Delta-T ThetaProbe Soil Moisture Sensor


Delta-T ThetaProbe

Purpose of instrument:-

The Delta-T ThetaProbe is used to measure the volumetric moisture content (qv) of the soil. This is defined by the equation:- 
              qv = (Vw/Vs) * 100%
where Vw is the volume of water and Vs is the total volume of the soil sample. A value of 0% indicates that the soil is completely dry soil, and 100% is pure water. Typically, for fully saturated soil, qv will be about 50%.

Principles of operation:-

The ThetaProbe works on the principle that the volumetric soil moisture content is related to the apparent dielectric constant (e) of the soil, with there being a linear correlation between qv and the square root of e. This relationship has been shown to be valid for many different soil types.

The probe consists of a waterproof housing, containing the electronics, with four stainless steel spikes attached to one end, which are inserted into the soil. A 100MHz sinusoidal signal is applied to the spikes, and changes in the impedance between them, due to changes in the dielectric constant of the soil, are detected. The probe then outputs an analogue voltage between 0 and 1 V DC, which is virtually proportional to the soil moisture content.

The precise relationship between soil moisture content and the square-root of e is dependent on the soil type, but after carrying out tests on the soil at Cardington, it was decided to use the manufacterers generalisied calibration coefficients for a mineral soil.

Further information on the ThetaProbes can be found on the Delta-T website: www.delta-t.co.uk

Sensor installation:-

The sensors were installed in the ground, by digging a large hole, and inserting the probes into the side wall of the hole at the desired depths. In this way, disturbance to the structure of the soil being measured, was kept to a minimum. The hole was then filled in, replacing the soil in the order that it came out, and the grass resown on top.