Soil Moisture, Temperature and Electrical Conductivity Sensor
The TDR-310W is a modification to the Acclima TDR-315N. Its shorter rods allow it fit in standard rockwool and coco coir blocks. 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. In greenhouse applications it is inserted horizontally into the sidewall of a rockwool or organic soil block.
3-element 10 cm stainless steel waveguide
5 m 3-conductor waterproof cable (standard)
Volumetric Water Content: 0% to 100%
Medium Temperature: -40 to +55 degrees C
Medium Permittivity: 1 to 100
Medium Bulk Electrical Conductivity: 0 to 6000 µS/cm
Pore Water EC (Hilhorst Model): 0 to 55000 µS/cm
20% to 80% Incident Wave Rise Time: 300 ps
Waveform Digitizing Resolution: 5 ps
Incident Wave Amplitude: 2.3 V
The recommended method of installation is to dig a vertical-walled trench with one dimension at least 12 inches deep. The depth of the trench should be a couple of inches below the depth at which the sensor is to be installed. Insert the waveguide rods horizontally into the vertical wall at the end of the trench. The insertion force may be quite severe in some soils. This can be reduced by deeply watering the soil before installation and/or by using a mechanical force multiplier to push the sensor into the sidewall.
Incorrect usage of this sensor voids our warranty.
It's recommended to have your VWC sensors placed in the same position throughout your pots to ensure accurate and consistent readings. The response curve shwon below is similar regardless of position.
If your cloth pot is too thick, you won't be able to get an accurate reading. If you are using a cloth pot a notice a constant VWC percentage, the probes may not be able to penetrate deep enough. In these instances we suggest the Vertical Positioning mentioned below.
Before inserting the sensor waveguide into the block, the block should be thoroughly hydrated.
Rock wool blocks are often inhomogeneous in density. Some portions of the block are more tightly packed than other portions. The denser volumes tend to be more hydrophobic and are difficult to wet. The less dense volumes tend to provide preferential paths for water to flow downward through the block creating wetter and drier regions throughout the material. If the blocks are not thoroughly hydrated there will be air inclusions in the block that may remain dry for weeks – thus limiting root growth and creating false VWC and PWEC readings. Even after thoroughly hydrating the block the saturation water in the inclusions drains out causing underestimation of the VWC reading. But on the average these errors can be compensated making the readings useful but not totally accurate. A recommended method of hydrating a rock wool block is to place it in a bucket containing about 12 inches of water. The block will float for about a minute then sink below the surface when it is fully hydrated. Allow another minute or so for all bubbling from the block to stop. Then lift the block out of the water by the bottom to avoid tearing the fibers at the top of the block.
Rock Wool and other greenhouse substrates are generally used at high water content levels – at or above the field capacity of the material. This causes the water to ‘sink’ toward the bottom of the material. Measurements taken 1 inch from the top of the block reported 20% water content. Measurements taken 4” from the top of the block showed 63% water content. For blocks with less water content the position of the average water content shifts to some extent, making it difficult to get high accuracy readings. What this infers is that the level at which the sensor waveguide is inserted into the block is critical if consistent and reasonably accurate measurements are needed.
Organic growing media is usually procured as loose material in bags or as dried, compressed bricks. When used in greenhouse applications it is normally placed into plastic bags to hold it together. The loose material needs to be compacted into the growing bag, otherwise it will not support the sensor well and air voids around the waveguide could occur and cause erroneous readings.
The organic material, whether loose or in brick form, is placed into a plastic ‘grow bag’. A typical bag has a volume of a little over 1 gallon and has small holes in its bottom. The bottom fold is also not sealed and will allow water to leak in and out. Place the filled bag into a bucket containing about 8-10 inches of water or nutrient solution and allow it to sink to the bottom while holding the top of the bag to keep it from capsizing. Allow the bag to sit in the water until the top surface is fully hydrated. This may several minutes and up to an hour for compressed material. Compact the wet material in the bag so that it is dense enough to support the sensor waveguide.
Coco and peat-based materials have a higher field capacity than rock wool and do not exhibit as much of a water gradient. But a mild gradient will form at saturation levels in the organic material. Hence it is needful to be consistent with the level of insertion into the bag wall. The insertion tool can be used to provide this consistency and to provide a flat void-free penetration into the soil. A common level for waveguide insertion is about 1.5 inches from the bottom of the substrate.
The TDR310W was designed for greenhouse applications, but it can be used in field applications as well. It is inherently accurate in all mineral soil types without soil-specific calibration.
The technology used to measure water content (VWC), and electrical conductivity (EC) makes assumptions about the type of medium it is in, so you need to select the correct calibration for your type of soil. We offer calibrations for Soil (Default), Rockwool, and several types of Coco. Have a medium that we don't yet? Send us an email at email@example.com so we can get a calibration profile created!
Frequently Asked Questions (FAQ)
What is the difference between the Bulk EC and PWEC (Pore water EC) readings? PWEC is the agricultural standard reading and a measurement for the electrical conductivity of water in the pores of your soil; Bulk EC is the electrical conductivity of the bulk soil (Soil, Air, Water).
Running into issues? Reach out to us at firstname.lastname@example.org or give us a call at 888-537-8573 during our business hours (M-F 9a-4p PT) so we can take care of you!