The SMT-100 soil moisture probe uses Time Domain Transmission (TDT) technology. The SMT-100 determines the volumetric water content and the soil temperature. It has a broad measurement range, it is maintenance free and frost resistant. With a robust design & manufacture and low power use, it is typically used for long-term observations (8+ years continuous use) and is well suited to IoT (Internet of Things) applications. The SMT-100 combines the advantages of the low-cost capacitance soil moisture sensor system with the accuracy of a TDR (time domain reflectometry) system. Like a TDR, it measures the travel time of a signal to determine the relative permittivity εr of the soil, and like a capacitance sensor, it converts εr into an easy to measure frequency. The SMT-100 utilises a ring oscillator to transform the signal’s travel time into a frequency.  

About the sensor

The SMT-100 is available in different variants in regards to the interface:

• SMT-100d with SDI-12 interface.
• SMT-100a with analogue voltage output (0-10V or other voltage ranges on request).
• SMT-100rs with RS-485 interface (ASCII, MODBUS, TBUS)
• SMT-100mA with 4-20mA output. Please note: the 4-20mA version can only measure soil moisture (not temperature)

SMT-100 soil moisture probe measurements are much less affected by soil temperature, EC and texture in comparison to capacitance/FDR sensors. Sensor-to-sensor variability is moderate and accurate VSW% measurements are also possible with the SMT-100 with out sensor-specific calibration.