Theory and Practice of yDielectric Soil Moisture

Measurement

Colin S. Campbell, Ph.D.p ,Decagon Devices, Inc.

Pullman, WA USA,

IntroductionIntroduction Decagon Devices Decagon Devices Started in 1983 supplying instrumentation for

measuring water potential Goal Develop robust instrumentation to take accurate

data AND fit within a budget

Vision In the future, measuring and modeling the natural , g g

environment will require more high quality, innovative, and inexpensive solutions

OutlineOutline

K+O2-

H- O2-

H-H-

O2-

H-

H-K+

O2-

H-

H- O2-

H-

H-

H-

H

H

Electromagnetic fieldsElectromagnetic fields Magnet and iron filings Magnet and iron filings

Electromagnetic fieldsElectromagnetic fields EM fields using electricity EM fields using electricity

Dielectric material: constantconstant

Properties of dielectric materialsProperties of dielectric materialsDielectric constant: Ability to store chargeDielectric constant: Ability to store charge

1 5 20 40 80

O i IOrganicmatter

Ice

water

Soil minerals

Air

Soil minerals

Properties of dielectric materialsProperties of dielectric materialsDielectric constant: Ability to store chargeDielectric constant: Ability to store charge

1 5 20 40 80

In soil, typically water (and air) is the only thing that

Mineral soil dielectric range

the only thing that changes

Soil

Dielectric Mixing Model: FYIDielectric Mixing Model: FYI The total dielectric of soil is made up of the The total dielectric of soil is made up of the

dielectric of each individual constituent The volume fractions, Vx, are weighting factors

that add to unity

ibiom

bom

bwa

bam

bm

b VVVVt

Where is dielectric permittivity, b is a constant around 0.5, and subscripts t, m, a, om, i, and w represent total, mineral soil, air,

i tt i d torganic matter, ice, and water.

Volumetric Water Content and Dielectric PermittivityDielectric Permittivity

Rearranging the equation shows Rearranging the equation shows water content, , is directly related to the total dielectric byto the total dielectric by

50

5.05.05.05.05.0

50

)(1 iiomomaamm VVVV

Take home points

5.05.0ww

t

Take home points Ideally, water content is a simple first-order function of dielectric

permittivity Generally, relationship is second-order in the real world

Therefore, instruments that measure dielectric permittivity of media can be calibrated to read water contentmedia can be calibrated to read water content

Calibrating dielectric to water contentto water content

a b a t b

t

Reasonably consistent calibration with soil typeProblems: High EC, clay activity, temperature, bulk density

Volume of sensitivityVolume of sensitivity

3030 cm

Volume of sensitivityVolume of sensitivity Basic principles Basic principles Highest sensitivity

closest to surface Design of sensor

30 cm

grods can change volume of influence

Wider needle spacing onlyspacing only increases volume of influence to a pointinfluence to a point

Measuring dielectric to get water contentcontent

M i Di l t iMeasuring Dielectric2 choices

Time Domain FrequencyDomainDomain

Dielectric Instruments: Time Domain ReflectometryDomain Reflectometry

tion

Refle

ct

Time

Kosuke Noborio, 1993

Dielectric Instruments: Time Domain ReflectometryDomain Reflectometry

Measures apparent length (L ) of probe from Measures apparent length (La) of probe from an EM wave propagated along metallic rods

L is related to and therefore La is related to and therefore

Kosuke Noborio, 1993

Problems with TDR?Problems with TDR? TDR is a good way to measure water TDR is a good way to measure water

content, why not use it? Cost Expensive to purchase extensive installationp p

Complexity System requires expertise to set up System requires expertise to set up

Power consumption Require significant battery capacity/solar panel

Frequency Domain (FDR or Capacitor)dielectric sensor basicsCapacitor)dielectric sensor basics

Typical CapacitorTypical Capacitor

CapacitorDielectric Material

++++

---

Positive Plate Negative Plate+++++

-----

+++++

-----

Electromagnetic Field

+++++

----

Example: How Capacitance Sensors Functionp p

Sensor (Side View)2 cm

1 cm

EM Field0 cm

z

Example: How Capacitance Sensors Functionp p

Sensor (Side View)2 cm

1 cm

EM Field0 cm

z

Calibration example: EC-5 SensorCalibration example: EC-5 Sensor

0.3

0.35

m3)

Sand (0.16, 0.65, 2.2, 7.6 dS/m)

Patterson (0.52, 0.83, 1.7, 5.3 dS/m)

Palouse(0.2, 0.7, 1.5 dS/m)

0.25

0.3

nten

t (m

3/m Palouse (0.2, 0.7, 1.5 dS/m)

Houston Black (0.53 dS/m)

0.15

0.2

Wat

er C

on

0.1

olum

etric

W

0

0.05

350 400 450 500 550 600 650 700 750

Vo

350 400 450 500 550 600 650 700 750Probe Output (mV)

Limits to dielectric measurement accuracyaccuracy

Two important factors Two important factors Temperature Electrical conductivity

H-H-

2

H- O2-

HH

O2-

H-

K+

O2- H-

O2-

H-

OH-

O

H-

HO2-

H-

H-

Buehler, 2011

Temperature sensitivityTemperature sensitivityTemperature Sensitivity Analysis of 6 (EP) and 70 (TE) MHz

