TDR moisture sensor

Time-domain reflectometry or TDR is a measurement technique which can also be used for measuring moisture by obtaining frequency-dependent electric and dielectric properties of materials and substances such as soil, agro-food, grain, snow, wood, or concrete.

Standard waveform analysis only provides the average moisture content along the sensor, while profile analysis provides spatial moisture content.

Standard waveform analysis

Functional principle of a TDR meter

In the waveform analysis, a voltage jump or pulse is injected into a coaxial cable by using a pulse generator. The coaxial cable carries the pulse to a probe (consisting of two or thre-wire parallel waveguides) placed in the soil or other medium. If the voltage jump changes over to the waveguide or probe, a partial reflection occurs. The advancing part is fully reflected at the end of the probe. An oscilloscope captures and displays the reflected waveform. Different reflections become visible in a TDR waveform, thus allowing a determination of the propagation velocity of the electromagnetic waves. The velocity of the pulse in the probe is measured and related to moisture content, with smaller velocities indicating an increase of moisture. An equivalent circuit diagram enables to draw conclusions on the relative permittivity of the material and the average moisture content.

Standard waveform analysis is used for partially automatic monitoring in several areas such as: hydrology, agriculture and construction.

Profile analysis

However the waveform analysis is unable to resolve a spatial water content profile. For this purpose, more sophisticated methods are required:

Moisture detection in buildings using profile analysis

• Subdivision: The waveguide is subdivided in individual segments by using PIN diodes. Disadvantages of this method are the dispersion occuring with increasing cable length, influence of the diode circuit on the signal and complex manufacturing.

• Alteration of the cross section: By partially altering the cross section of the waveguide, it is divided in individual segments. Altering the cross section creates artificial reflected waveforms at each intersection which help to differentiate the individual segments. However, an automated data analysis is difficult with this method since artifical interferences may not always be distinguished from real variances in the material.

• Length variation: This method uses several waveguides with different lenghts mounted parallel to each other. As a seperate waveguide must be connected for each area, the costs of this method are very high.

• Reconstruction algorithm: Another approach is to measure the propagation of the waves along the waveguide and to model it. By comparing measured and modeled TDR signals, conclusions on the distribution of moisture can be drawn. The accuracy of the reconstruction method and the complex algorithms is limited in practice by interfering factors such as a limited amplitude resolution of TDR instruments and by noise.

Profile analysis allows fully automatic measurement of the spatial moisture content and thus a leak monitoring of for example foundations or barriers of a landfill. This includes deep geological repositories in salt mines. Leaks can be detected and located quickly using this method.

See also

• Time-domain reflectometer
• Transmission line

References

Sources

• Cataldo, Andrea; De Benedetto, Egidio; Cannazza, Giuseppe (2011). Broadband Reflectometry for Enhanced Diagnostics and Monitoring Applications. Berlin, Heidelberg: Springer Press. ISBN 978-3-642-20233-9. {{cite book}}: Invalid |ref=harv (help)
• Evett, Steven R. (2003). "Soil Water Measurement by Time Domain Reflectometry" (PDF). USA: Encyclopedia of Water Science, Marcel Dekker, Inc. Retrieved 28 February 2014. {{cite web}}: Invalid |ref=harv (help)
• Trebbels, Dennis; Kern, Alois; Fellhauer, Felix; Huebner, Christof; Zengerle, Roland (07 July 2013). "Miniaturized FPGA-Based High-Resolution Time-Domain Reflectometer" (PDF). GER: Institute of Electrical and Electronics Engineers (IEEE). Retrieved 28 February 2014. {{cite web}}: Check date values in: |date= (help); Invalid |ref=harv (help)
• Jones, Scott B.; Wraith, Jon M.; Or, Dani (2002). "Time domain reflectometry measurement principles and applications" (PDF). USA: Institute of Electrical and Electronics Engineers (IEEE). Retrieved 28 February 2014. {{cite web}}: Invalid |ref=harv (help)

Further reading

Books

• Cataldo, Andrea / De Benedetto, Egidio / Cannazza, Giuseppe (2011). Broadband Reflectometry fo Enhanced Diagnostics and Monitoring Applications. Springer Press. ISBN 978-3-642-20233-9