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Enabling better global research outcomes in soil, plant & environmental monitoring.


The TDR-315 is a true Time Domain Reflectometer system (minus the coaxial cable) wrapped up in a small, rugged, permanently installable package. This model uses half the power yet it sacrifices none of the accuracy of its big brother the TDR-315. Acclima True TDR technology is a major, patented breakthrough in soil water and EC measurement technology.


  • Volumetric Water Content (0 to 100% with 0.1% resolution)
  • Permittivity (1 to 80 with 0.1 unit resolution)
  • Soil Electrical Conductivity a.k.a. Bulk EC (0 to 5000 μS/cm -resolution depends on reading range)
  • Soil Temperature (-40 to +60°C with 0.1 degree resolution)
  • Pore Water Electrical Conductivity (0 to 55000 μS/cm)


  • 10 meter, flexible, waterproof cable
  • 15 cm X 3.5mm stainless steel 3-element waveguide
  • Rugged, waterproof epoxy housing
  • Typical 350ps incident wave rise time applied directly to the soil (no bandwidth limiting coax cable).
  • Input bandwidth to the waveform digitiser is also unrestrained because of the absence of a coax cable.


The TDR-315L is available in a preconfigured IoT (Internet of Things) setup for continuous real-time solar energy for continuous photovoltaic performance monitoring.


A fully integrated digital TDR soil water content sensor, the 315H consumes a fraction of the energy than that of its predecessor, but still maintains a 150 pico-second rise time for superb accuracy.

The TDR-315L is a true waveform digitising Time Domain Reflectometer that derives soil permittivity and water content from the propagation time of an electromagnetic impulse conveyed along its waveguide. Like many of its expensive predecessors (conventional TDR mainframes) it contains an ultra-fast rise time step function generator, a waveform digitiser (200 GSPS) and a picosecond-resolution time base (5ps). Unlike these predecessors it eliminates the bandwidth-constraining coax cable, the bulky console, and 90-97% of the cost. Because it is a true time domain device its readings are not derived from current and voltage magnitudes and relationships and hence are not impacted by soil electrical conductivity and compaction. Its high immunity to EC allows credible application in non-saturated soils wetted with sea water. It uses the standard SDI-12 interface and is compatible with all data recorders that are version 1.3 compliant.

Operating Parameters:
Read time: 0.7 seconds
Voltage Requirements: 6 to 15 volts DC
Idle Current Consumption:  < 30 μA @ 6 to 15 VDC
Read Current Consumption: 170 mA @ 6 to 15 VDC
Idle Energy Consumption: 15 J per day at 6 VDC
Read Energy Consumption: 0.7 J per reading at 6 VDC, 1.4 J per reading @ 12 VDC
VWC and Permittivity Performance Specifications
Permittivity to VWC conversion method: Proprietary Dielectric Mixing Model with 0 to 100% range.
Closely follows Topp Equation to 46% VWC.
Permittivity Reporting Accuracy: ±1% of full scale 1 to 80 relative permittivity units
From 0 to 4000 μS/cm Bulk (in-soil) Electrical Conductivity
From –20°C to +50°C. The VWC reading will only report liquid water.
VWC Accuracy: Dependent upon soil type – but typically ±2%
<1% change with Bulk EC changes from 0 to 4000 μS/cm
Change in VWC with compaction follows only the change in soil volume.
Temperature: Precision thermistor located within 2 mm of the outer waveguide rod where it enters the epoxy housing.
Accuracy: Typical ±0.2°C, Worst Case ±0.5°C over -12 to +50°C
Electrical Conductivity: Calculated from long term (200 ns) amplitude of reflected wave using the ‘waveguide constant’. Derived in factory during calibration.
EC Accuracy: ±1%, ±35 μS/cm maximum error from 0 to 5000 μS/cm Bulk EC*.

*Note that the 1% error applies to higher values of EC while the 35 μS/cm error applies to very low values of EC.
Pore water EC is calculated from the Hilhorst model using an average pore size to represent all soils. Hence its accuracy is not specified. The reading does provide a ‘ballpark’ indication of the salinity of the water in the soil as opposed to the soil/water mixture or ‘Bulk EC’ property.

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Accepted article: Evaluation of a digital TDR for determination of permittivity and bulk electrical conductivity
Dr. Robert Shwartz | USDA Agricultural Research Service, Bushland, Texas | True TDR

Evaluation of a Direct-Coupled TDR for Determination of Soil Water Content & Bulk EC
Dr. Robert Shwartz | USDA Agricultural Research Service, Bushland, Texas | True TDR

Soil Water Sensors for Agriculture – Theory & Issues
Dr. Steve Evett | USDA Agricultural Research Service, Bushland, Texas | True TDR

Strategies to Improve Productivity in a Water Stressed Future
Dr. Steve Evett | USDA Agricultural Research Service, Bushland, Texas | True TDR


A time domain transmission sensor with TDR performance characteristics
J.M. Blonquist, S.B. Jones, B. Robinson | Utah State University | Acclima TDT

Using Soil Water Sensors to Improve Irrigation Management
J. Chavez | Colorado State University | Acclima TDT

In Soil & Down Hole Soil Water Sensors: Characteristics for Irrigation Management
R. Lascano, Dr. Steve Evett, M. Pelletier, R. Schwartz | USDA | Acclima TDT

Performance Evaluation of  Selected Soil Moisture Sensors
J. Chavez, J. Varble, A. Andales | Colorado State University | Acclima TDT