Sensors that use the Time Domain Reflectometry (TDR) are somewhat similar way to FDR probes, but the mechanics behind the measurement system are different. TDR sensors use parallel rods, acting as transmission lines. A voltage is applied to the rods and reflected back to the sensor for analysis. The speed or velocity of the voltage pulse along the rod is related to the apparent permittivity of the substrate (Blonquist 2005-A). In relatively wet soil the velocity of the pulse is slower than in drier soil.
Examples of this sensor include the Campbell Scientific CS650 and the IMKO Trime-Piko 32.
TDR probes are repeatable and do not require a large amount of maintenance. Since soil dielectric properties are affected by salt content, their readings can be affected by salinity. However, many probes independently measure EC and use this to compensate for the effect of salinity on moisture readings. TDR probes respond quickly to varying soil moisture.
Similar to capacitance and frequency domain sensors, TDR sensors must have good contact with the soil, because any air gaps will lead to erroneous measurements. The measurement volume of TDR sensors depends on the length of the rods. The probability of air gaps forming during installation increases with longer rods. Additionally, longer rods will measure a greater length of the soil profile and moisture content can vary significantly from shallow to deeper depths making interpretation of data difficult.
TDR sensors should not be used in high saline soils or soils with high bulk electrical conductivity or high attenuation. In soils with high EC values, the voltage pulse is not reflected back along the rod and, therefore, is not measured. The pulse is attenuated beyond the length of the rod. TDR probes have been found to have errors in soils with EC values of 1.32 dS/m (McIsaac 2010).