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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. Project Recap: Soil Moisture – agricultural, hydro-geological modeling and corrections to groundwater gravity measurements. TDR – time domain reflectometry

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Laboratory Testing and Calibration of Vertically Oriented TDR Soil Moisture Sensors

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

  2. Project Recap: • Soil Moisture – agricultural, hydro-geological modeling and corrections to groundwater gravity measurements. • TDR – time domain reflectometry • VWC – volumetric water content

  3. Principles of TDR Explanation of “Relative Permittivity”: Maxwell: Applications of Relative Permittivity: Ledieu Equation for VWC:

  4. TDR Waveforms

  5. Waveform Interpretation

  6. 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

  7. 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 waveguides • Possible Advantages: • Easy installation • Cost effective • Fewer TDR devices required to survey a large land area. Especially when less frequent measurements are needed.

  8. First Attempt!! • Results: • 38% error!! • Interesting variations in pulse reflection from connectors • Probe calibration needed • Probe connector offset constant needed

  9. Theory of Connector Calibration

  10. 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

  11. Results of Calibration • Electrical path length constant of waveguide connectors was used to correct previous data collected • Average % error decreased by nearly 32%

  12. 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

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