUS LOCATIONS THROUGHOUT U.S.A.

10 11 12 13 14 15 16 17

PERCENT MOISTURE CONTENT

Figure 28.

40
10 . 2 Density Versus Dielectric Constant

As mentioned earlier, a settling process was employed in the measurement procedure and

data were gathered to indicate the dependence of the measured values of dielectric parameters

upon the density or packing characteristics of the various grains. A summary of this data

is provided in tables 11, 12, and 13.

Table 11 Ratio of settled to dropped parameters of various corn samples at 1 MHz

.

Percent Density e' Tan 6 e" Grain Moisture Ratio Ratio Ratio Ratio

Corn X / 7/

iX < iX • Xw1 6 1.08 1.25

Corn 9ij . o 1X i X L. 1x . 1 .06 1 .31

KjV LLl 26. o

- 1. 11 1. 28 0 . 97 1 . 25 Pnrn liUX 11 29. 0 1..12 1. 16 1 . 08 1 .26 LOLU 30. 9 1. 13 1. 25 1 09 1 36

Corn 8. 5 1. 05 - 1. 07 1.06 1.13

Corn 10. 4 1.,02 1. 07 1.05 1.13

Corn 12. 9 1.,06 1. 10 1.07 1.17

Corn 15. 8 1.,08 1. 12 1.08 1.21

Corn 19. 1 1.,09 1. 16 1.06 1.23

Corn 22. 4 1..11 1. 19 1.04 1.25

Corn 25. 7 1..13 1. 20 1.06 1.27

Corn 28. 6 1..10 1. 25 1.07 1.33

Corn 30. 9 1..11 1. 23 1.00 1.23

Corn 33.,7 1..12 1. 27 1.05 1.34

Corn 39. 0 1..13 1. 44 1.13 1.62

Table 12. Ratio of settled to dropped parameters of various wheat samples at 1 MHz. Percent Density e' Tan 6 e" Grain Moisture Ratio Ratio Ratio Ratio

Wheat 10..7 1 .08 1. 12 1.09 1.21

Wheat 13..4 1 .05 1. 13 1.07 1.20

Wheat 14..3 1 .06 1.,09 1.07 1.16

Wheat 16..0 1 .06 1.,15 1.10 1.26

Wheat 16..6 1 .09 1.15 1.09 1.26

Wheat 18..4 1 .11 1.,13 1.10 1.24

Wheat 18,.5 1 .08 1..18 1.13 1.33

Wheat 21..2 1 .12 1.,27 1.16 1.47

Table 13.

Percent Density e' Tan 6 e" Grain Moisture Ratio Ratio Ratio Ratio

Soya 10.4 1.12 1.16 1.11 1.29

Soya 10.4 1.04 1.21 1.03 1.25

Soya 11.4 1.07 1.17 1.10 1.29

Soya 11.4 1.06 1.16 1.10 1.27

Soya 13.1 1.07 1.20 1.10 1.33

Soya 13.9 1.09 1.19 1.10 1.31

Soya 14.5 1.05 1.17 1.09 1.28

Soya 14.8 1.07 1.21 1.10 1.32

Soya 16.1 1.06 1.21 1.05 1.26

Soya 16.9 1.08 1.23 1.09 1.34

41
In tables 11-13 only the data from measurements at 1 MHz have been tabulated, and

similar comparisons may be made at other frequencies by referring to the tabulation in the

data section of this report. A few things seem immediately apparent from this data.

Evidently the settling process produces the greatest change in the loss or conductivity

portion of the complex dielectric constant since tbe ratios of settled to unsettled values

are higher for e" than for e'. This might allow e" to be used as a packing denisty indicator, which could in turn be used to correct e

1 for the density effect. Regarding e', it

appears that the higher moisture levels of all three materials are more subject to packing

density variations than the low moisture levels. This effect is most noticeable in the

second corn sample in table 11, because of the wider moisture range (8.5 to 39 percent).

However, over similar moisture ranges, the effect is about the same for the three materials

tested

.

10 . 3 Temperature Coefficient of e' and Tan 6

A few experiments were performed on corn, wheat, and soya to determine the temperature

coefficient of the dielectric parameters over the approximate range from 4 to 38°C.

Samples of each of these materials having different moisture contents were tested, and

moisture meter measurements were made before and after the tests to assure that significant

changes in percentage moisture content had not occurred. The following tables 14, 15, and

16 provide a summary of the test results. Most of the data taken were at 30 MHz. The

results are similar for wheat and soya. There was not adequate opportunity to work extensively enough with corn, and therefore the data are minimal. However, the tentative results

do suggest that the temperature coefficient for both e' and tan 6 is larger at lower frequencies. This is a topic needing more investigation for a wider variety of frequencies

and more samples of different moisture content. The technique used in these measurements

was to begin with the grain sample either at the low or the high temperature, and allow it to simply warm or cool to room temperature. For each measurement the grain was transferred

back and forth between the storage container and the dielectric sample holder, so that

essentially the sample holder was kept near room temperature at all times during the test. As might be expected, the grain temperature changes fairly rapidly at the beginning of a test when the temperature difference between the sample and the laboratory environment is

large. The later stages of a run proceed slowly, however; and as many as three hours were required for the sample to finally reach laboratory ambience. A possible source of error in

determining temperature coefficient in the region below room temperature is condensation of

moisture onto the grain from the surrounding air . Because additional moisture would tend

to raise the dielectric constant, it follows that the temperature coefficient determined

in this manner would be larger than that observed in the rang;;e above room temperature if

such errors were present. This did not appear to happen for e' of wheat , and soya as tables 14 and 16 show.

Table 14. Wheat measurement.

Moisture Measur ement Temperature Coefficient

Content Frequency Units/°C

% MHz Temp range (°C) tan 6" Temp range (°C)

13.4 30 + 0.020 4.5°C to 37"C - 0.0005

16.0 30 4- 0.022 6.4°C to 40 C C + 0.0003 4°C to 22.5°C + 0.0015 22.5°C to 40°C

18.4 30 + 0.022 5.2°C to 40°C - 0.0003 5°C to 23°C

+.0047 23°C to 36°C

21.2 30 + 0.019 3.2'C to 36.7°C + 0.0012 7°C to 34°C

42
Table 15. Corn measurement

.

Moisture Measurement Temperature Coefficient

Content Frequency Units/°C

% MHz Temp range (°C) tan 6 Temp range (°C)

34.6 1 (+)0.15 7 to 22 (+)0.02 7 to 22 34.6 30 (+) 0.0003 7 to 22 (+)0.004 7 to 22

Table 16. Soya measurement

.

Moisture Measurement Temperature Coefficient

Content Frequency Units/°C

% MHz Temp range (°C) tan 6 Temp range (°C)

10.4 30 + 0.028 25.5'C to 40°C + 0.0015 25.5°C to 40°C

11.4 30 + 0.028 2°C to 22 P C + 0.0011 2°C to 22°C

13.9 30 + 0.028 25°C to 36°C + 0.0015 25°C to 36°C

16.1 30 + 0.034 2°C to 22°C + 0.0022 2°C to 22°C

10.4 Effect of Sample Holder Size and Kernel. Dimensions

Using the three sample holders described in table 1, a series of measurements was made

to determine what effect the ratio of sample holder size to kernel size would have on measured e ' , and tan 6. The grain samples included wheat (16.6% moisture content), corn

(12.7% moisture content), and soya (9.8% moisture content). Data were obtained by dropping

the samples into the holder with no settling procedure employed. Each sample was dropped

ten times. Table 17 shows the average values for e' and tan 5 together with the standard

deviations. Also shown in the table is the A/K ratio. A is the annular distance in the

holder between the outer surface of the center conductor, and the inner surface of the

outer conductor. K is an average "diameter" dimension for a typical kernel of wheat, corn, or soya. In the case of wheat and soya, K is the average of two measurements (largest and

smallest) of a seed; and, in the case of corn, it was taken as the average of three measurements (length, width, and thickness) . These values of K were used to represent an average

dimension that could be used to compare with the sample holder dimension, recognizing that

the kernels of the various materials will be in random orientations when dropped into the

holder

.

These observations indicate a definite relationship between holder diameter and the

A/K ratio, and percentage differences for both e' and tan 6 are greater between the 2" and

3" holders than between the 3" and 4" holders (see table 18) . The percentage change is

similar for e' and tan 6, but it appears to be more pronounced in the e' values for corn. This is perhaps related to the larger kernel size compared to wheat or soy beans.

