1. 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

2. 6 MHz Probe Response

3. Variation of 6 MHz Readings with Temperature at high EC

4. 70 MHz Probe Calibration
Density 1.2 to 1.6 g/cm3

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

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

7. 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

8. Phase Difference Across R1
where

9. 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

10. 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

11. Comparison of Model and Measurements for 6 MHz Probe

12. 6 MHz Probe Temperature Dependence

13. Comparison of Model and Measurements for 70 MHz probe

14. 70 MHz Temperature Dependence

15. 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.