Penetration Depth of a Soil Moisture Profile Probe Working in Time-Domain Transmission Mode (original) (raw)
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Frequency Domain Probe Design for High Frequency Sensing of Soil Moisture
Agriculture, 2016
Accurate moisture sensing is an important need for many research programs as well as in control of industrial processes. This paper describes the development of a high accuracy frequency domain sensing probe for use in obtaining dielectric measurements of materials suitable for work ranging from 300 MHz to 1 GHz. The probe was developed to accommodate a wide range of permittivity's ranging from ε r = 2.5 to elevated permittivity's as high as ε r = 40. The design provides a well-matched interface between the soil and the interconnecting cables. A key advantage of the frequency domain approach is that a change of salt concentration has a significantly reduced effect on ε , versus the traditional time-domain reflectometry, TDR, measured apparent permittivity, Ka.
Design of the Probe Sensor for the TDR Soil Moisture Sensor System
The probe sensor is of two parallel plates type which is used to determine the moisture content of the soil. The probe sensor is connected to a resistance to time period converter circuit of the TDR soil moisture sensor system whose output time period depends upon the resistance of the soil which in turn depends upon the moisture content of the soil acting as medium between the plates of the probe sensor. The TDR probe sensor is designed and simulated using the Integrated Electro software in order to determine the effects of the parameters like length, thickness and gap between the plates on electric field and energy density. The simulation results are used to predict and determine the geometry of the probe sensor, the materials that should be used in making the plates of the probe sensor and coating the plates of the probe sensor for reducing the effects of the fringing field and noise in the environment.
DEStech Transactions on Materials Science and Engineering
This paper presents the results of a field study conducted at three different locations close to the campus of University of Texas at Arlington, Texas. Time Domain Reflectometry (TDR) and Time Domain Transmissivity (TDT) Sensors were installed at depths of 5 cm, 10 cm, 25 cm, 50 cm, and 100 cm below ground level. The soils at the three sites were tested for basic geotechnical properties such as size analysis, specific gravity, density, and hydraulic conductivity. In-situ infiltration tests were also performed to determine filed hydraulic conductivity. The observed moisture data response very well with recorded rainfall event and infiltration were greatly affected by rain intensity and duration.
Non-invasive probing of the near-surface soil moisture profile
Time domain reflectometry imaging (TDRI) is a technique for non-invasive measurement of moisture content distribution. A solution to the 2-D problem has previously been described, and here we explain a configuration that provides a practical system for measuring a 1-D vertical profile of soil moisture. The measurement system consists of an enhanced TDR transmission line and reflectometer together with specialist software, to translate measurements of propagation times into a moisture content profile. We demonstrate a soil moisture accuracy of better than 2% for each 20 mm layer to 60 mm depth, with increasing accuracy loss for greater depths. However the attainable accuracy is dependent on the soil type and the dielectric model used for translation of permittivity profile to soil moisture profile. A practical depth limit for the current configuration is 100 mm. Assumptions and limitations of the technique will also be described.
A manually controlled TDR soil moisture meter operating with 300 ps rise-time needle pulse
Irrigation Science, 1989
The principle of operation of a simple, manually controlled Time-Domain Reflectometer (TDR meter) for soil moisture measurements, which operates with a needle pulse of 300 ps rise-time, is described. A block diagram and constructions are also given. Construction of a switchless multiple sensor probe, having an inherent delay reference, is presented. Results of measurements of the soil dielectric constant as related to water content, for soils having different bulk densities, textures and humus content show a high correlation. The results agree closely with other investigators measurements with different, more expensive, TDR instruments. The general principle of microprocessor-controlled TDRoperated soil moisture meter is considered.
