time domain transmissometry (TDT)
Time Domain Transmissometry (TDT) is similar to TDR however it measures the transmission of a pulse along a looped, or closed circuit, rod. TDT measures the time from the start to the end of the loop for the propagation of an electrical pulse. Similar to TDR, a pulse measured via TDT will be slower in wetter soils than drier soils. The velocity of the pulse is related to the dielectric constant.
TDT is regarded as an accurate measurement principle, overcoming some of the limitations of TDR. The principle is new and has only been developed over the last 10 years. Therefore it has not been rigorously tested or applied like TDR. TDT needs to have good contact with the medium it is measuring. The volume of measurement can be relatively large due to sensor design (sensor length is 20cm). However, similar to other dielectric moisture measurement techniques such as capacitance and TDR, the field of influence is still within approximately 1cm of the probe.
soil moisture instrumentation
- Capacitance or frequency domain
- Time domain reflectometry (TDR)
- Cosmic-Ray (COSMOS)
- Ground penetrating radar (GPR)