From this brief experiment we see that there is a systematic variation in measured

values for e ' and tan 6 which depends on the ratio of sample holder diameter to kernel

size. We hypothesize that the measured effect is due to the packing geometry near the

metallic walls. The dielectric values should approach an upper bound asymptotically as A/K

increases. The fact that the apparent dielectric constant varies with A/K does not necessarily lead to a required diameter of the holder; but, when tables of moisture correlated

to apparent dielectric constant are constructed, the holder diameter should be specified or A/K must be uniformly large for independence of diameter.

43
Table 17. Effect of sample holder and kernel dimensions on observed

values of e

1 and tan 6 at 30 MHz, and standard deviation

of 10 drops.

Data taken August 18, 1976

Holder A/K Parameter

Diameter Ratio Grain e' a , Tan 6 a

e tano

5 ,08cm(2") 3,,3 Wheat(16.6%) 4..084 0..023 0,.0884 0 .0005

7,,62cm(3") 5.,0 Wheat (16.6%) 4.,192 0,.021 0,.0903 0..0005

10,,16cm(4") 6,,7 Wheat(16.6%) 4.,233 0 .020 0..0915 0 .0008

5,,08cm(2") 2, 1 Corn(12.7% 3.,490 0,.028 0..0781 0 .0007

7,,62cm(3") 3,,1 Corn(12.7%) 3,,573 0..038 0,.0799 0 .0007

10,,16cm(4") 4,,2 Corn(12.7%) 3.,702 0,.020 0 .0815 0 .0011

5 .08cm(2 M

) 2,.8 Soya(9.8%) 2..820 0 .026 0,.0682 0 .0008

7 ,62cm(3") 4,,2 Soya(9.8%) 2,,873 0 .021 0,.0693 0 .0007

10,,16cm(4") 5.,6 Soya(9.8%) 2..913 0 .017 0 .0711 0 .0004

Data taken September 30, 1976

Sept. 30 data (anodized center conductor and larger sample size)

5,,08cm(2") 3..3 Wheat (;i6%) 4,,034 0,,011 0,.0866 0 .0008

7,,62cm(3") 5.,0 Wheat Ci6%) 4,,150 0,,018 0,.0878 0 .0010

10,,16cm(4") 6.,7 Wheat Ci6%) 4,,214 0,.021 0,.0917 0,.0018

Table 18. Variation of z

' and tan 6 with sample holder size,

based on results in table 17.

Holders % Differences Between Holders

Larger-Smaller Grain e' Tan 6

7.62cir.-5.08cm Wheat 2.6 2.1

(3" - 2") Corn 2.4 2.3

Soya 1.9 1.6

10.16cm - 7.62cm Wheat 1.0 1.3

(4" - 3") Corn 3.6 2.0

Soya 1.4 2.6

10.16cm - 5.08cm Wheat 3.6 3.5

(4" - 2") Corn 6.0 4.4

Soya 3.3 4.3

10 . 5 Sample Size (Fill Level in Holder)

Referring again to the lower portion of table 17, additional data, taken at a later

date, are tabulated for the same wheat sample (~16%) as was used for the earlier observations of 8/18/76. The 9/30/76 data were taken using a larger sample size, thus filling the

holder to a greater depth, to determine if this would yield different values for e and

and tan 6. Had the earlier sample size not been adequate to completely fill the electric

field region of the sample holder, larger values for e' should have resulted. Because this was not the case (smaller values were observed because even in capped jars the grain dried

out a little) , it is concluded that the sample sizes were adequate to yield accurate values

for e

'

.

44
10 . 6 Contact Impedance

Also during the 9/30/76 observations an anodized center electrode was used in the 3"

sample holder to test whether contact impedance with the grain significantly affected the

measured dielectric values. Because the measured value of tan 6 only dropped from 0.0903

to 0.0878 and the standard deviation in tan 5 remained nearly unchanged, it is concluded

that variation of contact impedance is not a problem up to this moisture level. However, more testing should be done to verify this at higher moisture levels.

