“TDR is widely accepted as a standard real-time, in situ technique for determining soil and porous media water content owing to its ability to make relatively accurate permittivity estimates and to the exploitation of the significant permittivity contrast between water and other porous medium constituents. However, TDR applications may be limited due to high costs, user ability requirements, and problems when connecting TDR probes to long lengths of cable. The Acclima Digital TDT sensor has the potential to offer a more affordable alternative. The Acclima Digital TDT Sensor frequency bandwidth and permittivity estimates based on travel time measurements compare quite well to those of the Tektronix TDR and Campbell Scientific TDR100. The Acclima Digital TDT Sensor has the advantage over TDR in that signal transmitting and sampling hardware is located in the sensor head negating cable losses. TDT is also advantageous in that one-way travel time reduces signal attenuation in the sample (assuming sensor rods are the same length). Although the Acclima Digital TDT Sensor is presently geared for closed-loop irrigation control in turf grass where excavation is necessary for installation, refinement of the rod geometry for insertion (and perhaps conversion to a TDR measurement) will likely rank this TDT method alongside its TDR counterpart as an accepted laboratory and field standard for determining soil moisture content.”
A Time Domain Transmission Sensor with TDR Performance Characteristics
Research by J.M Blonquist Jr. S.B. Jones D.A. Robinson and Utah State University.