Soil Moisture Measurement Using SDI-12 Sensors

RTU Performs In-Situ Data Collection


Application summary: The Infinite ADU-500 RTU Data Logger can collect weather and soil moisture measurements on a remote site, powered only by a D-size battery. Measurements are transmitted via FTP while alerts are sent via SMS.

Soil Monitoring Requirements:

Soil moisture measurements are important for a number of applications and for a variety of different reasons. Applications include landslide studies, erosion, water shed studies, climate studies, predicting weather, flood warning, crop quality and yield optimization, irrigation, and soil remediation, naming just a few.

Measuring soil moisture is important in agriculture to help farmers manage their irrigation systems more efficiently. Not only are farmers generally able use less water to grow a crop, they can also increase yields and the quality of the crop by better management of soil moisture during critical plant growth stages.

To this end, in-situ soil electrical conductivity monitoring is key to these results because the salinity levels in soil moisture can have dramatic effects on crop health and yields. Over time, agricultural soils may become sodic or saline, and this may dramatically affect crop health and yields. However there are techniques that can remove the sodium to improve soil quality and increase crop production.

The main methods used to measure soil moisture content and electrical conductivity are: Time Domain Reflectometry (TDR), Time Domain Transmissometry (TDT), Coaxial Impedance Dielectric Reflectometry (CIDR) and Frequency Domain Reflectometry (FDR). Most soil sensors use the SDI-12 serial protocol, allowing users to connect multiple sensors to a data logger via a single cable.

RTU Datalogger Application:

The Infinite ADU-500 Autonomous RTU Data Logger, a D-size battery powered RTU/Data Logger, can address several SDI-12 soil sensors for a total of 48 measurement parameters (channels). Powered only by the internal 3.6V Lithium Thionyl battery, it can provide 12V excitation for all connected soil sensors. The restriction for the total current draw of the SDI-12 bus is:

(n-1)*IIDLE+IACT < 250mA (at 12V)


n: Nr. of sensors

IIDLE: Sensor idle current (quiescent current)

IACT: Sensor active current (during measurement)

Case study:

In the following example, ADU-500 is used to measure and log air temperature, humidity, precipitation and soil moisture. The ultra-low power HT200 sensor is used to measure air temperature and humidity. A tipping bucket rain gauge, connected to the unit digital counter, is used to for the precipitation measurement. SDI-12 soil sensors (6 pieces) are connected to the SDI-12 port, measuring soil moisture, temperature and conductivity at various soil depths. The ADU-500 RTU performs data collection according to the selected sampling rate (e.g. every 15 minutes). The sampled data is logged and sent to an internet server via FTP, according to the selected sending rate (e.g. every 6 hours). On alarm events, alert SMS are sent to predefined users.

The Lithium Thionyl battery lifetime is calculated for different sensor types.


Air temperature: [C°, -40-50°C

Air humidity: [% RH, 0-100%

Rainfall [mm

Parameters per SDI-12 sensor: Soil Moisture [wfv, Soil Conductivity [dS/m, Soil Temperature [C°

# of Recorded Channels: 21

Sampling Interval: 15 minutes

Logging rate: 1 (every sample = 15 minutes)

Sending rate: 24 (every 24 logs = 6 hours)

For more info on the Infinite ADU-500 RTU Data Logger, additional Infinite dataloggers, or to find the ideal solution for your application-specific needs, contact a CAS Data Logger Applications Specialist at 800-956-4437 or visit our website at

Contact Information:

CAS DataLoggers, Inc.

8437 Mayfield Rd.

Chesterland, Ohio 44026



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