Analysis of Coaxial Soil Cell in Reflection and Transmission

Accurate measurement of moisture content is a prime requirement in hydrological, geophysical and biogeochemical research as well as for material characterization and process control. Within these areas, accurate measurements of the surface area and bound water content is becoming increasingly important for providing answers to many fundamental questions ranging from characterization of cotton fiber maturity, to accurate characterization of soil water content in soil water conservation research to bio-plant water utilization to chemical reactions and diffusions of ionic species across membranes in cells as well as in the dense suspensions that occur in surface films. In these bound water materials, the errors in the traditional time-domain-reflectometer, “TDR”, exceed the range of the full span of the material’s permittivity that is being measured. Thus, there is a critical need to re-examine the TDR system and identify where the errors are to direct future research. One promising technique to address the increasing demands for higher accuracy water content measurements is utilization of electrical permittivity characterization of materials. This technique has enjoyed a strong following in the soil-science and geological community through measurements of apparent permittivity via time-domain-reflectometery as well in many process control applications. Recent research however, is indicating a need to increase the accuracy beyond that available from traditional TDR. The most logical pathway then becomes a transition from TDR based measurements to network analyzer measurements of absolute permittivity that will remove the adverse effects that high surface area soils and conductivity impart onto the measurements of apparent permittivity in traditional TDR applications. This research examines the theoretical basis behind the coaxial probe, from which the modern TDR probe originated from, to provide a basis on which to perform absolute permittivity measurements. The research reveals currently utilized formulations in accepted techniques for permittivity measurements which violate the underlying assumptions inherent in the basic models due to the TDR acting as an antenna by radiating energy off the end of the probe, rather than returning it back to the source as is the current assumption. To remove the effects of radiation from the experimental results obtain herein, this research utilized custom designed coaxial probes of various diameters and probe lengths by which to test the coaxial cell measurement technique for accuracy in determination of absolute permittivity. In doing so, the research reveals that the basic models available in the literature all omitted a key correction factor that is hypothesized by this research as being most likely due to fringe capacitance. To test this theory, a Poisson model of a coaxial cell was formulated to calculate the effective extra length provided by the fringe capacitance which is then used to correct the experimental results such that experimental measurements utilizing differing coaxial cell diameters and probe lengths, upon correction with the Poisson model derived correction factor, all produce the same results thereby lending support for the use of an augmented measurement technique, described herein, for measurement of absolute permittivity, as opposed to the traditional TDR measurement of apparent permittivity

Development of impedance spectroscopy based in-situ, self-calibrating, on-board wireless sensor with inbuilt metamaterial inspired small antenna for constituent detection in multi-phase mixtures like soil

Real time and accurate measurement of sub-surface soil moisture and nutrients is critical for agricultural and environmental studies. This work presents a novel on-board solution for a robust, accurate and self-calibrating soil moisture and nutrient sensor with inbuilt wireless transmission and reception capability that makes it ideally suited to act as a node in a network spread over a large area. The sensor works on the principle of soil impedance measurement by comparing the amplitude and phase of signals incident on and reflected from the soil in proximity of the sensor. The permittivity of the soil dielectric mixture which is calculated from these impedance measurements is used as input parameter to the dielectric mixing models which are used to estimate the ionic concentration in soil. The inbuilt wireless transceiver system is connected to a specially designed metamaterial inspired small antenna in order to reduce the sensor size while keeping the path losses to a minimum by using a low frequency. This composite right-left handed (CRLH) antenna for wireless transmission at 433 MHz doubles up as an underground, sensing element (external capacitor) and integrates with the on-board sensor for soil moisture and nutrient determination. The input impedance of the CRLH sensor, surrounded by the soil containing moisture and nutrient and other ions, is measured at multiple frequencies. It is shown that the change in moisture and ioinic-concentration can be successfully detected using the sensor. The inbuilt self-calibrating mechanism makes the sensor reliable at different environmental conditions and also useful for remote, underground and hand-held applications. A multi-power mode transceiver system has been designed to support the implementation of an energy efficient medium-access-control

