HIM soil moisture measurement with time-domain resonance method

In time-of-flight measurement, the waveguide used as a sensor is placed in the material under investigation in the form of a two- or three-core ribbon cable, one or more electrically conductive rods or strip lines (for example, horizontally inside tanks)[1]. The length of the waveguides can vary depending on the application and waveguide and can be between ten centimetres and 30 metres. A coaxial cable is connected to it, which connects the waveguide to a TDR measuring device containing a pulse generator and an oscilloscope. The coaxial cable is only used to transmit the pulse from the generator to the waveguide and has no function as a sensor.

At the beginning of the measurement, a pulse or voltage jump is applied to the coaxial cable by the pulse generator, which propagates along the cable. As soon as the pulse is transferred to the waveguide, there is a partial reflection of the signal. Through this partial reflection, the beginning of the sensor can be determined. The further propagation speed of the pulse is influenced by the humidity along the sensor. When the end of the sensor is reached, the impulse is completely reflected. The step response of a waveguide can be calculated over the time domain.[2]

The different reflections become visible in a TDR curve through the oscilloscope connected to the sensor. The TDR curve allows the speed of propagation of the electromagnetic waves to be determined. By comparing the original pulse with the reflected signal with the aid of an equivalent circuit, conclusions can be drawn about the relative permittivity of the material and thus the averaged moisture content.[2].

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