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

The TDR-305N is a modified version of the TDR-310N. The very short waveguide allows this sensor to make very shallow measurements and to be used in higher conductivity environments with a sacrifice in total measurement volume.

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Table of Contents


The TDR-305N is a 5cm waveguide version of the TDR-310N. The shorter waveguide allows this sensor to make very shallow measurements and to be used in significantly higher conductivity environments. 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. Being part of the Soil Smart Series means that its advanced waveform analysis works with all mineral soil types. 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 easy vertical installation in an augered hole using a 34 mm flat-bottomed auger. It mates with a 1” schedule 40 PVC pipe that acts as a handle in its installation to any desired depth. 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.


  • Housing designed for easy bore hole installation at any depth
  • Shorter waveguides are great for “check and go” type applications such as mobile readings on golf greens, other turf management, or limited space environments
  • Ideal for high conductivity environments
  • Soil Smart analysis works with all mineral soil types
  • 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 5 cm 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 6000 µS/cm
  • Medium Temperature:                                  -40 to +55 degrees C
  • Pore Water EC (Hilhorst Model):              0 to 55000 µS/cm

Measurement Performance

Parameter Min Max Units
   Range 1 85
   Resolution 0.1
   Repeatability (RMS deviation) 0.21
   Accuracy -4 +4 %FS[1]
   Stability with Bulk Electrical Conductivity (0-6000 uS/cm) -1 +1 %FS
   Range[2] 0 100 %
   Resolution 0.1 %
   Repeatability (RMS deviation) 0.5 %
   Accuracy -5 +5 %FS
   Stability with Bulk Electrical Conductivity (0-6000 uS/cm) -1 +1 %FS
   Range -40 +55 °C
   Resolution 0.1 °C
   Repeatability (RMS deviation) 0.01 °C
   Accuracy (+5 to +35 °C) -0.25 +0.25 °C
   Accuracy (-15 to +55 °C) -0.5 +0.5 °C
   Range 0 6000 uS/cm
   Resolution 1 uS/cm
   Repeatability (RMS deviation) 3 uS/cm
   Accuracy (0 – 1000 uS/cm) -25 +25 uS/cm
   Accuracy (1000 – 2500 uS/cm) -2.5 +2.5 %
   Accuracy (2500 – 6000 uS/cm) -5 +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 toallow 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.

Parameter Min Max Units
Supply Voltage (Measured between the red and white wires) -16 +16 Volts
SDI-12 Data Voltage (Blue-White wires) -16 +16 Volts
External Voltage Applied to sensor rod -0.3 +4 Volts
Electrostatic discharge, center rod IEC 61000−4−2 (ESD)  
Storage Temperature -40 +60 °C
-40 +140 °F

Operating Conditions

Parameter Min Max Units
Operating Supply Voltage 
(Prior to 2H 2022)
+15 Volts
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) < 10 uA
   Idle Current (-35 to +50 °C) < 50 uA
   Sensor read time 0.4 typical sec
   Sensor read current (Supply Voltage = 12V) 118 typical mA
   Sensor read current (Supply Voltage = 7V) 150 typical mA
   Sensor communications current 6 typical mA

SDI-12 Data Line Electrical Characteristics

Parameter Min Max Units
INPUT (when sensor is idle or receiving data)      
   Resistance to GND 160k 175k Ohms
   VIL (required input voltage in “marking” state) -1 1.3 V
   VIH (required input voltage in “spacing” state) 3.2 6 V
OUTPUT (when sensor is transmitting data)      
   Output impedance 1000 1250 Ohms
   VOL (output voltage in “marking” state) 0 0.25 V
   VOH (output voltage in “spacing” state) 4.7 5.2 V

Physical Characteristics

Dimensions (without cable) 15 cm x 3.3 cm
Weight (without cable) 121 g
Cable weight 32.7 g/m
Composition 316 Stainless Steel, Epoxy, ABS Plastic
Cable 3 copper conductor, 22 Ga., waterproof and UV resistant PVC jacket, 4.8mm overall diameter
Communication Protocol SDI-12 Version 1.4
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