Soil Moisture Measurement using TDT Technology

Dear Customer,

Accurate, Robust, Durable and Low Power, the SMT-100 TDT soil moisture sensors are ideal for use in remote, data-logging settings with continuous data collection via Internet of Things (IoT).

Soil Moisture Using Time Domain Transmission (TDT)

The SMT-100 soil moisture sensor uses Time Domain Transmission (TDT) technology. The SMT-100 measures the volumetric water content and the soil temperature. With a robust design & manufacture and low power use, it is typically used for long-term observations and is well suited to Internet of Things (IoT) applications.

SMT-100 combines the advantages of the low-cost capacitance sensor system with the accuracy of a Time Domain Reflectometry (TDR) 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 Volumetric Soil Water (VSW%) value independent of soil type. Therefore a site-specific calibration is rarely needed. Exceptions are specific situations such as substrates used in greenhouses.

Measurement Frequencies Affected by Salinity

Higher frequencies mean better accuracy and less influence of disturbing factors like soil texture, salinity & temperature. SMT-100 has a range between 150 MHz and 300 MHz, which is at least the double frequency compared with capacitance sensors.

The SMT100 is available in different outputs:

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

You can also use the SMT-100 in a Network using a Sensor-Node Integrated Package (SNiP):

The SNiP-SMT is a ‘Sensor Node Integrated Package’ for LoRaWAN or CAT-M1 communication of real-time soil moisture measurement for continuous soil monitoring.

The base SNiP-SMT integrates 1x S-NODE and 1x SMT-100 soil moisture sensor to a site’s unique network, communication and power requirements. The S-NODE can support an added 5x SMT-100 devices (Total: 6x SMT-100 sensors).

Typical applications taking advantage of IoT SNiPs include irrigation management of orchards, permanent plantings, sporting fields, public gardens and greenhouses.

See Further Specifications on the S-NODE in the SNiP.
See Further Specifications on the SMT-100 Sensor.

SMT-100 Instruments in Research

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 on Sensors using TDT Technology:

Bogena, H. R., Huisman, J. A., Schilling, B., Weuthen, A., & Vereecken, H. (2017). Effective Calibration of Low-Cost Soil Water Content Sensors. Sensors, 17(1), 208. https://doi.org/10.3390/s17010208

In the following Soil Moisture monitoring paper, more than 400 SMT-100 sensors were used:

Berthelin, R., Rinderer, M., Andreo, B., Baker, A., Kilian, D., Leonhardt, G., Lotz, A., Lichtenwoehrer, K., Mudarra, M., Padilla, I. Y., Pantoja Agreda, F., Rosolem, R., Vale, A., & Hartmann, A. (2020). A soil moisture monitoring network to characterize karstic recharge and evapotranspiration at five representative sites across the globe. Geoscientific Instrumentation, Methods and Data Systems, 9(1), 11–23. https://doi.org/10.5194/gi-9-11-2020

Another example of the efficiency of these probes has been shown in the following study in permeable pavements and green spaces (soil moisture and temperature monitoring):

Schaffitel, A., Schuetz, T., & Weiler, M. (2020). A distributed soil moisture, temperature and infiltrometer dataset for permeable pavements and green spaces. Earth System Science Data, 12(1), 501–517. https://doi.org/10.5194/essd-12-501-2020

We look forward to hearing from you in the future,

ICT International