10.7 Drop Test of Corn, Wheat, and Soya (Standard Deviation)

The data in table 17 also provide a means of estimating the standard deviation of

moisture meter measurements. To use an example, take the e' value for the 12.7 percent

corn obtained using the 4" sample holder. The standard deviation of the ten drops was

± 0.02 in e', or ± 0.5 percent. Referring to figure 6 and the percent moisture content versus e' plot at 30 MHz, the resulting error in determining the moisture content would

have been ± 1.7 percent. Similar evaluations can be derived from this data for the other

materials and for other measurement frequencies and levels of moisture content,

10.8 Overburden Effect on e' and Tan 5

During the process of taking data on e* versus percent moisture content of the various

materials, it was noticed that some change occurred as a result of setting the depth gauge

on top of the grain after it was dropped into the holder. Some further data were taken

which appear to confirm that the value of e' is affected by pressure on the sample. Using

the 3" sample holder and a wheat sample of 16.6 percent moisture content, the following

results were obtained as static weight was added to the surface of the grain:

Table

Weight on Sample (grams)

0 4.1253

171.4 4.1406

318.9 4.1544

613.9 4.1884

318.9 4.1866

171.4 4.1875

0 4.1864

19.

Depth Guage Reading

Tan 6 inches

0.0880

0.0888 1,.13 0.0896 1..16 0.0892 1..17 0.0885 1..17 0.0877 1..17 0.0871

Although there was practically no change in the depth gauge reading as a result of adding

the weight, a noticeable increase occurred in the value observed for dielectric constant;

but the change was not reversible upon removal of the weight as the table of data shows.

Such an effect might be important in an in situ measurement where, because of overburden,

grain moisture would artificially appear to increase at greater depths in a container.

10.9 Glass Bead Experiments

In order to gain some appreciation for the uniqueness of the dielectric behavior of

grain as opposed to any other particulate matter, some experimentation was done using a sample of glass beads in the 2" sample holder. The beads were not uniform in shape or size

although they were generally spherical having diameters averaging approximately 0.424 cm

(0.167") and variations of the order of 0.025 cm (± 0.01"). Dropping the sample of beads

into the holder eleven times produced a value for e' of 3.311 with a standard deviation, a, of 0.017 and a value for tan 8 of 0.0057 with a of 0.0008. Thus the repeatability of the

bead sample is comparable with the repeatability of the various samples of corn, wheat, and

soya

.

45
Considering the possibility that some stable particulate material such as glass beads

could be useful as a standard for electric moisture meter calibration, some further experimentation was done to learn what effect settling would have on the value obtained for e'. The sample was first dropped into the holder and values for e' and tan 6 observed, followed

by remeasurement of e' and tan 6 after repeated settling.

Repeated

Settling

Sample Condition

Dropped

'Settled

Settled

Settled

Settled

3.315

3.470

3.492

3.504

3.506

tan 3

0.0048

0.0045

a. 0062

0.0050

0.0047

Density

g/cc

1,507

1,577

1.591

1.591

1.591

After repeated settling the bead sample in the holder appeared to reach a limiting

value for e'. To see how reliable this limiting value might be, a different settling

technique was used. Instead of dropping the entire charge of beads into the sample holder

at one time and vibrating them afterward, the beads were poured into the holder in small

increments with settling done between each increment. This produced the following result

which can be compared to the previous data:

Density

e

tan 3/ g/cc

3,665 Q.0Q57 1.618

Thus by settling the sample incrementally the value of e' increased 10.5 percent

compared to the original dropped value. Comparing the change of density to the change in

e' shows that a density change of 7.4 percent was accompanied by a 10.5 percent change in

the observed value of e'. This is shown in more detail by the graph in figure 29. After

some initial settling the relationship between e' and density appears to be linear as shown

by the solid portion of the graph. The lower portion is shown as a broken line because of

the absence of data points in this region.

To utilize some material such as glass beads as a standard for electric moisture

meters would require careful research to determine such information as optimum bead material

and size, required degree of uniformity of bead size and shape, and application techniques

to be followed. Certainly any bead material selected would have to have an e' value in the same range as grain. Such material in the form of beads is difficult to locate.

