The graph above demonstrates the accuracy of TDR versus capacitance based sensors as salinity increases in the solution. In this study the sensors are submerged in water, thus WC should always be 100%. As the salinity of the solution increases, which results in an increase in EC, the accuracy of the Meter Group Teros 12 and Rika capacitance sensor declines precipitously. In contrast, the Growlink TDR sensor accurately reports WC despite the increase in salinity.
The two most common soil moisture sensors are capacitance based and TDR. A major drawback of capacitance based sensors is that the electrical conductivity of the soil throws off the capacitance measurements. Electrical conductivity of soil is also referred to as the salinity content of soil. This is even more of a drawback if you are feeding plants using a nutrient solution because nutrient solutions have a high salinity content. The higher the salinity, the less accurate the WC measurements become.
TDR stands for “Time-Domain Reflectometry”, and is a far superior technology when it comes to measuring water content. One of the greatest benefits to the TDR sensor is that propagation time is independent of the electrical conductivity of the substrate. This means that TDR sensors report the true volumetric water content independent of soil electrical conductivity.