Acclima Digital True TDR-315H Soil Moisture Sensor (SDI-12) Data Sheet

The TDR-315H soil moisture sensor is the replacement to the former Acclima TDR-315 and TDR-315L soil water content sensors. It is fully compatible with those sensors but incorporates several improvements.

Download the PDF Version of this Data Sheet

Click this link to download the data sheet.

Table of Contents

TDR315H Acclima Sensor


The TDR-315H is one of Acclima’s H-series sensors, which are very low power and suited perfectly for power-conscious applications. 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 longer waveguides provide a very large sampling volume and high accuracy of the waveform propagation.

Its form factor is designed for shallower horizontal installations where it is pushed 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.


  • Ultra-low power consumption and fast response time
  • Designed for easy use in shallower horizontal applications
  • Very large sampling volume with the wide spacing and the longer 15cm waveguides
  • High accuracy Acclima sensor configuration
  • Soil Smart analysis works with all mineral soil types
  • Incident Wave Amplitude: 400mV
  • 20% to 80% Incident Wave Rise Time: 150 ps
  • Waveform Digitizing Resolution: 5 ps
  • SDI-12 Interface
  • 3-element 15 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 2000 µS/cm (Ɵ, ɛ repeatability degrades above 2000 µS/cm BEC)
  • 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) 0.07
   Accuracy -2 +2 %FS[1]
   Stability with Bulk Electrical Conductivity (0-2000 uS/cm) -1 +1 %FS
   Range[2] 0 100 %
   Resolution 0.1 %
   Repeatability (RMS deviation) 0.07 %
   Accuracy -2 +2 %FS
   Stability with Bulk Electrical Conductivity (0-3000 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 2000 uS/cm
   Resolution 1 uS/cm
   Repeatability (RMS deviation) 3 uS/cm
   Accuracy (0 – 1000 uS/cm) -25 +25 uS/cm
   Accuracy (1000 – 2000 uS/cm) -2.5 +2.5 %

[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 +55 °C
-40 +140 °F

Operating Conditions

Parameter Min Max Units
Operating Supply Voltage
+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 0.25 typical sec
   Sensor read current (Supply Voltage = 12V) 36 typical mA
   Sensor read current (Supply Voltage = 7V) 88 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) 21 cm x 5.3 cm x 2 cm
Weight (without cable) 105 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
Scroll to Top