Issues Pertaining to Dielectric Assessment of Soil Moisture in a Coaxial Cell

The study considered a pre-existing coaxial cell design and attempted to improve upon the accuracy of the coaxial cell as a measurement device. The cell was modeled with a revised transmission line model. Simulations were conducted using MatLab and were compared to data taken from an Agilent Vector Network Analyzer using Root Mean Square Error (RMSE) analysis to estimate the relative permittivity of samples contained in the coaxial cell's sample chamber. It was found that while the new transmission line model was more rigorous in its representation of the coaxial cell, accuracy was still very limited. As a result, the coaxial cell should be redesigned to eliminate possible complications resulting from its current structure. A transmission line model may not be complex enough to properly represent the cell.School of Electrical & Computer Engineerin

Development of dielectric spectroscopic sensor for contaminant detection in a hydraulic fluid and a compressed air stream

A change in a fluid’s dielectric properties can be investigated using dielectric spectroscopy to gain valuable insight into the changing condition of the fluid. A dielectric spectroscopic sensor was developed using a cylindrical capacitive sensing unit with the fluid as the dielectric media. The sensor was used to estimate or detect contaminants in a hydraulic fluid and a compressed air stream. Tests were performed with a hydraulic fluid in which the dielectric sensor’s performance was evaluated in detecting iron powder and ISO medium test dust particles as contaminants in the fluid. Using iron powder as contaminants, two tests were performed with central electrodes of diameters 6.35 mm and 17.7 mm inch placed inside the capacitive dielectric sensor. The results from partial least squares (PLS) regression showed that the root mean square error of calibration (RMSEC) and the root mean square error of cross-validation (RMSECV) for a 6.35 mm (0.25-inch) diameter central electrode were 1.1 and 1.39 of adjusted ISO cleanliness code respectively. For a 17.7 mm (0.70-inch) diameter central electrode, the RMSEC and RMSECV values were 0.62 and 0.83 of adjusted ISO cleanliness code, respectively. Similarly, a test was performed using ISO test dust particles as contaminants with a central electrode of 17.7 mm diameter. The RMSEC and RMSECV values from the model for ISO test dust were 1.29 and 1.48 of adjusted ISO cleanliness code, respectively. Tests were also conducted to investigate the efficacy of dielectric spectroscopy in detecting water and oil droplets in a compressed air stream. Spray nozzles were used to produce fine droplets of deionized water and light lubricant oil. Multivariate statistical techniques, principal component analysis (PCA) and linear discriminant analysis (LDA), were used to develop statistical classifiers, which determined the performance of dielectric spectroscopic sensor in differentiating the dry compressed air from an air stream with entrained liquid droplets. Through model calibration and cross-validation, the classifiers were able to separate the two cases without any errors, validating the dielectric sensor’s ability to detect of liquid droplets in an air stream

Comparison of techniques for measuring the water content of soil and other porous media

The measurement of water in soil on a potential, gravimetric or volumetric basis is considered, with studies concentrating on the measurement of water by dielectric and neutron moderation methods. The ability of the time-domain reflectometry technique to measure water content simultaneously at different spatial locations is an important advantage of the technique. The reported apparent dielectric by the TRASE� time-domain reflectometer and Pyelab time-domain reflectometry systems is sensitive to change in extension cable length. In some soil, e.g. a commercial sand, the response to increasing extension length of extension cable is linear. For other soil a linear response occurs for certain lengths of cable at different moisture contents. A single model accounting for clay content, extension cable length, time-domain reflectometry system, probe type and inherent moisture conditions explained 62.2 % of variation from the control (0 m extension) cable. The extension cable causes a decrease in the returning electromagnetic-wave energy; leading to a decline in the slope used in automatic end-point determination. Calibration for each probe installation when the soil is saturated, and at small water contents is recommended. The ability of time-domain reflectometry, frequency-domain and neutron moderation techniques in measuring soil water content in a Brown Chromosol is examined. An in situ calibration, across a limited range of water contents, for the neutron moderation method is more sensitive to changing soil water content than the factory supplied 'universal' calibration. Comparison of the EnviroSCAN� frequency-domain system and the NMM count ratio indicates the frequency-domain technique is more sensitive to change in soil water conditions. The EnviroSCAN� system is well suited to continuous profile-based measurement of soil water content. Results with the time-domain reflectometry technique were disappointing, indicating the limited applicability of time-domain reflectometry in profile based soil water content measurement in heavy-textured soil, or soil with a large electrical conductivity. The method of auguring to a known depth and placement of the time-domain reflectometry probe into undisturbed soil is not recommended. A time-domain reflectometry system is adapted for in situ measurement of water in an iron ore stockpile. The laboratory calibration for water content of the processed iron ore compares favourably to a field calibration. In the field study, the 28 m extension cable used to connect the probes to the time-domain reflectometry affected the end-point determination of the time-domain reflectometry system. To account for this, 0.197 should be subtracted from the reported apparent dielectric before calculation of volumetric moisture content

