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The AQUAFLEX soil moisture probe uses a process called Time Domain Transmission (TDT).
An electrical pulse is sent along a transmission line, which is embedded within the AQUAFLEX sensor, and the electrical field around the transmission line interacts with the surrounding medium.
The speed and shape of this pulse is affected by the dielectric properties of the medium. Water has a much higher dielectric constant than most materials that constitute soil, so the presence of water has a profound impact on the electrical signal. This factor enables a soil moisture probe to measure moisture content.
TDT can be considered similar to TDR (Time Domain Reflectrometry), however, an AQUAFLEX soil moisture probe differs from traditional TDR in several ways.
- The sensor of an AQUAFLEX soil moisture probe consists of a transmission line that extends to the end of the sensor ribbon and then returns back to the sensor electronics. This means that AQUAFLEX measures the original pulse, not a reflection from an open metal rod.
- The AQUAFLEX pulse circulates many times through the sensor cable (US Patent 5148125) before the pulse delay is measured, resulting in extremely high precision in the time delay measurement.
- The AQUAFLEX soil moisture probe has a large measurement area. The sensor is 3 meters in length (10 feet). Whereas TDR measures a very small sample, AQUAFLEX spatially averages over a 50mmx3000mm cylinder of soil. This represents a sample size of approximately 6 litres. This sample size is significantly larger than other techniques and facilitates reliable and repeatable results.
- The shape of the pulse used by the AQUAFLEX electronics differs from that used by traditional TDR instrumentation, with the consequence that losses and noise in the transmission are reduced. As a result, an extremely good signal level can be achieved in a 3 metre long sensor.
- AQUAFLEX extracts information from the shape of the pulse, which gives a good indication of soil conductivity - information that is used to compensate the moisture measurement reading to maintain accuracy in conductive soils.
- AQUAFLEX includes automatic temperature compensation and also provides a soil temperature output.
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