The robust and accurate HydraProbe PROFESSIONAL probe is the optimal choice for your soil measurement campaign of the parameters soil moisture, temperature and electrical conductivity. With the patented measurement principle of coaxial impedance dielectric reflectometry and the operating frequency at 50 MHz, the HydraProbe offers excellent accuracy, exceptional comparability of values between different probes (+/-1.2%), virtually no drift and allows soil measurements over a wide range of soil types and salinity. The 4 measuring rods made of hardened stainless steel are stable and corrosion resistant and provide a measuring volume of 50-75 cm³. The probes can be easily connected to all standard data loggers (SDI-12, RS-485, Modbus) or for mobile applications with HydraGO and the free app the data can be stored directly on your mobile device.

Measuring principle

Standing electromagnetic waves at a radio frequency of 50 MHz are reflected from the soil, and the return signal is processed to generate soil moisture related variables such as the real dielectric permittivity. A separate thermistor in the base plate of the probes measures soil temperature. The HydraProbe is less affected by salts and temperature than TDR or other FDR soil sensors because of the delineation of the dielectric permittivity and operational frequency at 50 Mhz.  

References

The HydraProbe Sensors are part of the standard equipment of the U.S. Climate Reference Network (USCRN), a systematic and sustained network of climate monitoring stations across the US: https://www.ncei.noaa.gov/pub/data/uscrn/documentation/site/sensors/soil/Descriptions/Soil_StevensHydra-SDI-12.pdf 

Background

The Stevens HydraProbe is different than other soil sensing methods. It characterizes the ratio of the amplitudes of reflected radio waves at 50 MHz with a coaxial wave guide. A numerical solution to Maxwell’s equations first calculates the complex impedance of the soil and then delineates the real and imaginary dielectric permittivity (Seyfried 2004, Campbell 1990). The mathematical model that delineates the real and imaginary component from the impedance of the reflected signal resides in the microprocessor inside the digital HydraProbe. These computations are based on the work of J. E. Campbell at Dartmouth College (Campbell 1988, Campbell 1990, Kraft 1988). 

The HydraProbe from an electric and mathematical prospective can be referred to ratiometric coaxial impedance dielectric reflectometer and works similar to a vector network analyzer at a single frequency. The term “ratiometric” refers to the process by which the ratio of the reflected signal over incident signal is first calculated which eliminates any variability in the circuit boards from one probe to the next. This step is performed on several reflections. The term “coaxial” refers to the metal wave guild that get inserted into the soil. It has three outer tines with a single tine in the middle that the both receives and emits a radio frequency at 50 MHz. “Impedance” refers to the intensity of the reflected signal, and “dielectric reflectometer” refers to a reflected signal that is used to measure a dielectric.

Basing the soil moisture calibration on the real dielectric permittivity instead of the apparent permittivity has many advantages. Because the HydraProbe separates the real and imaginary components, the HydraProbe’s soil moisture calibrations are less affected by soil salinity, temperature, soil variability and inter sensor variability than most other electronic soil sensors. 

• Brochure Stevens Water HydraProbe