Acclima Digital True TDR-305W Soil Moisture Sensor (SDI-12) Data Sheet

The TDR-305W is a completely integrated TDR soil water content sensor (a.k.a., soil moisture sensor, probe, or meter) that yields accurate readings. It can be used in significantly higher conductivity environments and challenging soil environments. It's tuned for non-mineral soils often used in Controlled Environment Agriculture (CEA), such as rockwool and coco coir, yet still performs well in traditional mineral soils.

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Table of Contents


The TDR-305W is a 5cm waveguide version of the Acclima TDR-310W (10cm). The shorter waveguide allows this sensor to be used in significantly higher conductivity environments. Like the 310W, the Soil Smart analysis engine is also tuned for non-mineral soils often used in Controlled Environment Agriculture (CEA), such as rockwool and coco coir, yet still performs well in traditional mineral soils. It also has a much higher amplitude waveform than the H-series sensors, which consumes more power, but makes it very effective in taking measurements in challenging soil environments. 

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 primarily for ease-of-use in non-mineral media commonly used in vertical farming applications. 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.


  • Designed for easy use with rockwool blocks and even very small grow media
  • Good sampling volume with the wide spacing of the 5cm waveguides
  • Soil Smart analysis optimized for non-mineral soil types, while still works well in mineral soils
  • Ideal for high conductivity environments
  • Incident Wave Amplitude: 2.3 V
  • 20% to 80% Incident Wave Rise Time: 300 ps
  • Waveform Digitizing Resolution: 5 ps
  • SDI-12 Interface
  • 3-element 5cm stainless steel waveguide
  • 5m or 10m 3-conductor waterproof cable (standard)
  • Waterproof epoxy-filled housing

Measurement Functions

  • Volumetric Water Content:                          0% to 100%
  • Medium Permittivity:                                     1 to 85
  • Medium Bulk Electrical Conductivity:       0 to 9000 µS/cm
  • Medium Temperature:                            -40 to +55 degrees C
  • Pore Water EC (Hilhorst Model):              0 to 55000 µS/cm

Measurement Performance

   Repeatability (RMS deviation)0.21
   Stability with Bulk Electrical Conductivity (0-4500 uS/cm)-1+1
   Repeatability (RMS deviation)0.5%
   Stability with Bulk Electrical Conductivity (0-9000 uS/cm)-1+1%FS
   Repeatability (RMS deviation)0.01°C
   Accuracy (+5 to +35 °C)-0.25+0.25°C
   Accuracy (-15 to +50 °C)-0.5+0.5°C
   Repeatability (RMS deviation)3uS/cm
   Accuracy (0 – 1000 uS/cm)-25+25uS/cm
   Accuracy (1000 – 2500 uS/cm)-2.5+2.5%
   Accuracy (2500 – 6000 uS/cm)-5+5%
   Accuracy (6000 – 9000 uS/cm)-7.5+7.5%

[1] Percent Full scale, i.e., +/- 2 percentage points.

[2] VWC is calculated based on relative permittivity using the formula derived by Topp with minor modifications to allow readings in slurries and pure water. The relative permittivity of water varies with temperature, so to see a reading near 100%, the sensor must be fully immersed in water long enough to equilibrate temperature with the water. The water should be at 20.5C with at least 4cm of water around the rods on all sides, beyond the tips of the rods, and at least 1cm of the sensor body immersed. VWC readings higher than 100% are possible when the permittivity is higher than 80.

[3] The temperature sensing element is located next to one of the outer waveguide electrodes.

Absolute Maximum Ratings

Stresses beyond those specified below may cause permanent damage to the sensor. These are stress ratings only and operation at these levels is not implied.

Supply Voltage (Measured between the red and white wires)-16+16Volts
SDI-12 Data Voltage (Blue-White wires)-16+16Volts
External Voltage Applied to sensor rod-0.3+4Volts
Electrostatic discharge, center rodIEC 61000−4−2 (ESD) 
Storage Temperature-40+60°C

Operating Conditions


Operating Supply Voltage

(Prior to 2H 2022)





Operating Temperature (VWC errors due to ice)-30+55°C
Operating Temperature (VWC accurate, no ice allowed)0+55°C
   Idle Current (sensor powered but inactive, 20 °C)< 26uA
   Idle Current (-35 to +50 °C)< 60uA
   Sensor read time0.8 typicalsec
   Sensor read current (Supply Voltage = 12V)90 typicalmA
   Sensor read current (Supply Voltage = 7V)150 typicalmA
   Sensor communications current3 typicalmA

SDI-12 Data Line Electrical Characteristics

INPUT (when sensor is idle or receiving data)   
   Resistance to GND160k175kOhms
   VIL (required input voltage in “marking” state)-11.3V
   VIH (required input voltage in “spacing” state)3.26V
OUTPUT (when sensor is transmitting data)   
   Output impedance10001250Ohms
   VOL (output voltage in “marking” state)00.25V
   VOH (output voltage in “spacing” state)4.75.2V

Physical Characteristics

Dimensions (without cable)16 cm x 5.3 cm x 2 cm
Weight (without cable)94 g
Cable weight32.7 g/m
Composition316 Stainless Steel, Epoxy, ABS Plastic
Cable3 copper conductor, 22 Ga., waterproof and UV resistant PVC jacket, 4.8mm overall diameter
Communication ProtocolSDI-12 Version 1.4
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