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Acclima-TDR-315N Soil Moisture Sensor
The TDR-315N is a replacement to the former Acclima TDR-315L. It has a high voltage waveform output, which makes it effective in taking measurements when the soil electrical conductivity is high. It also consumes more power than the TDR-315H. It is a complete integrated time domain reflectometer that combines ultra-fast waveform generating and digitizing functions with a precision 5 pico-second resolution time base and highly sophisticated waveform digitizing and analyzing firmware that provides true time domain analysis of soil-propagated waveforms. Its form factor is designed for shallower horizontal installations where it is forced into the sidewall of a trench that has been dug to the desired measurement depth.
A probe guide is provided to ensure that the waveguide rods are parallel when pushing them into the soil. It provides reading data through a 3-wire SDI-12 interface and is compatible with any data recorder that is compliant with SDI-12 version 1.4 and earlier.
Key Features
Measurement Functions
About the TDR Sensor
Acclima’s TDR Sensors are Integrated Time Domain Reflectometers that contain an ultra-fast step function generator, an ultra-fast waveform digitizer and a precision time base. The step function generator and waveform digitizer are connected directly to a waveguide of length 5 cm, 10 cm or 15 cm without an intervening coaxial cable. The key to their operation is a patented waveform digitizing hardware and firmware set that has an effective digitizing rate of 200 billion samples per second. A step function generator launches a voltage step on the waveguide. A digitizer acquires a digital image of the incident wave and its returning reflections with a resolution of 5 trillionths of a second. The acquired wave image is then analyzed to determine the round-trip propagation time between the incident wave and the first reflected wave. Using the propagation equation that governs the speed of light through a medium the permittivity of the medium is then calculated as:
where t is the round trip propagation time, l is the length of the waveguide and 𝜺 is the relative permittivity of the medium though which the wave has passed. The fact that propagation time is independent of soil electrical conductivity leads to the fact that the calculated permittivity is also independent of soil electrical conductivity. This is the main advantage of digitized waveform time domain sensors over all non-time domain electronic sensors. Volumetric Water Content can be derived from permittivity using the Topp equation or a suitable dielectric mixing model. Electrical Conductivity in soils is affected by compaction and hence non-TDR sensors tend to be very sensitive to both installation compaction and follow-on soil settling. TDR sensors report the true volumetric water content independent of soil electrical conductivity, compaction and settling.
An 80 MHz floating point processor inside the sensor is used to process the waveform image. The total time required to send a measurement command to the sensor, acquire the digitized waveform image, process the image, and calculate measurement data is less than one second.
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