Delta-T Devices Ltd.

- Model ML3 thetaProbe - Soil Moisture Sensor


Highly accurate yet easy to use: With the ML3 ThetaProbe Soil Moisture Sensor it’s easy to make reliable, accurate soil moisture measurements. Simply insert the soil moisture probe into the soil, connect to your data logger or readout unit, provide 5-14 V DC at 18 mA, and within seconds you can be logging soil moisture. The ML3 ThetaProbe is robust, buriable and maintenance free (environmentally protected to IP68). It can be easily installed at depth into augered holes using the optional extension tubes. The ML3’s excellent salinity response has been characterised at EC values up to 2000 mS.m-1. It also has a wide operating temperature range, allowing it to be used at temperatures as low as -40 ºC (non-flexing cables). The ML3 includes a thermistor sensor for the measurement of soil temperature (probe must be fully buried).

  • Class leading ± 1% soil moisture accuracy
  • Can be buried or used as portable probe
  • Easy data logger connection (0 – 1 V DC)

With its 1% accuracy the ThetaProbe has set the standard for soil moisture measurement for more than 20 years.

The ML3 can be logged by any Delta-T data logger; it is compatible with any logger that can provide 5 – 15 V DC excitation power and will accept the 0 – 1 V output signal. For portable applications the ML3 connects to the HH2 Moisture Meter and they can be ordered together as the convenient ThetaKit – see below (please note that the HH2 Meter does not provide temperature readings).

The ML3 ThetaProbe is buriable and its cylindrical shape makes it easy to install in augered holes (45 mm diameter). The tough stainless steel rods can be replaced if bent or damaged. The threaded base of the sensor can be connected to optional 50 cm and 100 cm extension tubes.

Typical applications include irrigation, hydrology, civil engineering, soil water profiling, pollution monitoring and forestry. Because the ThetaProbe has a predictable saline response (which actually declines as salinity increases) it can be used even in very saline soils – up to 2000 mS.m-1.

The ThetaProbe can also be used in non-soil media.


Supplied in a convenient carry case, the ThetaKit contains everything needed to makes accurate spot readings of soil moisture content. The kit comes complete with a dedicated HH2 Moisture Meter for instant readout, an insertion tool for penetrating hard soils, and a set of spare replacement rods.

ThetaKits are an excellent choice for turf grass applications.

Please note that the HH2 Meter does not provide temperature readout or recording.

  • Measurement: Volumetric water content and soil temperature
  • Accuracy: ± 0.01 m3.m-3 (1%)
    • With soil-specific calibration
      • ± 0.5°C (0 to 40°C for temp sensor)
      • ± 0.75°C (-20 to +60°C for temp sensor)
  • Soil moisture measurement range:
    • Full accuracy over: 0 to 0.5 m3.m-3
    • Full range : 0 to 1.0 m3.m-3
  • Salinity range:
    • 50 to 500 mS.m-1
    • Salinity errors < 0.035 m3.m-3 from 0.05 to 0.4 m3.m-3. Can be calibrated up to 2000 mS.m-1
  • Temperature range: Full accuracy over: 0 to 40°C
  • Output:
    • 0 to 1.0 V differential: Corresponding to 0 to ~0.6 m3.m-3
    • Resistance 5.8Ω to 28Ω for temp sensor
  • Power requirement:
    • 5-14 V, ~18 mA for 1s
    • Minimum 5.5 V with 100 m cable
  • Environmental: IP68 (-40 to +70°C)
    • Sample volume:
      ~60 x 30 mm diameter
    • Sample volume is weighted towards soil immediately surrounding the rods
  • Dimensions and weight:
    • Overall: 158 x 40 mm diameter
    • Rods: 60 mm x 3.2 mm diameter
    • (Rods are replaceable)
    • Weight: 0.1 kg (exc. cable)
  • Sensor calibrations:
    • Individual sensors are interchangeable
    • Recalibration advised every 3 years (depending on use)
  • Soil calibrations: Generalised Mineral and Organic soil calibrations are supplied