Comparison of techniques for measuring the water content of soil and other porous media

The measurement of water in soil on a potential, gravimetric or volumetric basis is considered, with studies concentrating on the measurement of water by dielectric and neutron moderation methods. The ability of the time-domain reflectometry technique to measure water content simultaneously at different spatial locations is an important advantage of the technique. The reported apparent dielectric by the TRASE� time-domain reflectometer and Pyelab time-domain reflectometry systems is sensitive to change in extension cable length. In some soil, e.g. a commercial sand, the response to increasing extension length of extension cable is linear. For other soil a linear response occurs for certain lengths of cable at different moisture contents. A single model accounting for clay content, extension cable length, time-domain reflectometry system, probe type and inherent moisture conditions explained 62.2 % of variation from the control (0 m extension) cable. The extension cable causes a decrease in the returning electromagnetic-wave energy; leading to a decline in the slope used in automatic end-point determination. Calibration for each probe installation when the soil is saturated, and at small water contents is recommended. The ability of time-domain reflectometry, frequency-domain and neutron moderation techniques in measuring soil water content in a Brown Chromosol is examined. An in situ calibration, across a limited range of water contents, for the neutron moderation method is more sensitive to changing soil water content than the factory supplied 'universal' calibration. Comparison of the EnviroSCAN� frequency-domain system and the NMM count ratio indicates the frequency-domain technique is more sensitive to change in soil water conditions. The EnviroSCAN� system is well suited to continuous profile-based measurement of soil water content. Results with the time-domain reflectometry technique were disappointing, indicating the limited applicability of time-domain reflectometry in profile based soil water content measurement in heavy-textured soil, or soil with a large electrical conductivity. The method of auguring to a known depth and placement of the time-domain reflectometry probe into undisturbed soil is not recommended. A time-domain reflectometry system is adapted for in situ measurement of water in an iron ore stockpile. The laboratory calibration for water content of the processed iron ore compares favourably to a field calibration. In the field study, the 28 m extension cable used to connect the probes to the time-domain reflectometry affected the end-point determination of the time-domain reflectometry system. To account for this, 0.197 should be subtracted from the reported apparent dielectric before calculation of volumetric moisture content

