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.