SMT-100

The SMT-100 soil moisture probes uses a 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.

SMT-100 combines the advantages of the low-cost capacitance 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 utilizes a ring oscillator to transform the signal’s travel time into a frequency. The resulting frequency (>100 MHz) is high enough to operate well even in clay soils. The SMT-100 measures the VSW% value (volumetric soil water) independent of soil type. Therefore a site-specific calibration is rarely needed. Exceptions are specific situations such as substrates used in greenhouses. This is not the case for capacitance sensors.

Measurement Frequencies Affected by Salinity

Higher frequencies mean better accuracy and less influence of disturbing factors like salinity (See picture above). The SMT-100 has a range between 150 MHz and 300 MHz, which is at least the double frequency compared with capacitance sensors but can still be offered at a competitive price and with other advantage (low power consumption, inexpensive, easy to install).

Then SMT-100 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 WITHOUT sensor-specific calibration. Capacitance/FDR sensors are very affected by those soil parameters and need constant in-situ calibration as well they are usually more fragile. That is why we generally suggest our customer not to use capacitance sensors but prefer TDT, TDR or Standing Wave ones.

The SMT100 is available in 3 different variants in regard to the interface:

• SMT100 SDI-12 (SDI-12 interface)
• SMT100 analog voltage output (0-10V or other voltage ranges on request)
• SMT100 RS-485 (ASCII, MODBUS, TBUS)

Compact, functional and robust: the PCB board based design allows for an economic design, and the blade shape facilitates installation. The case and cable are water sealed. The SMT-100 is connected to a logger or handheld for data management.

Advantages

• Frost resistant
• Reliable measurement
• Maintenance free
• Long lifetime (more than 8 years operation)
• Fast response
• Low influence of salinity and pH
• Cost efficient

There are many scientific papers referencing the SMT100. Of particular interest is paper from a well-known group of the Research Center Jülich in Germany: http://ictinternational.com/content/uploads/2019/10/sensors-17-00208-v2.pdf

Main benefits of the SMT100 are:

• High accuracy (see above mentioned paper)
• Accurate temperature sensor included
• Low sensor to sensor variability (see above mentioned paper)
• Exceptional durability (electronics is completely moulded in epoxy resin)
• High measurement frequency (150-300 MHz compared to about 70 MHz alternatives): Why is a higher frequency so important: less susceptibility to salinity induced measurement errors
• High quality polyurethane cable suited for long term operation in soil (microbe resistant, water resistant)
• Convenient standard cable length of 10m
• Very robust sensor blade
• Best price performance ratio to support large scale instrumentation and large scale wireless sensor networks
• Based on over 25 years of research and development experience in the area of electromagnetic moisture measurement

More recent references which may help:

https://www.geosci-instrum-method-data-syst.net/9/11/2020/gi-9-11-2020-discussion.html (in total 400 sensors in this paper)

https://www.earth-syst-sci-data.net/12/501/2020/

You can also use the SMT-100 in a preconfigured IoT system:

The SMT-100 is available in a preconfigured IoT (Internet of Things) setup for continuous real-time soil moisture measurement and monitoring.