A Small-Scale Multifunctional Heat-Pulse Sensor for Soil Water Content and Electrical Conductivity

Robert Heinse, Kelly Lewis and Scott Jones

Environmental Soil Physics GroupDep. Plants, Soils and Biometeorology, Utah State University

Western Society of Soil Science 2006, Park City, UT

RationaleMultifunctional sensor

Complex measurementsIntegrated interpretation

Water content and electrical conductivity Heterogeneity at a wide range of scalesSampling volumeHeat-pulse methodDirect current EC

Objectives

A

B

M

N

Thermocouple

Heater

27 mmSense water content using a physical property independent of salinity, and measure electrical conductivityDesign, construct and test a sensor providing estimates of

water content electrical conductivity

at a comparable sampling volume by pairing two dual-probe heat-pulse (DPHP) sensors

dual-probe heat-pulse sensor (Ham and Benson, 2004)

Heat Pulse TheoryKnight and Kluitenberg (2004) model for

predicting volumetric water content :

mtt /0=ε c = specific heat capacityρ = bulk densityq’ = rate at which heat energy is releasedt0 = heater firing timetm = time to max tempr = apparent probe spacing∆T = Change in Temp

ww

sbv c

ccρρρθ −

=

)192

7128

5241

2411(' 5432

20 εεεε

πρ −−−−

∆≈

Tretqc

Prediction of the solution electrical conductivity σw:

Archie (1942)

where: σw… pore water conductivityS… saturationΦ… porositya, m, n… parameters

Measurement of the bulk electrical conductivity σ:

σσ nw

mw Sa −−Φ=

11111

2

−

⎥⎦

⎤⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛−−⎟⎟

⎠

⎞⎜⎜⎝

⎛−=

BNBMANAM rrrrVIπ

σM N BA

I

V

A

B

M

N

Electrical Conductivity Theory

ImplementationShallow sample cell (2cm tall)Water addition/removal capability

porous plate at the bottom

DPHP and DPHP/4-electrodeAggregated clays

0.25—1 mm1—2 mm

0.4 0.5 0.6 0.7 0.8

0.4

0.5

0.6

0.7

0.8

DP

HP

θ [c

m3 c

m-3

]

(A)

0.25--1 mm

Syringe Volumetric Water Content [cm3 cm-3]0.4 0.5 0.6 0.7 0.8

0.4

0.5

0.6

0.7

0.8

DP

HP

θ [c

m3 c

m-3

]

(B)

1--2 mm

Syringe Volumetric Water Content [cm3 cm-3]

Change to cell water content!

Water Content Prediction

Saturated measurements with a solute pulse

15:00 18:00 21:000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Time

Spe

cific

Con

duct

ivity

[S m

-1]

Application of 0.68 Sm-1 Potassium Bromide Solution

Application of 0.04 Sm-1

Tap Water

σσ nw

mw Sa −−Φ=

Saturated measurements with a solute pulse

0 0.01 0.02 0.03 0.04 0.05 0.06 0.070

0.01

0.02

0.03

0.04

0.05

0.06

0.07

σw [S m-1]

Est

imat

ed σ

w [S

m-1

] salinizing

desalinizing

Unsaturated static measurement

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80

0.01

0.02

0.03

0.04

0.05

0.06

Syringe Volumetric Water Content [m3 m-3]

Spe

cific

Con

duct

ivity

[S m

-1]

Solution EC

Predicted solution EC

1--2 mm Ceramic aggregatemodeled and measured bulk EC

1--2 mm Glass beadsmodeled and measured bulk EC

σσ nw

mw Sa −−Φ=

Nutrient monitoring

0 50 100 150 200 250 300

0.05

0.1

0.15

0.2

0.25

0.3

Time [hr]

Sol

utio

n E

lect

rical

Con

duct

ivity

[S m

-1]

Slow release fertilizer

12 4 6

3 5

Slow release fertilizer

12 4 6

3 51

1

3

3

2

2

-4

-4

-3

-3

-3-2

-2 -2

-2-1

-1

-1

0

0 0

11 22

2 33 101 -101

y [c

m]

0 2 4 6 8 10-2

-1.5

-1

-0.5

0

5

-5

-4 -4

-3

-3

-3

-2

-2

-2

-1

-10

0

0

1 1

-1-1

-1 -12

-2

2-1

2

x [cm]

y [c

m]

0 2 4 6 8 10-2

-1.5

-1

-0.5

0

A NM B

Sensitivity distribution

FactorsElectrode spacingPair spacingDepth???

Summary

Integration of heat-pulse water content and 4-electrode electrical conductivity sensing In-situ predictions in comparable sampling volumeMonitor fluxes of water and solutes

Poster presentation: “Measuring Electrical Conductivity Using A Low-Power Data-Logging System” by Lewis et al.

Measurement Accuracy

0 3 6 9 12 15 1810-5

100

105

Geo

met

ry F

acto

r [m

-1]

Spacing between AM [mm]0 3 6 9 12 15 18

100

Pot

entia

l Mea

sure

d ov

er M

N [V

]

0 3 6 9 12 15 18

100

0 3 6 9 12 15 1810-4

10-3

10-2

10-1

100

0.0004 S m-1

0.04 S m-1

4 S m-1