Meet the HydraProbe. A rugged soil sensor with patented technology to measure the three most significant soil parameters—moisture, electrical conductivity* and temperature.
The HydraProbe is the most scientifically researched soil sensor available and is depended on by the USDA, NOAA, farmers, leading irrigation companies, and universities for over 25 years. It has been engineered to handle the terrain you want to measure and provides data you can trust year after year.

* Select models only.

The Science 
Behind HydraProbe

HydraProbe was originally developed by the physics department at Dartmouth College. It’s “dielectric impedance” measurement principle differs from TDR, capacitance, and frequency soil sensors by taking into account the energy storage and energy loss across the soil area using a 50 MHz radio frequency wave.

Unlike other soil sensors, this unique, patented method separates the energy storage (real dielectric permittivity) from the energy losses (imaginary dielectric permittivity). Complex mathematical computations performed by an onboard microprocessor process the reflected signal measurements to accurately determine the soil’s dielectric permittivities—the key parameters behind the soil moisture and bulk EC measurement.

The HydraProbe’s detailed mathematical and signal characterization of the dielectric spectrum helps factor out errors in the soil moisture measurement such as temperature effects, errors due to salinity, and soil type. Low inner-sensor variability means there is no need for sensor-specific calibrations.

This method has passed t­he most rigorous scientific peer review from dozens of journals such as t­he Vadose Zone Journal, American Geophysical Union, and T­he Journal of Soil Science Society of America.


Strong, non-bending, non-corrosive stainless steel tines


Fully potted electronics—immersible in water.

Up to 10 year warranty

5 years on the STANDARD version, 10 years on the PROFESSIONAL and PROFESSIONAL-ET


Durable 18 gauge, UV-resistant high-density polyethylene cable can remain buried or be exposed to the elements.

Maintains accuracy for years with no calibration.

Patented Sensor Technology

HydraProbe uses unique “Coaxial Impedance Dielectric Reflectometry” to provide consistent long-term accuracy of moisture, salinity and temperature in any soil type. This also provides low inter-sensor variability, so every sensor measures the same without the need to calibrate.






Continual, long-term data without calibration.


Consistent research-grade accuracy every season, every location.


Forget calibrating, ignore the soil type. Just set it and forget it.


Durable stainless steel tines, fully potted components and a 5-year warranty.


Continual, long-term data without calibration.

  • Stable—no sensor drift, ensuring continual accuracy.
  • Patented technology that accurately measures moisture and electrical conductivity permits more accurate optimization of watering and fertilization than with just moisture.
  • Depended on by the USDA, NOAA, leading irrigation companies, and many universities for over 20 years. Used by NASA for ground truthing of satellite-based soil imaging.
  • Soil moisture calibration has been rigorously peer-reviewed, making it one of the most trusted soil sensors available.


Consistent research-grade accuracy every season, every location.

  • Unparalleled spatial and temporal measurement consistency. No sensor-to-sensor variations across locations, seasons, soil types or moisture range.
  • Instant measurement of the 3 most significant soil parameters simultaneously—moisture, salinity and temperature.
  • Unlike most TDR or capacitance-based sensors, HydraProbe is less sensitive to changes in temperature, salinity, and soil mineralogy.


Forget calibrating, ignore the soil type. Just set it and forget it.

  • Repeatable accuracy and stability without the need for calibration in most soils.
  • Digital sensor using the SDI-12 protocol—no setup, just connect to data logger. Compatible with any SDI-12 capable data logger.
  • Zero maintenance required.


Durable stainless steel tines, fully potted components and a 5-year warranty.

  • Can remain in-situ indefinitely, or relocated and redeployed without worry.
  • Ideal for remote locations, harsh environments and applications where data is critical.
  • Enables measurement of native (undisturbed) soil, even hard-packed clay.
  • Industry-leading 5-year warranty.


3 New Models


  • VWC (% Moisture)
  • Temperature
  • 3 soil calibrations
  • 5-year warranty


Everything the STANDARD model has plus:

  • Electrical Conductivity (EC)
  • Thermal compensation
  • Pore water EC
  • 5 soil calibrations
  • 10-year warranty
  • NIST Traceability


Everything the PROFESSIONAL model has plus:

  • Extended temperature range

Used in more water supply forecast and climatological networks than any other soil sensor

Prevalence of Soil Measurement Technologies in Soil Moisture Networks

HydraProbe Installations Worldwide - Dec. 2016

1-20 21-100 101-300 301-1,000 1,001-5,000 5,001-20,000 >20,000

The HydraGO lets you take HydraProbe to go.

Take soil measurements anywhere, without the effort or expense of setting up a permanent soil monitoring system. Your smartphone communicates wirelessly with the HydraGO using Bluetooth.

Simply insert the probe into the soil, and tap on the “Sample” button in the app. The location of each measurement is recorded along with the soil measurement data. All data can be saved and emailed as a .CSV for analysis in Excel.

