Modification of Digital Tdt Sensor for Direct Insertion in Field Settings.

Acclima’s TDT is a Digital TDT sensor that is among the most accurate devices for soil water content measurements.  The Digital TDT sensor estimates dielectric permittivity by measuring the transmission travel time along its U-shaped rods. In spite of the high accuracy and the affordability of the Digital TDT sensor, the gap between the U-shaped transmission rods requires packing of soil around its rods after locating the sensor in place, such that it is impossible to directly insert it into the soil profile. This limitation has prevented its widespread use in vadose monitoring especially where limited soil disturbance is desired. In this study we modified the TDT sensor by covering the space between two transmission rods and within the inner transmission rod with polycarbonate sheet. Using the known permittivity value of the sheet and other sensor materials, we construct a numerical model and laboratory experiment where we compared the permittivity values of covered and un-covered sensors at different soil water content. The comparison showed a linear relation between two sets of permittivity values, except at very low water content. Therefore, a calibration equation which only is a function of permittivity of material used to modify the TDT sensor can be derived to convert the permittivity values of modified TDT sensor to soil water contents, and enables use of the modified sensor for undisturbed in-situ measurements.