11. CONCLUSIONS

Based upon the data accumulated using the equipment and methods described in this

report, the following conclusions have been reached:

1. In utilizing dielectric constant, e', as an indicator of percent moisture content

for the three materials tested, frequencies in the vicinity of 1 MHz or below

provide greater sensitivity or measurement resolution than frequencies in the

vicinity of 200 MHz. This is illustrated in figures 5 and 6 where the sensitivity

is 0.6 e' units per one percent change in moisture content at 1 MHz compared to 0.12 e' units per one percent change in moisture content at 2Q0 MHz.

2. The measured value of e

1 is a function of the packing density of the sample of

material in the holder. For measurement repeatability it is important that

density be kept constant from one sample to another. From these experiments the

dropping technique appeared to provide better density repeatability than the

settling process devised for this experiment.

3. The measured value of e' approaches a limiting maximum value as the density

increases and also approaches a maximum value.

46
DIELECTRIC CONSTANT OF GLASS BEADS

VS DENSITY FOR 2" SAMPLE HOLDER

3.7

3.6

3.3

3.2

-rr

is > — I

i f /

4/

/

*/

t V

*

4 —

*-

*< - 0i1.50 1.55 1.60 1.64

DENSITY g/cc

Figure 29.

47
4. The ratio of sample holder diameter to kernel size has an apparent effect upon

the observed value of dielectric constant up to an A/K ratio of approximately 5.

5. The sample volumes used in these experiments permitted the sample holder center

conductor to be covered to a depth equal to about two center conductor diameters

and were adequate to permit e' measurement which was free from errors due to underfilling

.

6. Adding an anodized coating to the center conductor of a sample holder produced no significant change in the measured value of complex dielectric constant for wheat

at 16% moisture content.

7 . The effect of temperature on the observed values for dielectric constant was

small at frequencies of 1 and 30 MHz. Temperature coefficients of the order of + 0.02 e

1 units per degree centigrade and 0.01 tan 5 units per degree centigrade were observed for wheat, corn, and soya over the range from 4 to 40 degrees

centigrade.

8. An apparent increase in dielectric constant, e' , due to surface weight on the

grain sample was observed. The addition of approximately 600 grams to the sur- face of the sample produced a change in dielectric constant change of + 1.5%.

This change was not accompanied by a measurable change in tan 6, and only a very

small change in density. Also the change in dielectric constant was not reversible upon removal of the weight.

9. The e' versus percent moisture relationship at higher moisture levels (>25%) for corn was continuous with the data at levels below 25%, and did not pose a problem

for the passive circuit equipment used. For meters utilizing an active oscillator

circuit, the low Q values of a test cell when filled with high moisture grain

could be a problem.

10. There is an apparent difference in the behavior of the dielectric parameters

depending on growing conditions and hybrid seed type. This conclusion is weak,

however, because of the doubt as to the accuracy of the moisture level percentages

in the test samples.

Much of this work supports the practices in current use by the U.S.D.A. in its grain

inspection procedures. Of specific note are the practice of dropping the sample into the

holder, and the development of special calibration charts for particular geographic areas.

Areas of possible future interest not covered by this report include a study of the

effects of other parameters on the accuracy of moisture measurement.. Included among the

other parameters are kernel shape, and the presence of other constitutents including protein,

carbohydrate, fat, and fiber content, bound and unbound water, and possible changes due to enzymatic processes. Another approach might also be the study of dielectric constant as a function of the density of a single kernel as distinguished from bulk density.

It also appears desirable to extend the work to include measurments at frequencies

below 1 MHz because the values for e' are continuing to increase with decreasing frequency.

This enhances the prospect for still higher resolution which is an important consideration

in choosing the optimum measurement frequency.

48
1 2 . ACKNOWLEDGMENT

We would especially like to thank Mr. Hayward Hunt and his staff at the U.S. Department

of Agriculture Grain Standardization Laboratory in Beltsville, Maryland, for their support

in providing grain samples to us which had been analyzed by their oven-dry facility. Also

their cooperation and advice were very helpful.

It is appropriate to acknowledge the work of others whose labors have preceded ours in

this subject area. Many have contributed, and a complete bibliography of their work would

be a project in itself. However, because of the similarity of effort and the nearly common

objectives, we would especially like to reference the work of those at the Agricultural

Research Service of the U. S. Department of Agriculture at the University of Nebraska,

specifically S. 0. Nelson, R. E. Stetson, J. L. Jorgensen, and A. R. Edison [10,11]. In many respects our work has been similar, and their papers have been most valuable. A recent paper [12] by Nelson gives a good bibliography.