Technical Specifications





Real dielectric permittivity (isolated) ± 0.5% or ± 0.2 dielectric units1 to 80 where 1 = air, 80 = distilled water0.001
Soil moisture for inorganic & mineral soil± 0.01 WFV for most soils

± 0.03 max for fine textured soils*
From completely dry to fully saturated (from 0% to 100% of saturation)0.001
Bulk electrical conductivity± 2.0% or 0.02 S/m whichever is typically greater0 to 1.5 S/m0.001
Temperature**± 0.3°C-10°C to +60°C0.1°C
Inter-sensor variability ± 0.012 WFV (θ m3 m-3)n/a




Power supply9-20 VDC9-20 VDC
Power consumption1 mA idle / 10 mA active for 2 seconds during duty cycle10 mA idle / 30 mA active
Cable3-wire: power, ground, data4-wire: power, ground, com+, com-
Max. cable length60 m (197 ft.)1,219 m (4,000 ft.) Non-spliced: 304.8 m (1,000 ft.)
Baud Rate12009600
Communication protocolSDI-12 Standard v. 1.2Custom or open spec
AddressingSerial; allows multiple sensors to be connected to any RS485 or SDI-12 data logger via a single cable.


Operating temperature range
  • Standard temperature probe range: -10°C to +60°C
  • Standard extended temperature probe range: -30°C to 60°C**
  • Extra extended temperature probe range: -40°C to 65°C**
Storage temperature range-40°C to +65°C
Water resistanceTolerates continuous full immersion
Cable18 gauge (22 gauge for RS-485 version), UV resistant, direct burial
Vibration and shock resistanceExcellent; potted components in PVC housing and 304 grade stainless steel tines


Length4.9” (124 mm)
Diameter1.6” (42 mm)
Optional slim housing version available: 1.4" (35.8 mm)
Weight7 oz. (200 g)
Optional slim housing version available: 6.5 oz. (184 g)
Cable weight0.86 oz/ft (80g/m)
Sensing volume (cylindrical region)Length: 2.2” (5.7 cm)
Diameter: 1.2” (3.0 cm)

Measurement parameters

1Voltage 1
2Voltage 2
3Voltage 3
4Voltage 4
5Voltage 5
6Soil Temperature in Celsius
7Soil Temperature in Fahrenheit
8Water fraction by volume
9Loss Tangent
10Soil Conductivity (temperature corrected) in Siemens / meter
11Real dialectric permittivity
12Real dielectric permittivity (temperature corrected)
13Imaginary dialectric permittivity
13Imaginary dialectric permittivity (temperature corrected)
15Soil conductivity in Seimens / meter
16Diode Temperature in Celsius
17Saved for future development
18ADC Reading 1
19ADC Reading 2
20ADC Reading 3
21ADC Reading 4
22ADC Reading 5

* Accuracy may vary with some soil textures.

** For standard extended temperature range and extra extended temperature range probes, contact us to inquire.


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Download the Stevens Soil Resource Guide

Written by the soil experts at Stevens, our soil resource guide contains a wealth of information and will benefit anyone involved with soil. Whether you’re a soil scientist, a farmer or a soil researcher, this 52 page book is a fantastic reference and source of up-to-date theories, practices and advice. 


Soil Geomorphology

Soil Properties

Salinity / Electrical Conductivity (EC)

Dielectric Permittivity

Soil Monitoring Applications

Soil Moisture and Irrigation

Soil Sensor Technologies

Soil Sensor Calibration

Sensor Accuracy

…and much more!

Soil Resource Guide Download
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Scientific Studies

Title Main Author Pub. Date Jornal Reference Application
Dielectric Loss and Calibration of the HydraProbe Soil Water Sensor Seyfried, M. S. 2005 Seyfried, M. S., L. E. Grant, E. Du, and K. Humes, Dielectric Loss and Calibration of the HydraProbe Soil Water Sensor Derivation of the HydraProbe's general soil moisture calibration
Estimating root zone soil moisture at distant sites using MODIS NDVI and EVI in a semi arid region of southwestern USA Schnur, M. T. 2010 Ecological Informatics. doi:10.1016 / j.ecoinf.2010.05.001 Using HydraProbe soil sensor to assess regional effects on vegetation and root zone soil moisture in arid lands.
The NOAA Hydrometeorology Testbed Soil Moisture Observing Networks: Design, Instrumentation, and Preliminary Results Zamora, R. J. 2011 Journal of Atmospheric and Oceanic Technology, 28, 1129-1140. doi:10.1175/201OJTECHA1465.1 Using HydraProbe to forecast floods and assess flood risk.
Evaluation of Lichtenecker's Mixing Model for Predicting Permittivitty of Soil at 50 MHz Leao, T. P., E. P. 2015 American Society of Agricultural and Biological Engineers, 58 (1), 83-91. doi:10.13031/trans.58.1 0720 Dielectric Mixing and dielectric permittivity of heterogeneous materials.
Soil Moisture for Hydrlogical Applications: Open Questions and New Opportunities Brocca, L. C. 2017 Advances in Hydro-Meteorological Monitoring, Special Issue of Water, 9 (140). doi:10.3390/w9020140 Soil moisture and its effect on climate, drought and regional weather.
Climate Models Predict Increasing Temperature Variability in Poor Countries Bathiany, S. V. 2018 Science Advances, 4(5). doi:10.1126/sciadv.aar5809 Using soil moisture measurements to make improved climate models.
Incorporating Antecedent Soil Moisture into Streamflow Forecasting Abdoul Oubeidillah 2019 Hydrology 2019, 6(2), 50 Monitoring soil moisture to improve streamflow predictions.
Synthetic Aperture Radar (SAR) Compact Polarimetry for Soil Moisture Retrieval Amine Merzouki, Heather McNairn 2019 Remote Sens. 2019, 11, 2227 Examining whether Compact Polarimetry can accurately estimate surface soil moisture over bare fields.