Soil moisture - Frequency domain (SMT100)
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Parameter to be measured:
Soil moisture, permittivity, soil temperature
Method:
combination of Frequency Domain Reflectometry and Time domain Reflectometry
Equipment:
- SMT-100-sensor (TRUEBNER Instruments) with customizable cable length
Advantages:
- "quasi-TDR" method supposedly yields better results in clay soils
- Low cost
- Easy to install
- compatible with many loggers (RS485, SDI-12 or analogue protocol)
Disadvantages:
- Soil specific calibration required for highest accuracy (common with other capacitance-based devices)
What to watch out for:
- good contact between probe and medium required
- coarse soil may require extra tool for penetrating the soil before installing the sensor
- starting from an initial internal resistance of approx. 60 kOhms, we observed a very heterogenous deterioration of sensors. After 5 years of use, some of them had not changed, but most of them showed a much lower internal resistance , down to a few kOhms only in standby-mode). All sensors still produced reasonable values, but the increased current drained our batteries or even made the voltage drop. Our solution was to switch them off during measurements with a relay-like MOSFET-circuit.
- for SDI-12 version, when the ground cable of a single sensor is damaged, it may put higher voltage on the data cable, preventing all other sensors to communicate
Price:
SDI-variant: 200 € (2021, Germany)
Links
Manufacturer:
Projects that used the above equipment: