Highly accurate soil moisture sensor

Soil moisture sensor ML3 ThetaProbe

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)

Difference with old model ML2x

・Built-in temperature sensor with accuracy of ±0.5℃

・Can be extended in steps of 5, 10, and 25m using an extension cable (optional)

・The body color is white to reduce the effects of radiant heat.

・Reduce measurement error in soil with high salt concentration

Principle of operation of TheataProbe

ML3 is Delta-T’s unique soil moisture sensor based on the ADR method. The high frequency generated from the built-in 100MHz high frequency oscillator is sent to the stainless steel rod array in the center of the sensor through the specially designed analog board, and transmitted to the three arrays equipped around the sensor through the soil. The amplitude difference between the interference reflected waves caused by the difference in impedance between the array and the soil is output as a voltage, and the dielectric constant is measured.
ML3 is calibrated one by one with a medium of known dielectric constant at the time of manufacture, so it has high reliability, and its output is 0 to 1VDC for the dielectric constant ε range of 1 to 32. This corresponds to a volumetric water content of 0-0.5m3/m3 for typical mineral soils.

Readout unit & Data Loggers

HH2 (Readout unit)

Other Information

Lookup Table




Measurement Volumetric water content and soil temperature
Accuracy ± 0.01 m3.m-3 (1%)
± 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
Calibration formula V:voltage Mineral soil:
θ:m3/m3 Organic soil:
Temperature range Full accuracy over: 0 to 40°C 
Output 0 to 1.0 V differential 
Resistance for temp sensor 5.8kΩ〜28kΩ
Power requirement 5-14 V, ~18 mA for 1s
Output 0 to 1.0 V differential
Operation Environment -20℃〜+60℃
Overall: 158 x 40 mm diameter
Rods: 60 mm x 3.2 mm diameter (Rods are replaceable)
Weight: 0.1 kg (exc. cable)
Environmental IP68
Cable optional