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@ARTICLE{Qu:150312,
      author       = {Qu, Wei and Bogena, Heye and Huisman, Johan Alexander and
                      Vereecken, Harry},
      title        = {{C}alibration of a {N}ovel {L}ow-{C}ost {S}oil {W}ater
                      {C}ontent {S}ensor {B}ased on a {R}ing {O}scillator},
      journal      = {Vadose zone journal},
      volume       = {12},
      number       = {2},
      issn         = {1539-1663},
      address      = {Madison, Wis.},
      publisher    = {SSSA},
      reportid     = {FZJ-2014-00382},
      pages        = {10},
      year         = {2012},
      abstract     = {Wireless sensor networks are becoming more popular to
                      monitor spatial and temporal variability of soil water
                      content. The aim of this paper is to calibrate and test the
                      newly developed SPADE sensor for wireless sensor network
                      applications. During calibration, the measured sensor output
                      is related to dielectric permittivity using an empirical
                      calibration in liquids with well-defined permittivities.
                      Using these calibration measurements, we have evaluated
                      sensor-to-sensor variability and compared the accuracy of a
                      universal calibration between sensor output and permittivity
                      with a sensor-specific calibration. Our results showed that
                      the sensor-to-sensor variability of the SPADE sensor is
                      larger than sensor noise, and that a sensor-specific
                      calibration can improve the accuracy of soil water content
                      estimation as compared to a single universal calibration. To
                      quantify the effect of temperature on the sensor output, we
                      have derived a temperature correction function in a
                      temperature range from 5 to 40 °C. The results showed an
                      underestimation for low temperatures (5 to 25 °C) and an
                      overestimation for high temperatures (25 to 40 °C). A
                      second order polynomial function was fitted to the
                      measurements with good agreement (R2 = 0.9831, RMSE =
                      0.1489). The transferability of the temperature correction
                      function from reference liquids to soils was verified using
                      saturated coarse sand and a silty loam soil sample. The
                      temperature-corrected SPADE sensor measurements in soil
                      showed good agreement with TDR measurements and corresponded
                      well with theoretical predictions from a dielectric mixing
                      model.},
      cin          = {IBG-3},
      ddc          = {550},
      cid          = {I:(DE-Juel1)IBG-3-20101118},
      pnm          = {246 - Modelling and Monitoring Terrestrial Systems: Methods
                      and Technologies (POF2-246)},
      pid          = {G:(DE-HGF)POF2-246},
      typ          = {PUB:(DE-HGF)16},
      UT           = {WOS:000319393200014},
      doi          = {10.2136/vzj2012.0139},
      url          = {https://juser.fz-juelich.de/record/150312},
}