Time Domain Reflectometry - TDR
New TDR developmentsThe PRENART TDR SYSTEMS are used for in situ measurement of volumetric water content of soils.
The TDR measurements are carried out with the new TDR100 instrument made by Campbell Scientific Inc., USA, combined with a handheld PC. The data acquisition and analysis programme AutoTDR has been developed specifically for the rugged handheld Allegro field PC. However, the AutoTDR programme will also run on other IBM compatible PC's. The TDR system has been developed in collaboration with The Danish Institute of Plant and Soil Science, where it has been used extensively for several years.
The portable TDR system in combination with the multiplexed probe system allows the installation of an adequate number of inexpensive measuring probes in areas with variable soil conditions. The TDR instrument can be used to service probe systems installed at many sites.
TURNKEY TDR-systemA complete TDR system including a compact portable TDR instrument, a handheld computer and probe systems has been developed. The system has been developed both for the inexperienced and the experienced users of TDR. The beginner will appreciate the easy to use software taking care of data analysis and storage. The experienced user will appreciate the flexibility of the AutoTDR software.
The complete system includes the new compact TDR 100 instrument from Campbell Scientific Inc., USA, combined with the rugged waterproof handheld Allegro field PC from Juniper Systems running the AutoTDR software and 5 sets of measuring probes including the lengths 20 cm, 50 cm and 90 cm. The TDR 100 instrument is carried in a convenient hipsack. Using the Allegro field PC only 1 - 2 minutes is required for making measurements on each probe set.
An important advantage of the portable TDR system is the possibility of including a large number of inexpensive measuring probes at sites with heterogeneous soil conditions. Also available with the system is a portable TDR probe with 20 cm rods especially useful for irrigation scheduling in parks and golf courses.
TDR measurementsThe TDR instrument generates an electric pulse with a short rise-time and large frequency content. The pulse propagates along the cable and measuring probe. The propagation velocity along the probe is inversely proportional to the water content of the soil surrounding the probe. A small reflection where the cable is connected to the probe rods marks the beginning of the probe. The open end of the measuring probe reflects all the remaining energy in the pulse. The analysis software identifies the beginning and end reflections and calculates the travel time (or apparent length of the measuring probe). The travel time or apparent probe length is first converted into the dielectric constant for the soil/water mixture and then into volumetric water content using the 'Topp' equation.
Examples of TDR measurements of water contentThe dielectric constant value for water is approximately 20 times that for soil solids. This large difference accounts for the high sensitivity of TDR measurements to changes in soil water content. The propagation velocity in dry soils is larger than in wet soils. Examples of TDR measurements made in a dry and a wet soil respectively are shown Below.
PRENART Turnkey TDR-Kit
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