The Acclima TDR-310S series sensors are genuine Time Domain Reflectometers – complete with an integrated 350ps rise-time step function generator, a 10 cm waveguide, a 5ps resolution waveform digitiser and a precision time base that measures waveform temporal features as short as 5ps and as long as 20ns.
The built-in algorithms in its floating point microprocessor search the digitised waveform and capture the incident and reflected wave time ordinates and from these the permittivity of the medium surrounding the waveguide is calculated using the first-principals-based EM propagation equation. From that a proprietary dielectric mixing model calculates the water content of the medium. The mixing model matches the Topp Equation very closely up to around 46% water content but behaves in a more representative fashion up to 100% water content – providing the assurance that these TDR sensors can be used in all applications including slurries where water content may reach 100%. A precision thermistor is thermally coupled to the waveguide and reports medium temperature with ± 0.2°C accuracy. The waveguide electrodes are also used to measure the Electrical Conductivity of the medium. This property is also reported along with water content, permittivity and temperature.
|Dimensions||15 cm x 3.5cm x 3.5cm (round)|
|Construction||Type 304 stainless steel, epoxy, ABS sensor head box|
|Cable||3 conductor, 22 Ga., Water proof and UV resistant PVC jacket, 10 meter|
|Operating Temp.||-20°C to 50°C|
|Storage Temp.||-20°C to 75°C|
|Volumetric Water Content||0 to 100%|
|Absolute VWC Accuracy||±2% typical|
|VWC Temp Stability||± 1% of full scale 1°C – 50°C|
|VWC Soil EC Stability||± 2% of full scale 0 to 5 dS/m BEC|
|Temp Reporting Accuracy||± 0.2°C, 0 to 50°C|
|EC Reporting Accuracy||± 2.5% 100uS/cm 0 to 10,000 uS/cm BEC|
|Technology||Waveform Digitising Time Domain Reflectometer|
|Effective Acquisition Bandwidth||200 Giga-sample/sec.|
|Propagation Time Resolution||5 ps|
|Waveform Propagation Resolution||1.5 mm in air, 0.16 mm in water|
|Waveguide Length||10 cm|
|Permittivity to VWC Calculation||Modified Dielectric Mixing Model|
|Propagation Waveform Bandwidth||3.5 GHz|
|Communication Protocol||SDI-12 Revision 1.3|
|Maximum Cable Length||60 meters|
|Maximum Devices per Cable||50|
|Operating Voltage Range||6–15 VDC|
|Listening/Sleep Mode Current||31 uA typical, 35 uA max|
|Communications Current||6mA @ 6v, 3.5mA @ 12v|
|Read Moisture Comm Time||425 ms total for each read cycle|
|Moisture Sense Current||170mA|
|Soil Moisture Sense Time||2.0 seconds|
The device uses the standard SDI-12 interface, Version 1.3 and can be used with any Data Recorder or readout device that supports that standard. The integration of the Time Domain Reflectometer into a single package provides several advantages over conventional high performance TDR systems using a mainframe, coaxial cable and waveguide probe:
1. Conventional TDR systems use a TDR console coupled to a waveguide probe with a coaxial cable. An expensive high-performance coaxial cable is required to reduce the unwanted reflections from the impedance irregularities within the cable. Further the cable acts as a low pass filter that removes information from both the incident and reflected waves. The absence of the coaxial cable coupling the TDR electronics to the waveguide of the TDR-315 allows the full bandwidth of the step function generator to be applied to the waveguide and also facilitates an unfiltered capture of the reflected wave.
2. Conventional TDR systems cannot be deployed for continuous season – long field measurements without risking damage and a nuisance to crop managers. The Acclima TDR sensors can be completely installed in the root zone (without incurring root and compaction disturbances) and can remain there indefinitely. Soil tillage can be performed over the installation as long as the tillage depth does not reach within 2 inches of the sensor.
3. Power consumption of each TDR sensor is reasonably low allowing for low cost alkaline battery operation or rechargeable batteries coupled with a solar panel.
4. The cost of the cost of these Acclima TDR sensors is a small fraction of the cost of a conventional TDR set capable of equivalent performance.
5. All measurements are performed automatically without the need for setting up the console, cable, probe, PC and software. The “first-principles-based” calculations are also performed automatically and the multiple data points are delivered within 2 seconds in industry standard metrics.