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Time domain reflectometry

Video: TDR (1:00 min)
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Example TDR signals
  

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In time domain reflectometry, a voltage pulse is sent to a two- or three-rod probe, which is being inserted into a soil. The electromagnetic field, generated between and around the rods, travels along the probe. The velocity and the spatial distribution of the electromagnetic field propagating perpendicularly to the probe depends on the dielectric properties of the soil which in turn depends primarily on the soil water content. The pulse is reflected at the front end of the probe and travels back to the instrument where it is being recorded. The pulse is getting slowed down with increasing soil water content because the water molecules are aligned to the electromagnetic field. Turning the H2O dipoles slows the propagating electromagnetic wave down. The travel time of the pulse as well as the dielectric properties of the dry soil and of water are used to calculate the actual soil water content.

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