Development of a dielectric based soil moisture sensor

Agricultural Engineerin

Soil Moisture Sensing via Swept Frequency Based Microwave Sensors

There is a need for low-cost, high-accuracy measurement of water content in various materials. This study assesses the performance of a new microwave swept frequency domain instrument (SFI) that has promise to provide a low-cost, high-accuracy alternative to the traditional and more expensive time domain reflectometry (TDR). The technique obtains permittivity measurements of soils in the frequency domain utilizing a through transmission configuration, transmissometry, which provides a frequency domain transmissometry measurement (FDT). The measurement is comparable to time domain transmissometry (TDT) with the added advantage of also being able to separately quantify the real and imaginary portions of the complex permittivity so that the measured bulk permittivity is more accurate that the measurement TDR provides where the apparent permittivity is impacted by the signal loss, which can be significant in heavier soils. The experimental SFI was compared with a high-end 12 GHz TDR/TDT system across a range of soils at varying soil water contents and densities. As propagation delay is the fundamental measurement of interest to the well-established TDR or TDT technique; the first set of tests utilized precision propagation delay lines to test the accuracy of the SFI instrument’s ability to resolve propagation delays across the expected range of delays that a soil probe would present when subjected to the expected range of soil types and soil moisture typical to an agronomic cropping system. The results of the precision-delay line testing suggests the instrument is capable of predicting propagation delays with a RMSE of +/−105 ps across the range of delays ranging from 0 to 12,000 ps with a coefficient of determination of r2 = 0.998. The second phase of tests noted the rich history of TDR for prediction of soil moisture and leveraged this history by utilizing TDT measured with a high-end Hewlett Packard TDR/TDT instrument to directly benchmark the SFI instrument over a range of soil types, at varying levels of moisture. This testing protocol was developed to provide the best possible comparison between SFI to TDT than would otherwise be possible by using soil moisture as the bench mark, due to variations in soil density between soil water content levels which are known to impact the calibration between TDR’s estimate of soil water content from the measured propagation delay which is converted to an apparent permittivity measurement. This experimental decision, to compare propagation delay of TDT to FDT, effectively removes the errors due to variations in packing density from the evaluation and provides a direct comparison between the SFI instrument and the time domain technique of TDT. The tests utilized three soils (a sand, an Acuff loam and an Olton clay-loam) that were packed to varying bulk densities and prepared to provide a range of water contents and electrical conductivities by which to compare the performance of the SFI technology to TDT measurements of propagation delay. For each sample tested, the SFI instrument and the TDT both performed the measurements on the exact same probe, thereby both instruments were measuring the exact same soil/soil-probe response to ensure the most accurate means to compare the SFI instrument to a high-end TDT instrument. Test results provided an estimated instrumental accuracy for the SFI of +/−0.98% of full scale, RMSE basis, for the precision delay lines and +/−1.32% when the SFI was evaluated on loam and clay loam soils, in comparison to TDT as the bench-mark. Results from both experiments provide evidence that the low-cost SFI approach is a viable alternative to conventional TDR/TDT for high accuracy applications

Study of moisture in concrete utilizing the effect on the electromagnetic fields at UHF frequency on an embedded transmission line

A thesis submitted to the Faculty of Creative Arts and Technologies, University of Luton, in partial fulfilment of the requirements for the degree Doctor of Philosophy.The aim of the research was to find an effective, reliable and cost-effective method for long-term monitoring of moisture in concrete structures. The slow diffusion rate of moisture through concrete requires that monitoring should be done over time scales of several years without periodic re-calibration. The solution arrived at was to use a quasi-coaxial transmission line, termed a cagecoaxial transmission line, as the sensing element. The transmission line, terminated in a short circuit, is encapsulated in a porous dielectric medium. It was found that the microstructure of the encapsulating medium had to be similar to the concrete in terms of capillary characteristics in order to track the moisture content of the material under test. The moisture in the encapsulating medium would change the electrical length of the transmission line by increasing the relative permittivity of the medium. The method used makes it possible to measure moisture levels to full saturation. Moisture content can be measured in terms of a percentage of saturation, which will be of considerable help as an early warning system of possible frost damage. A mathematical model was derived to calculate the relative permittivity in terms of moisture content in concrete. It was shown that to calculate the total permittivity of a solid porous medium with a dielectric mix formula, the formula must be expanded to include air, water and solid, before realistic values for the permittivity of the ingredients could be assigned. A dielectric mix formula was derived to account for the liquid to solid boundary effect on the permittivity of water in a solid porous material. The foundations were laid for the development of a reliable and cost-effective probe based on an oscillator, operating around 1 GHz, using the transmission line as a tuning element. The frequency of oscillation is a function of the apparent length, determined by the permittivity and therefore the moisture content, in the transmission line dielectric material. A method to convert this frequency to a format that can be monitored on a data logger system is described. The high oscillation frequency eliminates the effect of ionic conduction from dissolved substances