Connecting and Programming the EC-5 With the LI-8100A Soil CO2 Flux System

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LI-COR offers two soil moisture probes for use with the LI-8100A Automated Soil CO2 Flux System. The ECH2O model EC-5 (Decagon Devices, Inc., Pullman, WA, USA) provides a voltage signal that corresponds to the volumetric water content of the soil. This value is a ratio, often expressed as θv = m3 H2O/m3 soil. Among other things, soil moisture can help explain the magnitude of soil CO2 fluxes measured with the LI-8100A system.

Connection to the LI-8100A System

There are two options for soil moisture probe connection. If the probe will be used with a single chamber and the LI-8100A, it must have bare wire leads in order to connect to a voltage port on the Auxiliary Sensor Interface (V1, V2, V3, or V4). If the probe will be used with a multiplexed system, it must have an 8-pin connector to attach directly to any port on an 8100-104 Long Term Chamber (V2, V3, or V4). The part numbers for these options are shown below.

Part Number Model Connector Connects To
8100-202 EC-5 Bare Wire Leads Auxiliary Sensor Interface
8150-202 EC-5 8-pin Connector 8100-104 Long-term Chamber

The 8100-202 EC-5 soil moisture probe has three wires. The red wire is connected to V4(+), the unshielded ground wire is connected to V4 GND, and the white wire is connected to V OUT 5 VDC SWITCHED(+).

Programming the LI-8100A for Soil Moisture Measurements

In the LI-8100A Windows® Application Software, the programming for the soil moisture probes will need to be configured. In the measurement setup, select one of the voltage input tabs (V1- V4 for single chamber measurements; V2-V4 with the multiplexer) that corresponds with your sensor. Select “Soil Moisture Probe” for this port.

The EC-5 probe has a polynomial calibration when powered by a 5V excitation. This polynomial will not fit in the LI-8100A programming, but the final values can be calculated in SoilFluxPro after the data have been collected.

For this reason, it is best to enter a slope of 1 and an offset of 0. This will record the sensor output in volts, which can later be used to calculate volumetric water content in SoilFluxPro using Transform > Decagon_EC-5(X). SoilFluxPro uses the following equation:

1‑1VWC = -0.314V2 + 1.16V – 0.612

It is also possible to perform a soil-specific calibration to obtain a more accurate equation; refer to the original manufacturer (Decagon Devices) for more information. Switched vs. Constant Excitation When configuring a port, “Soil Moisture Probe” is selected instead of “General Purpose Input” for one reason. Since continuous excitation may cause the sensor to exceed regulations on electromagnetic emissions, the LI-8100A provides an excitation voltage to the sensor in pulses. This excitation pulse is turned on for 10 seconds at a time to allow the sensor to stabilize before taking a sample. When a measurement is active, the soil moisture sampling occurs while the chamber is closing. When a measurement is not active, a sampling occurs once every 70 seconds. The data from that measurement is then kept until the next pulse, or until the next chamber measurement. The assumption is that volumetric water content of the soil will not change drastically within that time.