SMT100 Soil Moisture and Soil Temperature Probe

The SMT100 soil moisture probe uses Time Domain Transmission (TDT) technology. The SMT100 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 SMT100 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 SMT100 utilises a ring oscillator to transform the signal’s travel time into a frequency.

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

• SMT100 SDI-12 (SDI-12 interface).
• SMT100 analogue voltage output (0-10V or other voltage ranges on request).
• SMT100 RS-485 (ASCII, MODBUS, TBUS)
• SMT100 4-20mA. Please note: the 4-20mA version can only measure soil moisture (not temperature)

SMT100 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 SMT100 with out sensor-specific calibration.

Soil volumetric water content (VWC):	Using factory calibration up to ± 3% (VWC) in mineral soils with moderate salinity from 0 to 50% VWC Using medium specific calibration up to ± 1% (VWC)
Temperature:	Typical ± 0.2°C, max. ± 0.4°C over full range (analog version ± 0.8°C)
Resolution	0.1% volumetric water content or better 0.01°C or better (analog version 0.2°C)
Measurement Range:	0 to 60% volumetric water content (up to 100% volumetric water content with limited accuracy) Temperature: -40 to +80°C (analog version -40 to +60°C)
Digital output options	RS-485 with TBUS RS-485 with Modbus RS-485 with ASCII SDI-12
Analogue Outputs	0 – 10 V (other voltage ranges on request) 4-20 mA (only volumetric water content)
Power	4-24 V DC, up to 40 mA during measurement (analogue version 12 – 24 V DC for 0 – 10 V output)
Measurement time	digital versions: less than 50 ms analogue versions: less than 500 ms
Cable length	10m
Sensor dimensions	18.2 cm x 3 cm x 1.2 cm

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