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RTU Data Logging of SDI-12 Soil Moisture and Environmental Variables

Infinite Informatics Collects All Data Points on Location

Soil moisture content is an important parameter for a number of industries that have far reaching implications if the data gathered is not accurate and reliable.  A few of the industries in need of this data would be geologists studying landslide potential, civil engineers designing water sheds or the National Weather Center predicting floods.  All of these have impacts on human society but none are as important as agricultural studies designed to optimize crop quality and field yield so we can continue to feed the burgeoning population with the limited land and resources planet Earth has.

This Application Note is focused on how the Infinite Informatics ADU-500 Autonomous RTU Data Logger captures the required data through its SDI-12 sensor channels at a remote location without mains power or functioning network.  The ADU-500 will also transmit the data files via FTP and can send alerts via SMS when readings are outside of a desired range.

Soil Monitoring Requirements

Measuring soil moisture is important in agriculture to help farmers manage irrigation systems more efficiently allowing them to use less water while increasing yields.  When soil moisture data is compared to the environmental data farmers can predict how much water will be needed at any time to maximize plant growth during various stages of growth.  Most soil sensors use the SDI-12 serial protocol, allowing users to connect multiple sensors to a data logger via a single cable.

In Situ (in place) soil moisture sensors test electrical conductivity by passing a small electrical current between 2 electrodes essentially testing the salinity levels present.  These results are important because the level of salt in soil moisture has dramatic effects on crop health and yields because agricultural soils may become sodic (higher Sodium than usual). There are techniques that can remove the sodium to improve soil quality and increase crop production but identifying where these soil conditions are and when they occur are a challenge to say the least.

There are 3 main methods used to measure soil moisture content and electrical conductivity.  Time Domain Reflectometry (TDR) identifies the presence of various substances by sending out an electric signal and measuring how long it takes to receive the signal back.  Time Domain Transmissometry (TDT) is similar to TDR except that a pulse of electricity is used rather than a continuous signal from a closed loop. Frequency Domain Reflectometry (FDR) is similar to TDR and TDT except that the amplitude response of the sensor signal is measured and Coaxial Impedance Dielectric Reflectometry (CIDR) is an FDR sensor that measures the frequency response of the outgoing signal to determine conductivity.

RTU Data Logger Application Case study

15_adu500_soil_moisture_diagramIn 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 is connected to the digital counter for precipitation data, and 6 soil moisture sensors are connected to the SDI-12 port measuring soil moisture via conductivity at various soil depths. The ADU-500 RTU performs data collection according to the selected sampling rate every 15 minutes per the configuration program then the sampled data is logged and sent to an internet server via FTP at a pre-determined time. Alarm events generate alert SMS text messages that are sent to predefined users so they can take corrective action if necessary.

The Infinite ADU-500 is an autonomous data logger with RTU (remote telemetry unit) that can record data from up to 2 analog sensors and 3 digital sensors along with several SDI-12 sensors for a total of 53 measurement channels providing a flexible cost-effective platform for important remote data logging applications.  Powered only by the internal 3.6V Lithium Thionyl battery, it can provide 12V excitation for all connected SDI-12 sensors. The restriction for the total current draw of the SDI-12 bus is:

(n-1)*IIDLE+IACT < 250mA (at 12V)
where,
n: No. of sensors
IIDLE: Sensor idle current (quiescent current)
IACT: Sensor active current (during measurement)

The Lithium Thionyl battery lifetime is calculated for different sensor types below:

Manufacturer

Stevens

Decagon

  Acclima

Campbell Scientific

Soil sensor

Hydra Probe II

5TE / GS3

  SEN-SDI

CS650 / CS655

Type

CIDR

FDR

    TDT

TDR

Nr. of sensors

6

6

6

6

IIDLE [mA]

1

0.3

0.015

0.135

IACT [mA]

10

10 / 25

30

45

Warm up time [sec]

3

3

3

3

Measurement time [sec]

2

0.15

0.45

0.003

Battery Lifetime [Years]1)

1.2

5 / 4

2.5

6.6

1) Calculations are based on the sensor manufacturer’s specifications.