G. S. Campbell, C. S. Campbell, D. R. Cobos and M. G. Buehler Modeling Temperature and Salinity Response of Dielectric Soil Moisture Sensors G. S. Campbell, C. S. Campbell, D. R. Cobos and M. G. Buehler Decagon Devices, Inc. Pullman, WA

6 MHz Probe Response

Variation of 6 MHz Readings with Temperature at high EC

70 MHz Probe Calibration Density 1.2 to 1.6 g/cm3

Objectives Model effects of Water Content and EC on probe response Model effects of temperature on probe response Compare model results with measurements

Approach Schwank, M. and T. R. Green. 2007. Simulated effects of soil temperature and salinity on capacitance sensor measurements. (www.mdpi.org/sensors a free, refereed, on line journal) Analysis of the Sentek LC Oscillator soil moisture sensor

Equivalent Circuit for Probe-Soil System R1 is drive resistor C1 is the probe-soil coupling C2 is soil capacitance R2 is soil resistance C3 is stray capacitance

Phase Difference Across R1 where

Deriving Circuit Parameters from Soil and the Probe Properties Fs probe-soil geometric factor Fp coupling cap. geometric factor εo free space permittivity εb bulk soil permittivity εp probe (FR4) permittivity σb bulk soil electrical conductivity

Assumptions for Simulations Dielectric permittivity of soil fit to the Topp Eq. (b = 0.61) EC computed from Mualem-Friedman : Permittivity decreases 0.5%/C EC increases 2%/C

Comparison of Model and Measurements for 6 MHz Probe

6 MHz Probe Temperature Dependence

Comparison of Model and Measurements for 70 MHz probe

70 MHz Temperature Dependence

Conclusions The model appears to account for observed responses to moisture, salinity and temperature Salinity sensitivity is directly related to frequency. Increasing frequency reduces sensitivity. Positive temperature response comes from probe response to salinity. When salinity errors are low, temperature sensitivity is low.