Humidity measurement with time domain reflectometry

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To measure the moisture content of a substance that can time-domain reflectometry ( English Time Domain Reflectometry , short TDR ) may be used.

The average moisture content along a sensor is determined by measuring the transit time of an electrical signal. The punctual moisture content along a sensor can be determined by adapting the sensor, complex algorithms and a corresponding reconstruction process (profile measurement).

How a TDR measuring device works

Runtime measurement

A TDR waveguide with exposed copper wires

When measuring the transit time, the waveguide used as a sensor is placed in the material to be examined in the form of a two or three-core ribbon cable , one or more electrically conductive rods or strip lines (e.g. horizontally inside tanks). The length of the waveguide can vary depending on the application and waveguide and be between ten centimeters and 30 meters. A coaxial cable is connected to this and connects the waveguide with a TDR measuring device, which contains a pulse generator and an oscilloscope . The coaxial cable only serves to transmit the impulse from the generator to the waveguide and has no function as a sensor.

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

The various reflections are visible in a TDR curve through the oscilloscope connected to the sensor. The TDR curve enables the speed of propagation of electromagnetic waves to be determined . By comparing the original pulse with the reflected signal with the help of an equivalent circuit diagram , conclusions can be drawn about the relative permittivity of the material and thus the averaged moisture content.

The transit time measurement is used for the fully automatic measurement of the average moisture content in various areas, such as in hydrology , agriculture and construction technology ( see also construction moisture ). Manual measurements with handheld devices are also possible. Information on the spatial distribution of water content enables, for example: efficient fertilization and irrigation , monitoring of the setting process in concrete and cement , measurement of the level of liquids in a container , detection of landslide risks due to excessively high soil moisture content and monitoring of soil remediation processes .

Profile measurement

A transit time measurement, as described above, is not sufficient for numerous applications if the point-by-point moisture content along the sensor has to be determined instead of the average moisture content. There are several possible solutions for determining the moisture content distribution in a material:

Moisture detection in buildings by means of profile measurement
  • Profile reconstruction: The most modern and widely used approach is to measure the wave propagation along the sensor and to model the reflected pulse. Profile reconstruction algorithms with time domain reflectometry reconstruct the moisture content along the waveguide from a recorded TDR curve. This method does not require any changes to the waveguides.
However, the algorithms available for profile reconstruction are limited to certain materials and soil types. In practice, the accuracy of the reconstruction process and the complex algorithms is limited by disruptive factors such as the limited amplitude resolution of the TDR instruments and by noise .
  • Changing the cross section: With the help of changes in the cross section , the waveguide is divided into individual areas. For this purpose, the cross-section is enlarged or reduced in each case at the corresponding points. Artificial reflections are generated at the changes in cross-section by changing the wave impedance , which make the subdivision visible in the signal.
Automated measurement data evaluation is difficult with this method, since the artificial disturbances cannot always be distinguished from real changes in the material. As a result, this method is only used to a limited extent.
  • Subdivision: The waveguide is divided into individual segments with the help of pin diodes . The pin diodes create artificial reflections. These reflections can be used to subdivide the TDR curve.
Disadvantages of this method are the increased attenuation of the impulse ( dispersion ) that occurs with increasing cable length , the influence of the diode circuit on the signal and the complex, compared to other methods, expensive and manual production of these special waveguides due to the complexity of the construction.
  • Length variation: With this method, several waveguides of different lengths are mounted parallel to each other. Each waveguide thus covers its own area. A localization of the moisture or determination of the point moisture content is thus possible without changes to the waveguides.
Since a separate waveguide has to be connected for each area, the effort and thus the costs for this method is very high. This method is rarely used due to the high installation effort and material costs.

The profile measurement enables a fully automatic measurement of the point moisture content or localization of moisture and thus a tightness monitoring of, for example, foundations , floors or barriers of a landfill . This also includes nuclear repositories in salt mines .

See also

literature

  • Christof Hübner, Stefan Schlaeger, Klaus Kupfer: Spatial Water Content Measurement with Time-Domain Reflectometry . In: Elmar von Wagner (ed.): Tm - technical measuring . tape 74 , no. 5 , May 2007, pp. 316–326 , doi : 10.1524 / teme.2007.74.5.316 .
  • Andrea Cataldo, Egidio De Benedetto, Giuseppe Cannazza: Broadband Reflectometry for Enhanced Diagnostics and Monitoring Applications 1st edition Springer-Verlag, Berlin and Heidelberg 2011, ISBN 978-3-642-20232-2 .
  • Udo Kaatze, Christof Hübner: Electromagnetic techniques for moisture content determination of materials . In: Measurement Science and Technology . tape 21 , no. 8 , August 2012, doi : 10.1088 / 0957-0233 / 21/8/082001 .

Web links

Commons : Time Domain Reflectometry  - collection of images, videos, and audio files

Individual evidence

  1. ↑ a b c Mathis Nussberger: Soil moisture determination with TDR: Single-rod probes and profile reconstruction algorithms. (PDF) Dissertation, Eidgenössische Technische Hochschule Zurich (ETH Zurich), 2005, accessed on March 27, 2014 .
  2. ↑ a b Christof Hübner, Stefan Schlaeger, Klaus Kupfer: Spatial Water Content Measurement with Time-Domain Reflectometry . In: Elmar von Wagner (ed.): Tm - technical measuring . tape 74 , no. 5 , May 2007, pp. 316–326 , doi : 10.1524 / teme.2007.74.5.316 .
  3. ↑ Dennis Trebbels, Alois Kern, Felix Fellhauer, Christof Hübner, Roland Zengerle: IEEE Transaction On Instrumentations And Measurement . Ed .: Institute of Electrical and Electronics Engineers [IEEE]. 1st edition. tape 62 , Issue 7, July 2013, ISSN  0018-9456 , Miniaturized FPGA-Based High-Resolution Time-Domain Reflectometer, p. 2101-2113 ( imtek.de [PDF]).
  4. ↑ a b c Christof Hübner: Development of high-frequency measurement methods for determining soil and snow moisture . Ed .: Dissertation, Karlsruhe Institute of Technology [KIT]. 1st edition. 1999, cable sensor, p. 109–170 ( bibliothek.fzk.de [PDF]).
  5. ↑ K. Kupfer, E. Trinks, Th. Schäfer: TDR sensors for controlling landfill seals in salt mines. (PDF) Materials Research and Testing Institute at the Bauhaus University Weimar, November 18, 2004, accessed on February 14, 2014 .