Product Information

Theta Probe ML3

The ML3 Theta Probe is a capacitive soil moisture sensor developed and produced by Delta T Devices. Besides the soil moisture it also measures the soil temperature. Its sealed plastic body is attached to four sensing rods which insert directly into the soil for taking readings. A waterproof plug connects to a choice of signal cables. Both extension cables and extension tubes can be used. The sensor can be installed horizontal as well as vertical, at the surface as well as in deeper soil horizons.

The ML3 is powered by a data logger. The soil moisture output signal of the sensor is a differential analogue voltage signal (0 … 1.0 V DC ≈ 0 … 60 % Volume) and the soil temperature is provided as a resistant signal. The data logger stores the input signals and converts the soil moisture using the supplied general soil calibrations. It can also be calibrated for specific soils.

Besides all Delta-T Loggers the ML3 may be connected to any customary differential logger. The manual provides a detailed description how to configure. Additional it can be operated as a mobile sensor using the Moisture Meter HH2. Both devices are available as ThetaKit. (Note: the HH2 does not display or store ML3 temperature readings.)

The ML3 is a stable working measuring instrument with a low conductivity and temperature response. To quantify the effect on the results a clear overview is given in the manual how the output changes with different salt content or increasing temperature. As the salinity increases the effect on the measuring value decreases. Thus, the sensor operates in soils with high salt content as well (with special calibration up to 2000 mS/m).

The ML3 determines soil moisture by measuring the relative permittivity (εr) of the soil. Being a capacitive sensor the capacity of the integrated capacitor is calculated. When the relative permittivity changes the capacity changes too. Using regression formulas, e.g. Topp and Davis, εr can be converted into soil moisture. Typical application fields of soil moisture sensors are: irrigation control, hydrology, civil engineering, monitoring of contaminants, agriculture and forestry.



  • high accuracy
  • soil specific calibration
  • low energy consumption
  • low sensitivity to salinity
  • operates in soils up to 500 mS/m (with special calibration up to 2000 mS/m)
  • adaptable cable system

Technical Specifications

Technical Specification

Volumetric Water Content

±1 % vol over 0 … 50 % vol and 0 … 40 °C using soil specific calibration

Measuring range:

0 … 100 % vol (over 50 … 100 % vol with reduced accuracy)

Conductivity:±3.5 % vol over 50 … 500 mS/m and 0 … 50 % vol

(2000 mS/m with soil specific calibration)

Output signal:

0 … 1 V differential (≈ 0 … 60 % vol)


Accuracy:±0,5 °C over 0 … 40 °C
Operating range:-20 ... +60 °C



Order Numbers + Accessories


Ordering Details:

Art.-Nr.:  115000              

FDR soil moisture measurement system with plug

Art.-Nr.:  115001 FDR soil moisture measurement system without plug




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