laboratory testing and calibration of vertically oriented tdr soil moisture sensors

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Laboratory Testing and Calibration of Vertically Oriented TDR Soil Moisture Sensors By: Phillip McFarland

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Laboratory Testing and Calibration of Vertically Oriented TDR Soil Moisture Sensors. By: Phillip McFarland. Project Recap: Soil Moisture – agricultural, hydro-geological modeling and corrections to groundwater gravity measurements. TDR – time domain reflectometry - PowerPoint PPT Presentation

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Page 1: Laboratory Testing and Calibration of Vertically Oriented TDR Soil Moisture Sensors

Laboratory Testing and Calibration of Vertically Oriented

TDR Soil Moisture Sensors

By: Phillip McFarland

Page 2: Laboratory Testing and Calibration of Vertically Oriented TDR Soil Moisture Sensors

Project Recap:

•Soil Moisture – agricultural, hydro-geological modeling and corrections to groundwater gravity measurements.

•TDR – time domain reflectometry

•VWC – volumetric water content

θ =VwVt= VwVs +Vp

Page 3: Laboratory Testing and Calibration of Vertically Oriented TDR Soil Moisture Sensors

Principles of TDR

c = 1εoμo

→ εo =1c 2μo

v p =1ε 'μ '

→ ε'= 1v p2μ '

εra ≡ε 'εo= c

2μov p2μ '

assume :μo = μ '

εra =cv p

εra =ctL

t =L εrac

v p =Lt

θ =0.115 εra −0.176

Maxwell: Explanation of “Relative Permittivity”:

Applications of Relative Permittivity:

Ledieu Equation for VWC:

Page 4: Laboratory Testing and Calibration of Vertically Oriented TDR Soil Moisture Sensors

TDR Waveforms

Page 5: Laboratory Testing and Calibration of Vertically Oriented TDR Soil Moisture Sensors

Waveform Interpretation

Page 6: Laboratory Testing and Calibration of Vertically Oriented TDR Soil Moisture Sensors

Horizontally Oriented Soil Moisture Sensor Probes

•Proven technology

•Very accurate VWC measurement

•Little or no calibration required

•Destructive to soil

•In situ TDR device required

•Costly

•Limited sample volume per probe

Page 7: Laboratory Testing and Calibration of Vertically Oriented TDR Soil Moisture Sensors

Development of a Vertically Oriented TDR Probe System

Challenges:•Calibration of probe connectors•Larger sample volume•Average VWC over several layers v. local measurement at each layer•Unknown soil constituents may deflect probe installation (i.e. buried stones)•Best material for waveguidesPossible Advantages:•Easy installation•Cost effective•Fewer TDR devices required to survey a large land area. Especially when less frequent measurements are needed.

Page 8: Laboratory Testing and Calibration of Vertically Oriented TDR Soil Moisture Sensors

First Attempt!!

Results:•38% error!!•Interesting variations in pulse reflection from connectors

•Probe calibration needed•Probe connector offset constant needed

Page 9: Laboratory Testing and Calibration of Vertically Oriented TDR Soil Moisture Sensors

Theory of Connector Calibration

twater = to +(Le εrawater (T))

c

tair = to +(Le εraair

c

to water = twater − t'watertoair = tair − t'air

t'=Le εrac

Page 10: Laboratory Testing and Calibration of Vertically Oriented TDR Soil Moisture Sensors

Calibration Method

•20 probe pairs used

•4 pairs @ lengths: 28, 24, 20, 16 and 8 in.

•TDR measurements made in air and DI water

•2 measurements made with each pair of waveguides in each medium with each connector

•160 TDR measurements total made

•Relative permittivity of water and air known

•Measurements compared with theoretical values

•Probe constant isolated empirically

Page 11: Laboratory Testing and Calibration of Vertically Oriented TDR Soil Moisture Sensors

Results of Calibration

c = Le 't

Leo '= cto

Avg.%error ≈ 6%

Leo '= 0.071m

•Electrical path length constant of waveguide connectors was used to correct previous data collected

•Average % error decreased by nearly 32%

Page 12: Laboratory Testing and Calibration of Vertically Oriented TDR Soil Moisture Sensors

What’s Next•Develop a field ready model

•Further test existing and new probes and connectors in a wider variety of conditions

•Implement a comprehensive soil moisture system around gravity measurement sites

•Use reliable soil moisture information to correct gravity ground water measurements