How does the ThetaProbe work?

The ThetaProbe is a unique soil moisture sensor that measures soil properties using a standing wave measurement technique.

The probe generates a 100MHz sinusoidal signal which is applied to a specially designed internal transmission line that extends into the soil by means of an array of 4 stainless steel rods. The impedance of this array has two components; the dielectric constant of the soil and its ionic conductivity. The 100MHz frequency has been chosen to minimise the effect of ionic conductivity, so that the array impedance depends almost solely on the soil's dielectric constant. Because the dielectric constant of water (80) is significantly greater than other soil matrix materials (3 - 5) and air (1), the dielectric constant of soil is determined primarily by its soil water content.

The impedance of the array affects the reflection of the 100MHz signal and these reflections combine with the applied signal to set up a standing wave along the transmission line. Finally, the ThetaProbe detects the difference in the standing wave voltage at two points along the transmission line, and the analogue output corresponding to this is a sensitive and precise measure of soil moisture content.

Work published by Knight, Topp, Whalley, White, Zegelin and others, shows virtually linear correlation between the square root of the dielectric constant, and percent volumetric moisture content, (q ). This has been documented for many soil types.

Each ThetaProbe is calibrated during manufacture in media of known dielectric constant to make the probes interchangeable. The output signal is 0 -1V DC for a range of soil dielectric constant, S , between 1 and 32, which represents 0 to 0.5m³m³ volumetric soil moisture content for generalised mineral soils.

Further information

• ThetaProbe ML2: Principles of Operation and Applications J.D. Miller & G.J. Gaskin.
• Special Considerations When Measuring The Moisture Content of Clay Soils Using Dielectric Sensors Delta-T Application Note

The ThetaProbe has been jointly developed with the
Macaulay Land Use Research Institute (MLURI).