CAPACITANCE SENSOR FOR SOIL MOISTURE MEASUREMENT

Moisture content is an important parameter that controls the performance of earth materials. Because satisfactory techniques were not available, this study was undertaken to develop new instrumentation that could be permanently installed in highway substructures for long-term monitoring of moisture content. Moisture content determination requires that the amount of water and the dry weight in a given volume of soil be obtained. This study considered only the amount of water. A concept that involved the relationship between the amount of water in soil and the dielectric constant, an electrical property, was investigated. The electrical capacitance (C) of a pair of electrodes with moist material between them is related to to the dielectric constant (K) by C equals (Alpha times Epsilon) K, where Alpha is a constant based on the geometry of the electrodes and Epsilon is the permittivity of the vacuum. So that instrumentation problems encountered in the measurement of capacitance and dielectric constant with laboratory methods used in the field could be overcome, the sensor electrodes and soil were incorporated in the resonance circuit whose frequency of oscillation is a function of the circuit components. The measured output is the difference between the frequency in soil and in an electonic reference substituted for the soil. The curcuit design was first tested with the electrodes forming a container into which soil could be placed. Several configurations of the embedded sensor were designed and tested. The capacitance concept was useful for sensing change in the amount of moisture per unit volume of soil.

Media Info

  • Media Type: Print
  • Features: Figures; References;
  • Pagination: pp 64-76
  • Monograph Title: FROST, MOISTURE, AND EROSION
  • Serial:

Subject/Index Terms

Filing Info

  • Accession Number: 00099655
  • Record Type: Publication
  • ISBN: 0309023807
  • Files: TRIS, TRB
  • Created Date: Nov 5 1975 12:00AM