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Acclima-TDR-315N Soil Moisture Sensor
The TDR-315N is a replacement to the former Acclima TDR-315L. It has a high voltage waveform output, which makes it effective in taking measurements when the soil electrical conductivity is high. It also consumes more power than the TDR-315H. 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 shallower horizontal installations where it is forced 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.
- SDI-12 Interface
- 3-element 15 cm stainless steel waveguide
- 5 m 3-conductor waterproof cable (standard)
- Waterproof Epoxy-filled Housing
- 20% to 80% Incident Wave Rise Time: 300 ps
- Waveform Digitizing Resolution: 5 ps
- Incident Wave Amplitude: 2.3 V
- Volumetric Water Content: 0% to 100%
- Medium Permittivity: 1 to 100
- Medium Bulk Electrical Conductivity: 0 to 3000 µS/cm
- Medium Temperature: -40 to +55 degrees C
- Pore Water EC (Hilhorst Model): 0 to 55000 µS/cm
About the TDR Sensor
Acclima’s TDR Sensors are Integrated Time Domain Reflectometers that contain an ultra-fast step function generator, an ultra-fast waveform digitizer and a precision time base. The step function generator and waveform digitizer are connected directly to a waveguide of length 5 cm, 10 cm or 15 cm without an intervening coaxial cable. The key to their operation is a patented waveform digitizing hardware and firmware set that has an effective digitizing rate of 200 billion samples per second. A step function generator launches a voltage step on the waveguide. A digitizer acquires a digital image of the incident wave and its returning reflections with a resolution of 5 trillionths of a second. The acquired wave image is then analyzed to determine the round-trip propagation time between the incident wave and the first reflected wave. Using the propagation equation that governs the speed of light through a medium the permittivity of the medium is then calculated as:
where t is the round trip propagation time, l is the length of the waveguide and 𝜺 is the relative permittivity of the medium though which the wave has passed. The fact that propagation time is independent of soil electrical conductivity leads to the fact that the calculated permittivity is also independent of soil electrical conductivity. This is the main advantage of digitized waveform time domain sensors over all non-time domain electronic sensors. Volumetric Water Content can be derived from permittivity using the Topp equation or a suitable dielectric mixing model. Electrical Conductivity in soils is affected by compaction and hence non-TDR sensors tend to be very sensitive to both installation compaction and follow-on soil settling. TDR sensors report the true volumetric water content independent of soil electrical conductivity, compaction and settling.
An 80 MHz floating point processor inside the sensor is used to process the waveform image. The total time required to send a measurement command to the sensor, acquire the digitized waveform image, process the image, and calculate measurement data is less than one second.
TDR Sensor Models
Acclima offers several models of sensors. These models are available in families that are optimized for different purposes. For example, the TDRxxxH family is optimized for low power operation and can operate at low voltages, while the TDRxxxN family is optimized for high output power and works better in highly saline soils. Each family may have several models that are optimized for different installation methods or environments.
Sensor Connection and Setup
Acclima’s TDR sensors use the industry standard SDI-12 communications protocol and are compatible with any Data Recorder, Reader or Wireless Device that incorporates a compliant SDI-12 Port. When sensors are connected to a data recorder, it is recommended to follow the sensor configuration procedures for the data recorder. These procedures should include connecting the sensor and setting an address.
The sensor cable includes 3 wires:
- red = power (Required operating voltage depends on sensor – see datasheet)
- blue = bi-directional data line
- white = return/common/ground for data and power
If a cable is used to connect multiple sensors to the data recorder and if concurrent readings are to be taken the voltage drop in long cables may become an issue. For example: 5 sensors operating simultaneously could draw up to 400 mA of current and cause a voltage drop of 3.25 volts in 250 feet of 22-gauge wire. If the voltage at the sensor drops to the lower operating limit, the sensor may misread or fail to report data. To avoid this do not use concurrent commands or ensure that long cables are of sufficient wire gauge to handle the current loads without significant voltage drops. The SDI-12 command “aV!” can be used to measure the sensor supply voltage at the sensor and help diagnose power issues. However, the “aV!” command is not concurrent, and so it cannot measure the loading effects of concurrent measurements.
The wiring diagram for the sensor connection to the data recorder is shown below. All sensors are connected in parallel to the terminals on the data recorder or to a cable connected to the data recorder.
Note that if the data recorder and the sensor power supply are separate units, the white wire will need to be connected to the negative power supply terminal and to the data ground or common terminal on the data recorder.
The SDI-12 protocol uses an address to distinguish between multiple sensors all connected to the same wire. These must be set uniquely for each sensor wired to a common SDI-12 communications port. Sometimes a single communications port will have multiple connectors to accommodate easily connecting sensors, so care should be taken to understand the configuration of the data reader or recorder.
SDI-12 addresses consist of only a single character. TDR sensors usually ship from the factory programmed with address ‘0’. However, this can be changed to any one of 62 possible addresses including:
- Numbers 0-9
- Uppercase letters A-Z
- Lowercase letters a-z
SDI-12 data recorders and readers should provide a method of assigning addresses to sensors. Consult the manual for these products to find the appropriate procedure.
The Acclima SDI-12 Reader provides a very convenient way to check sensor operation and to assign addresses to sensors. With a few button pushes a sensor can be re-addressed. At the same time the reader can be used to take both readings and waveform images from the sensor to assure that the sensor is installed tightly in the soil and reporting properly. GPS coordinates can also be provided from the reader to document the location of the sensor. After each sensor has been individually addressed the full set of sensors can be connected to the data recorder for operation.
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