Measuring moisture content using time-domain reflectometry
To measure the moisture content of a material can be used, the time domain reflectometry (English Time Domain Reflectometry, TDR short ).
The average moisture content along a sensor is obtained with simple methods of evaluation ( runtime measurement). The selective moisture content along a sensor can be achieved by adjustments of the sensor, complex algorithms and a corresponding reconstruction methods determine (profile measurement).
Transit time measurement
In the propagation time measurement is the as the sensor used waveguide in the form of a two-or three-wire flat cable, one or more electrically conductive bars or lines ( for example horizontally in the interior of the tank ) were placed in the examined material. The length of the waveguide may vary depending on use and waveguide and be between ten centimeters and 30 meters. Because a coaxial cable is connected, which connects the waveguide with a TDR meter, which includes a pulse generator and an oscilloscope. The coaxial cable is used only for the transmission of 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 step is applied to the coaxial cable from the pulse generator, which propagates along the cable. When the pulse is transferred to the waveguide, there will be a partial reflection of the signal. Through this partial reflection of the beginning of the sensor can be determined. Further propagation speed of the pulse is influenced by the moisture along the sensor. When reaching the sensor end of the pulse is reflected. The step response of a waveguide can be calculated over the time period.
The various reflections are visible through the device connected to the sensor in a TDR oscilloscope curve. TDR curve enables the determination of the propagation velocity of the electromagnetic waves. By comparing the original pulse with the reflected signal with the help of an equivalent circuit can draw conclusions on the relative permittivity of the material and, thus, include the average moisture content.
The term measurement is used for the fully automatic measurement of the average moisture content in various fields such as hydrology, agriculture and construction (see also building moisture ). This manual measurements with hand-held devices are also possible. Information on the spatial water content distributions allow for example: Efficient fertilizing and watering, monitoring the solidification process in concrete and cement, measuring the level of liquids in a container, recognition of landslide risks due to high moisture content of soils and monitoring of soil remediation processes.
Profile measurement
A runtime measurement, as described above, is not sufficient for many applications if it can be determined instead of the average moisture content of the localized moisture content along the sensor. To determine the moisture content distribution in a material, there are several possible solutions:
- Profile Reconstruction: The most advanced and most widely used approach is to measure the wave propagation along the sensor and to model the reflected pulse. Algorithms for profile reconstruction to reconstruct the time-domain reflectometry moisture content along the waveguide of a recorded TDR waveform. This method does not require any changes to the waveguides.
- Change of the cross section: With the aid of changes in the cross section of the waveguide is divided into individual areas. To the cross section at the corresponding positions in each case increased or decreased. The cross-section changes artificial reflections produced by changing the wave impedance which visualize the subdivision in the signal.
- Division: In this case the waveguide with the aid of pin diodes is divided into individual segments. The pin diodes generate artificial reflections. Due to this reflection, the TDR waveform may be divided.
- Length variation: a plurality of different lengths of fiber to be mounted in parallel with this method. Each waveguide thus covers a separate area. Localization of humidity or moisture content determination of punctual is possible without changes to the waveguides.
The profile measurement allows fully automatic measurement of moisture content or selective localization of moisture and thus a leak monitoring of, for example foundations, soles or barriers of a landfill. This includes nuclear waste repository in salt mines.