Also allow us to make special mention of two laboratory assistants whose work was so vital. Jocelyn Spencer and Douglas Tamura. both put forth excellent work through some long

and tedious hours.

49
13. BIBLIOGRAPHY

[I] Young, James H., Moisture, Chapter 7, Instrumentation and Measurement for Environmental

Sciences, Zachery A. Henry, editor, Spec. Pub. 0375 , pp. 7-01-7-10 (American Society

of Agricultural Engineers, 1975)

.

[2] Zeleny, L., and Hunt, W. H., Moisture Measurement in Grain, (American Society of

Agricultural Engineers, Proc . of 1962 Winter Meeting, Paper No. 62-926).

[3] Whinnery, J. R., Jamison, H. W., and Robbins, T. E., Coaxial-line Discontinuities,

Proc. I.R.E., pp. 695-709 (Nov. 1944).

[4] Somlo, P. I., The Discontinuity Capacitance and the Effective Position of a Shielded Open Circuit in a Coaxial Line, Proc. Inst. Radio and Elec . Eng., Australia,

28, No. 1, pp. 7-9 (Jan. 1967).

[5] Ramo, S., Whinnery, J. R., Van Duzer, L., Fields and Waves in Communication Electronics

(John Wiley & Sons, Inc., New York, 1965, Library of Congress No. 65-19477), Table

8.09, after p. 444.

[6] Woods, D., A Precision Dual Bridge for the Standardization of Admittance at Very

High Frequencies, Proc. IEE (London), 104C , Mono. 244R, pp. 506-521 (June 1957).

[7] Huntley, L. E., A Self-calibrating Instrument for Measuring Conductance at Radio

Frequencies, J. Res. Nat. Bur. Stand. (U.S.), 69C (Eng. and Instr.), No. 2,

115-126 (April-June 1965).

[8] Buckley, F., and Maryott, A. A., Tables of Dielectric Dispersion Data for Pure

Liquids and Dilute Solutions, Nat. Bur. Stand. (U.S.), Circular 589 (Issued

Nov. 1, 1958).

[9] Von Hippel, A. R., Dielectrics and Waves (John Wiley & Sons, Inc., New York, 1954,

p. 231, Library of Congress Card No. 54-11020).

[10] Jorgenson, J. L., Edison, A. R. , Nelson, S. 0., and Stetson, L. E., A Bridge Method

for Dielectric Measurements of Grain and Seed in the 50-250 MHz Range, Trans. A.S.A.E.

13(1) , pp. 18-20, 24 (1070).

[II] Nelson, S. 0., Dielectric Properties of Grain and Seed in the 1 to 50 MG Range, Trans.

A.S.A.E., 8(1), pp. 38-48 (1965).

The following paper by S. 0. Nelson provides an extensive bibliography.

[12] Nelson, S. 0., Use of Electrical Properties for Grain-moisture Measurement, Journal of

Microwave Power, 12(1) , pp. 67-72 (1977).

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66
NBS-1 14A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 78-897

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

ELECTRICAL CHARACTERISTICS OF CORN, WHEAT, AND SOYA

IN THE 1-200 MHz RANGE

5. Publication Date October 1978

6. Performing Organization Code

7. author(S) R< N> Jones> H> E> Bussey, W. E. Little, and 8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2762275

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP) 13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

A set of coaxial sample holders together with a measurement and data

reduction technique has been developed and applied to the study of the

dielectric properties (e*=e l -jeM

) of wheat, corn, and soya over the 1 to

200 MHz range. Particular attention was given to the behavior of the

dielectric properties as a function of percent moisture content, frequency, and packing density. Data were also taken to evaluate the

dependence of dielectric properties on temperature and sample holder

configuration. Some study was also devoted to the correlation between

dielectric constant (e'), loss factor (e M ), loss tangent (e"/e')> and

percent moisture content. Particular emphasis was devoted to a study

of high moisture corn (up to 40%)

.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Dielectric properties; grain; loss tangent; moisture.

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