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Description  more details

The Stevens Hydra Probe II soil sensor is an in-situ soil probe that measures 21 different soil parameters simultaneously with digital output. The Hydra Probe II soil sensor instantly calculates soil moisture, electrical conductivity/salinity, and temperature as well as supplying voltage outputs for research applications.

Stevens also offers an "Extended Temperature Range" version of the Hydra Probe II soil sensor which can measure down to -22° F (-30° C) for research, cold-climate, high-altitude, arctic applications or any other measurement situation where there will be significant below-freezing ground temperatures.

For more information on soil sensors such as the Hydra Probe II soil sensor and monitoring information, please visit Soilsensor.com.


  • Instantly measure soil moisture, conductivity/salinity, temperature, and many other parameters
  • Optimize soil analysis and watering
  • Enables measurement of native (undisturbed) soil
  • Over ten years of field proven science
  • Measure flow of water from topsoil to sub-root zone
  • Performs well in high-salinity soil
  • Easier monitoring of remote sites
  • Review real-time soil data and trends from the office


  • Instantaneous sensor response
  • Serial addressable: multiple units on one cable
  • Maintenance-free
  • Easily linked to wireless systems
  • No calibration required for most soils
  • Compatible with most data logging systems
  • Digital or analog output
  • Compact, rugged for years of in-soil use - backed by a five-year performance warranty from Stevens

Hydra Probe data add science to soil management for better understanding of soil conditions. The result is optimized analysis of soil for scientific study and for enhanced quality and yield of turf and crops.

Hydra Probe sensing technology has been deployed over 10 years by the USDA and is used by NASA for ground truthing of satellite-based soil imaging.

A compact, rugged design with potted internal components makes the Hydra Probe easier to deploy than competing sensors and ideal for remote and harsh conditions. Durable construction makes it possible for the units to remain in the field for many years, maintenance-free.

The defined sensing area allows accurate measurements in regions where there are strong soil moisture gradients, such as near the soil surface. Response time to changing soil conditions is immediate, and calibration is as simple as selecting a soil type (sand, silt, loam or clay).

Each Hydra Probe is serial addressable, allowing for multiple sensors to be connected to any RS485 or SDI-12 data logger via a single cable. Sensor data can also be sent directly to a radio modem through a small RS485-RS232 converter.

The Hydra Probe uses an electromagnetic signal propagated from the center tine of the probe to measure multiple parameters. The voltages recorded depend on the type of electrical properties of the soil. On-board software converts the raw voltage to standard units of measurement for each parameter. With a standard database or spreadsheet, managers can view real-time soil snapshots or long-term soil trends.

Applications include:

  • Long/short-term soil monitoring
  • Spot checking of soil
  • Golf green management
  • Sports turf management
  • Precision agriculture/fertigation
  • Geotechnical measurement
  • Weather/climate studies
  • Watershed management
  • Flood control forecasting
  • Forest tinder conditions

Hydra Probe II Technical Specifications


Measurements Range Accuracy
Dielectric Constant 1 to 80 where:
1 = air
80 = distilled water 
± 1.5% or 0.2 whichever is typically greater
Soil Moisture for inorganic and mineral soil From completly dry to fully saturated ± 0.01 WFV for most soils
± 0.03 max for fine textured soils*
Conductivity 0.01 to 1.5 S/m ± 2.0% or 0.005 S/m whichever is typically greater 
Temperature -10° to +55° C

Extended temperature range probe available
-30° to +55° C, please contact Stevens for details 
± 0.3° C

*Accuracy may vary with some soil textures

Hydra Probe Electrical Operation

  SDI-12 RS-485
Electrical 9-20 VDC 9-20 VDC
Communication Protocol SDI-12 Standard v1.2 Custom or open spec
Maximum Suggested Cable Length 60 meters / 197 ft 1,219 meters / 4,000 ft
(max. non-spliced 304.8 m / 1,000 ft )
Cable 3-wire: power, ground, data 4-wire: power, ground, com+, com-
Power <1 mA idle /  30 mA active <10 mA idle / 30 mA active
Baud Rate 1200 9600
Intersensor Variability ± 0.012 WFV
(Θ m3 m-3)
± 0.012 WFV (Θ m3 m-3)

Hydra Probe Environmental Parameters:

  • Operating Temperatures:
    • In soils: freezing to +55° C
    • Standard temperature probe range: -10° C to +55° C
    • Extended temperature probe range: -30° C to +55° C
  • Storage Temperatures: -40° C to +55° C
  • Water Resistance: tolerates continuous full immersion
  • Cable: 22 gauge, UV resistant, direct burial
  • Vibration and shock resistant with potted components in PVC housing and 304 grade stainless steel tines


Hydra Probe Physical Parameters:

  • Length: 4.9 in (12.4 cm)
  • Diameter: 1.6 in (4.2 cm)
  • Sensing Volume (Cylindrical region)
    • Length: 2.2 in (5.7 cm)
    • Diameter: 1.2 in (3.0 cm)
  • Weight: 200 g (cable: 0.08 kg/m)


Hydra Probe Measurement Parameters:

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


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