Time-Domain Reflectometry

Time Domain Reflectometry (TDR) is widely used to measure soil water content, bulk electrical conductivity, and rock mass deformation. TDR measurements are nondestructive and offer excellent accuracy and precision.

Time domain reflectometry systems based around the TDR100 consist of a reflectometer, datalogger, coaxial multiplexer(s), cables, and probes. A TDR100-based system is generally used for longer-term projects. These systems offer significant benefits when large numbers of probes must be monitored continuously.

Time domain reflectometry (TDR) systems were historically used to determine where a break occurred in an attached cable. A reflectometer generates an electromagnetic pulse that is applied to a coaxial system. The elapsed travel time and pulse reflection are measured, which traditionally indicated the length of the cable. Because soil moisture affects the travel time, Campbell Scientific can use this technology to make accurate measurements of soil volumetric water content. TDR systems can also be used for rock mass deformation studies.

The datalogger controls operation of the reflectometer and multiplexers. Using the TDR instruction, the datalogger can be configured to output volumetric water content, bulk electrical conductivity, or waveform values. Dataloggers typicallly used in our contemporary TDR100 system include the CR800, CR850, CR1000, and CR3000. Detailed descriptions of our dataloggers are available in the datalogger section.