Archived: Using the Stevens Hydra Probe with NI Wireless Sensor Networks (WSN)

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This document describes the use of the Stevens Water Monitoring Systems Hydra Probe with the NI Wireless Sensor Networks (WSN) system for wireless soil moisture measurements. This document is one in a series of documents describing how to use specific sensor products with the NI WSN system to wirelessly enable a variety of applications, such as environmental monitoring, climate studies, resource monitoring, etc. For more information on using other sensors with the NI WSN system, please refer to the WSN Sensor Solutions


Stevens Water Monitoring Systems, Inc Hydra Probe

Stevens Water Monitoring Systems, Inc. is a manufacturer of sensor systems that help optimize management of water resources. The Stevens Hydra Probe is a capacitive sensor capable of measuring the soil moisture, conductivity, temperature, and the soil’s dielectric constant. These measurements can then be used to analyze a wide range of soil conditions, including the soil’s salinity and moisture content.

Some applications for the Hydra Probe include weather and climatological monitoring, water shed management, irrigation monitoring and control, management of golf course greens and sports Turf, and maintenance of wine vineyards. Also, Hydra Probe applications include land slide studies, erosion, predicting weather and flood warning, crop quality and yield optimization, and soil remediation. 

Figure 1: The Stevens Hydra Probe for soil moisture measurement

The Hydra Probe outputs an electromagnetic radio wave at 50 MHz and measures at high frequencies its reflections off the surrounding soil. The Hydra Probe then uses Stevens patented algorithms to convert the signal response of the standing radio wave into the dielectric constants of the sand, silt, clay and loam soil in which it is placed. The complex dielectric permitivity is related to the soil’s electrical capacitance and electrical conductivity.

The probe component of the Hydra Probe consists of a tine assembly made up of four metal rods that extend out of the base plate of the sensor. These tines are marine grade stainless steel, and act as the radio wave guide. Electromagnetic waves at a radio frequency are transmitted and received by the center tine.  When in use, the Hydra Probe is buried in the soil of interest on its side, with the tines parallel to the ground surface.  The measurement region is the soil in the cylindrical volume that resides between the stainless tines. 

The sensor head or body of the Hydra Probe contains all of the electrical components. The outer casing is ABS and the internal electronics are permanently potted with a rock-hard epoxy resin, both of which protect the internal electrical circuitry from the corrosive and reactive properties of soil in which the sensor is placed.

The Stevens Hydra Probe is sold in two models: the analog Hydra Probe which outputs four analog voltages and  the Hydra Probe II, a digital version which incorporates a microprocessor with SDI-12 and RS-485 serial outputs.   In this paper, we discuss the analog output model, which has the advantage of a faster measurement rate. This could be useful in applications where you need to take measurements several times a second, for instance measuring the hydrology of sands in the surf zone between waves.

When working with the analog Hydra Probe, a software algorithm is used to convert the returned analog voltages to the following soil measurements: the real and imaginary dielectric constant, temperature, temperature corrected real and imaginary dielectric constants, water content, soil salinity (indicative of nitrate levels), soil conductivity, temperature corrected soil conductivity and temperature corrected soil water conductivity.

Wireless Soil Moisture Measurement

By combining the Hydra Probe with the NI Wireless Sensor Network (WSN), you can easily build and deploy a distributed wireless soil monitoring system.  The NI WSN-3202 voltage node can remotely read the four analog voltages returned by the Hydra Probe, and transmit them wirelessly back through the NI Wireless Sensor Network to your PC for calculation and analysis of the parameters of interest. This allows remote long and short-term soil monitoring.

The addition of multiple WSN-3202 voltage nodes facilitates the use of multiple Hydra Probes.   Since the inter-sensor variability of the Stevens Hydra Probe is very low, a direct comparison of data from multiple probes in a soil column or in a watershed can be done. A number of Hydra Probes can be buried at different depths to provide a comprehensive image of the soil moisture and conditions on different planes. Adding WSN voltage nodes to the system also allows a user to incorporate a number of other environmental sensors for complete weather or environmental applications.

Connecting the Hydra Probe to the NI WSN-3202 Node

The Stevens Hydra Probe can be monitored with the NI WSN-3202 voltage input module, which can acquire all four analog outputs of the analog Hydra Probe.  The Hydra Probe requires a power supply of 7 – 30 Vdc, with a maximum current draw of 40 mA.  Because this exceeds the power available on the sensor power output of the WSN-3202, you must provide an external power supply, such as a solar panel or external battery.

The Hydra Probe sensor cable contains 7 color coded 18 AWG copper wires and a silver shielded cable drain wire.  The four voltage data wires, V1, V2, V3, and V4, need to be wired into four analog input channels of the WSN-3202, as shown in Figure 2 below.  The external power supply is connected to the red and black wires.  The black and yellow ground wires may be connected and grounded together, or grounded separately as shown in the diagram of Figure 2.

The three  voltage outputs VI, V2, and V3 will range from 0 and 2.5 V while V4 will be between 0.1 to 0.8 V. You will therefore want to select an appropriate measurement range when configuring the WSN-3202 for data acquisition.  These 4 voltages will then need to be processed in software by a series of algorithms to obtain the soil parameters of interest. This is discussed in the next section.

Figure 2.  Connecting the Hydra Probe to the NI WSN-3202


 Hydra Probe Cable LeadConnection
BlackPower Ground
RedPower 7-30 VDC (12 VDC Ideal)
BlueV1 Range 0-2.5 V
BrownV2 Range 0-2.5 V
GreenV3 Range 0-2.5 V
WhiteV4 Range 0-1 V
Yellow & Silver Drain wireReference Ground

Table 1.  Hydra Probe Wire Labels

Programming NI WSN for use with the Hydra Probe

Using LV on a host PC with the NI WSN-3202 with the Hydra Probe

The LabVIEW Project Explorer Window can be used on a host computer to configure the WSN-3202 node for acquisition of the Hydra Probe's four analog voltages.  Right click on the node in the Project Explorer to configure its Properties. Under the Channels tab, configure all channels for Analog Input and a Range of -5 to 5 Volts, since the four analog voltages are ranged from 0-2.5 V. Under the Node tab, set the node sample interval (seconds) hardware configuration to an interval appropriate for your application.  The higher the sample interval value, the less often the WSN-3202 will take a reading from the Hydra Probe. 

You can read the Hydra Probe’s voltages into LabVIEW by reading the current value of the shared variables available from the NI WSN-3202, seen as AI0 through AI3, in the image below. LabVIEW code can then be added to call the Stevens Hydra.dll. This DLL is a proprietary function library which will take the 4 analog voltages and process them into the complex soil moisture readings and meaningful data. For convenience, this function library had been packaged into a custom subVI, which also takes the soil type an input variable.  Refer to the link in the Related Links section to download this example, along with the Stevens DLL.

Figure 3.  Reading the voltage of the Hydra Probe in LabVIEW

Using LabVIEW WSN Embedded Programs on the NI WSN-3202 with the Hydra Probe

With LabVIEW WSN and programmable version of the WSN-3202, you can download and run LabVIEW VIs on the WSN node for local data processing and control.  For example, you could add custom data filtering, customized triggering or event detection, or perform local alarming in the node itself.  Another possible use is to utilize a digital output of the WSN-3202 to connect and disconnect the external power supply to the Hydra Probe, under LabVIEW WSN control, in order to minimize power draw by the Hydra Probe.

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