Sensors, 2020
Soil volumetric water content (θ) is a parameter describing one of the most important factors conditioning proper plant growth. Monitoring soil moisture is of particular importance in the rational use of water resources for irrigation, especially during periods of water scarcity. This paper presents a method of measuring soil moisture in the vicinity of the plant root system by means of a probe designed to be mounted on a mobile device used for precise plant irrigation. Due to the specific field conditions of the measurement, the design of the probe was proposed as a monopole antenna. Electromagnetic simulations of the probe were carried out with Ansys HFSS software to optimise its dimensions. Then a prototype of the probe was manufactured to conduct laboratory measurements with the use of a vector network analyser (VNA) working in the 20 kHz to 8 GHz frequency range. The VNA analyser was configured to work in the time-domain reflectometry (TDR) mode. From measurements of the time d...
2010
A special method of overlapping measurements was proposed to increase the vertical resolution of a TDR tube soil moisture sensor)probe Trime FM3. An inversion algorithm has been developed to derive soil moisture profiles along single TDR sensor)probe, which is based on the hypothesis of linear contribution of soil moisture content of layers enclosed in a cylindrical soil volume. This model was tested during experimentation on an artificial soil monolith assembled from 18 homogeneous horizontal discrete macro layers of 5 cm of height fabricated from dark)chestnut soil and artificially moistened to different values. For the first step, a series of overlapping measurements was produced using a TDR device, with step size of 1cm. Measured values were then compared to calculations based on the proposed model using moisture values of discrete macro layers as input parameters. At this stage the best agreement between measured moisture values and proposed model was achieved with a height of ...
Measurement, 2013
In this paper studies on the impact of the location of Time-Domain Reflectometry probes in soil samples on the moisture measurement are presented. In particular, we were interested if the commonly accepted assumption that moisture measurements performed by TDR probes correctly average the value of the soil water content in a soil sample. Soil samples having different physical properties, both undisturbed and disturbed, were used. Our results show that the soil moisture measurement depends on the placement of the TDR probe in the sample, and a TDR probe placed vertically measures the arithmetic mean of soil moisture for the whole sample, for both the wetting and the drying cycles. This result is general and does not depend on the analyzed sample.
Computers and Electronics in Agriculture, 2018
Frequency Domain Analysis (FDA), as an approach, has been developed for the measurement of soil dielectric constants. As it stands, the standard dielectric of dry soil is much less than the dielectric of soil exposed to water, and the volume of water present significantly affects the propagation of electromagnetic waves. With this in mind, this paper proposes a plan for the design and implementation of two distinct isolated probe structures for the measurement of water contents within the soil at different levels. Accordingly, Probe A is used to determine the water level present in soil at four different depths. Probe A does this by utilizing pairs of parabolic copper sections fixed horizontally and isolated over the outer surface of an access tube. Probe B, on the other hand, makes use of two steel rings buried vertically inside of an access tube, and is used to determine the water contents of soil on two levels. To do so, a fixed frequency square-wave is transmitted to measure the soil capacitance in which the probe sensors are connected to an Arduino microcontroller which also included air humidity, air temperature, and soil temperature sensors. During the experimental assessment of both probes, the results are loaded onto an SD memory card and are then compared with the results of other commercial sensors installed in the same irrigated plot. The soil moisture monitoring station used is powered by a photovoltaic (PV) module of 10 W 12 V and a storage battery of 12 Ah. The experimental monitoring station used to assess the efficiency of both probe designs was set up in a Mediterranean semiarid zone in the Southeast of Spain.
Calibration of a TDR probe for low soil water content measurements
Sensors and Actuators A-physical, 2008
Time domain reflectometry (TDR) probes are increasingly used for field and laboratory estimation of soil water content. Usual calibration of TDR probes for the determination of soil water content uses two media: air for low and water for high values of dielectric permittivity, although the measured range of dielectric permittivity in soil is much smaller as compared to the range implied by the calibration media. The use of air for calibration of short TDR probes gives calibration errors due to overlapping incident and reflected pulses in the reflectogram, which result in their relative shift in time. This phenomenon, named the convolution effect, can be avoided by the application of selected calibration media. The presented approach minimizes of dielectric permittivity measurement errors by choosing the calibration media with dielectric permittivity values close to the limits of the measurement range and the possibility to use TDR probes of various lengths. The comparison of errors of TDR apparent dielectric permittivity measurement in three sandy soils, based on the probe calibrations in various media, is also presented.