ECH2O Probes

0.006

0.004

3 C-1

)

EP Si L EP Sd L EP STE Si L TE Sd L TE S

0.002

T (m

3 m-3

00 0.1 0.2 0.3 0.4

m

/ T

-0.002Measured Vol. Water Content (m3 m-3)

No temperature dependence in field studystudy

55%

90 cm

120 cm

45%

50%

3 )

30 cm

90 cm

150 cm40%

45%

nten

t (m

3m

-3

60 cm

60 cm

30 cm30%

35%

c W

ater

Con 90 cm

20%

25%

Vol

umet

ri

Hard Pan

120 cm

15%4/28/07 5/8/07 5/18/07 5/28/07 6/7/07 6/17/07 6/27/07 7/7/07 7/17/07 7/27/07 8/6/07

Hard Pan

150 cm

Diurnal fluctuations: Real or caused by temperature? (toe slope site)temperature? (toe slope site)

60%

150 cm50%

30 cm60 cm

120 cm

40%

m-3

)

20%

30%

VWC

(m3

m

90 cm10%

0%

Dielectric Constant – EC Dependence

250

pTDR frequency rangeECH2O frequency (70 MHz)

200

TAN

T

Temp = 25°CREAL

EC[S/cm]=1.00E-4

150

C C

ON

ST

O i t ti

EC[dS/m]=0.10

50

100

ELEC

TRIC Orientation

PolarizationIon Dipole

Polarization

0

50

DIE

IMAGINARY

1E+3 1E+4 1E+5 1E+6 1E+7 1E+8 1E+9 1E+10 1E+11 1E+12FREQUENCY, f (Hz)DielConst_Water_1330.xls

Buehler, 2011, ISEMA

Dielectric Constant – EC Dependence

250

pTDR frequency rangeECH2O frequency (70 MHz)

200

TAN

T

Temp = 25°CREAL

EC[S/cm]=1.00E-3

EC[dS/ ] 1 00150

C C

ON

ST

O i t ti

EC[dS/m]=1.00

50

100

ELEC

TRI Orientation

PolarizationIon Dipole

Polarization

0

50

DIE

IMAGINARY

1E+3 1E+4 1E+5 1E+6 1E+7 1E+8 1E+9 1E+10 1E+11 1E+12FREQUENCY, f (Hz)DielConst_Water_1330.xls

Buehler, 2011, ISEMA

Dielectric Constant – EC Dependence

250

pTDR frequency rangeECH2O frequency (70 MHz)

200

TAN

T

Temp = 25°C

REAL EC[S/cm]=1.00E-2

EC[dS/ ] 10 00150

C C

ON

ST

O i t ti

EC[dS/m]=10.00

50

100

ELEC

TRI Orientation

PolarizationIon Dipole

Polarization

0

50

DIE

IMAGINARY

1E+3 1E+4 1E+5 1E+6 1E+7 1E+8 1E+9 1E+10 1E+11 1E+12FREQUENCY, f (Hz)DielConst_Water_1330.xls

Buehler, 2011, ISEMA

Dielectric Constant – EC Dependence

250

pTDR frequency rangeECH2O frequency (70 MHz)

200

TAN

T

Temp = 25°C

EC[S/cm]=1.00E-1

EC[dS/ ] 100 00150

C C

ON

ST REAL

O i t ti

EC[dS/m]=100.00

50

100

ELEC

TRI Orientation

PolarizationIon DipolePolarization

0

50

DIE

IMAGINARY

1E+3 1E+4 1E+5 1E+6 1E+7 1E+8 1E+9 1E+10 1E+11 1E+12FREQUENCY, f (Hz)DielConst_Water_1330.xls

Buehler, 2011, ISEMA

EC-5 Sensor shows no significant EC effect in mineral soileffect in mineral soil

0.3

0.35

m3)

Sand (0.16, 0.65, 2.2, 7.6 dS/m)

Patterson (0.52, 0.83, 1.7, 5.3 dS/m)

Palouse(0.2, 0.7, 1.5 dS/m)

0.25

0.3

nten

t (m

3/m Palouse (0.2, 0.7, 1.5 dS/m)

Houston Black (0.53 dS/m)

0.15

0.2

Wat

er C

on

0.1

olum

etric

W

0

0.05

350 400 450 500 550 600 650 700 750

Vo

350 400 450 500 550 600 650 700 750Probe Output (mV)

ConclusionsConclusions Measuring charge storing capacity of soil Measuring charge storing capacity of soil

(dielectric) gives approximation of water contentcontent

TDR is a good measurement but has some drawbacksdrawbacks

FDR (capacitance) can also measure dielectric effectivelyeffectively

Other factors like temperature and EC can h lib ti b t t i t i lchange sensor calibration, but not in typical

situations

ReferencesReferences Noborio K 1993 Time Domain Noborio, K. 1993. Time Domain

Reflectometry (TDR): Measurements of d l l d fwater content and electrical conductivity of

soil. Dept. of Soil and Crop Sci., Texas A&M p p ,University.