Collection: TDR soil moisture sensors

In time-of-flight measurement, the waveguide used as a sensor in the form of a two- or three-wire ribbon cable, one or more electrically conductive rods, or striplines (for example, horizontally inside tanks)[1] is placed in the material to be examined. The length of the waveguide can vary depending on the application and the waveguide, and can range from ten centimeters to 30 meters. A coaxial cable is connected to it, which connects the waveguide to a TDR measuring device, which contains a pulse generator and an oscilloscope. The coaxial cable serves only to transmit the pulse from the generator to the waveguide and does not function as a sensor.

At the beginning of the measurement, the pulse generator applies a pulse or voltage step to the coaxial cable, which propagates along the cable. As the pulse passes through the waveguide, a partial reflection of the signal occurs. This partial reflection allows the beginning of the sensor to be determined. The further propagation speed of the pulse is influenced by the humidity along the sensor. Upon reaching the end of the sensor, the pulse is completely reflected. The step response of a waveguide can be calculated over the time domain.[2]

The various reflections are visualized in a TDR curve by the oscilloscope connected to the sensor. The TDR curve allows the propagation speed of the electromagnetic waves to be determined. By comparing the original pulse with the reflected signal using an equivalent circuit, conclusions can be drawn about the relative permittivity of the material and thus the average moisture content.[2]

Time-of-flight measurement is used for fully automated measurement of average moisture content in various fields, such as hydrology, agriculture, and construction engineering (see also building moisture).[3] Manual measurements with handheld devices are also possible. Information on spatial water content distributions enables, for example, efficient fertilization and irrigation, monitoring the setting process in concrete and cement, measuring the fill level of liquids in a container, detecting landslide risks due to excessive soil moisture content, and monitoring soil remediation processes.[1]