Delta-T Devices Ltd.

- Model ML3 - Soil Moisture and Temperature Sensor


The new ThetaProbe adds temperature measurement to its class‐leading soil moisture measurement capabilities. The ML3 ThetaProbe makes it easy to take reliable, accurate soil moisture measurements. Simply insert the probe into the soil, connect your data logger or meter, provide 5‐14V DC at 18mA, and within seconds you can be measuring soil moisture. The ThetaProbe’s class‐leading ± 1% accuracy, stability and reliability have made it the preferred choice for thousands of researchers worldwide.

  • Soil moisture with ± 1% accuracy
  • New built‐in temp measurement 
  • New extendable cable system
  • Improved performance in saline soils
  • Buriable – IP68

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

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

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 5-15V DC at 20mA, 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 degrees C (non-flexing cables).

The ML3 includes a thermistor sensor for the measurement of soil temperature of soil temperature with soil moisture (probe must be fully buried).

Data logging and readout

The ML3 can be logged by any Delta-T data logger; it is compatible with any logger that can provide 5 – 15V DC excitation power and will accept the 0 – 1V 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).

Easy installation

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


The addition of a thermistor sensor to the ML3 enables the simultaneous logging of soil temperature with soil moisture at depth. A new cabling system makes it simple to extend or replace cables, offering a choice of 1m, 5m, 10m and 25m extensions. ML3 cables and connectors are buriable and environmentally protected to IP68.

The salinity response of the ML3 has been improved ‐ its output has been characterised at EC values up to 2000 mS.m‐1.

The temperature range has also been improved with tests demonstrating that the ML3 can operate down to ‐40 degrees C (non‐flexing cables).

  • 4‐rod arrangement
  • Replacement rods
  • Compact cylindrical shape

  • 3 rods are arranged around a central rod. This creates a defined cylindrical zone of measurement, 60mm long x approximately 30mm diameter.
  • Made of 3mm diameter, resilient, 304 austenitic stainless steel, with sharpened tips. Rods have threaded ends that screw into ThetaProbe body. The exposed rod length is 60mm.
  • The ThetaProbe has a 40mm diameter body, with threaded end. Extension tubes (0.5 and 1.0m) screw onto this thread. Case sealed to IP68. The overall length is 170mm.

  • Retains soil closer to central rod in case of drying and cracking (other designs, and particularly flat PCB sensors, don’t do this)
  • Measurements can be made close to the soil surface
  • Withstand repeated insertion in soil. Can be replaced at low cost if bent or damaged
  • Highly resistant to corrosion
  • Sharp, narrow rods minimise errors due to soil compaction by the rods
  • Easy to insert and remove from augered holes
  • Rapid attachment of extension tubes
  • Handy size for portable use
  • Rugged, waterproof and buriable

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 ThetaKitcontains 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.

Water content

  • Accuracy: ±0.01 m3.m-3
  • Range: 0 to 0.5 m3.m-3
  • Salinity range:
    • < 0.035 m3.m-3
    • 50 to 500 mS.m-1
  • Temperature
  • Accuracy: ± 0.5°C
  • Range: -20°to 40°C
  • Output: 0 to 1.0V differential
  • Power: 5 to 14V, 18mA for 1s
  • Sample vol.: ~60 x 30mm diameter
  • Size: 170 x 40mm diameter
  • Environmental: IP68, -40 to +70°C