Factory Calibration Settings

As the maximum water storage capacity largely depends on the soil type, the sensors come factory calibrated with a so called “air and water calibration”. An Air/Water calibration is a simple calibration method, suitable if the probe is used to monitor trends rather than absolute volumetric values.

If absolute volumetric values are required, Green Shield offers additional calibration methods.

Calibration

Volumetric water content, θ, is defined mathematically as:
where Vw is the volume of water and VT = Vsoil + Vvoid = Vsoil + Vwater + Vair is the total volume (that is soil volume + water volume + air space).

To simplify, below saturation, we can consider the total volume as constant and equivalent to the value of dry soil. Adding water to dry soil, in most cases, will not change the total volume. Water will fill the voids in between the soil particles. Once all voids are completely filled with water, the soil is saturated.

As an example, 10 dm3 of coarse, dry sand can be mixed with up to 4 litres of water without increasing the total volume. Consequently, the maximum volumetric water content is 0.4 or 40%.

The TBSSPP1 can be operated with different methods of calibration. The utilized calibration mode can be selected using an Extended SDI-12 command.

Air / Water Calibration

As the maximum water storage capacity largely depends on the soil type, the TBSSPP1 comes factory calibrated with a so called “air and water calibration”. This means that a measurement value of 0% corresponds with the sensor placed in air and a value of 100% corresponds with the sensor placed in water. This is a very basic calibration method, not taking into account any soil specific properties.

The dielectric constant of dry soil is higher than the dielectric constant of air consequently an air/water calibrated probe will deliver measurement results higher than 0% when placed in dry soil. As an example:

dry sand: 26%
dry volcanic scoria:    22%
dry potting soil: 15%

Nevertheless, for many purposes, when soil moisture monitoring is rather about observing trends than measurement of absolute values, an air/water calibration is sufficient.

The calibration is long term stable, however it can be repeated any time by placing the sensor in air and sending the extended SDI-12 commands (please see calibration sheets) for air calibration and then placing the sensor in water and sending the extended SDI-12 command for water calibration.

The table shows the measurement response of an air/water calibrated probe in sand compared to the true, absolute volumetric value.

Calibration

As the maximum water storage capacity largely depends on the soil type, the sensors come factory calibrated.

Learn more

Technical Specs

Green Shield uses state of the art technology and combines it with scientific information to give you full control.

Learn more

How to use Green Shield

Learn how to install and remove your censors.

Learn more