Applications of Thermo-TDR Sensors for Soil Physical Measurements

Advanced sensors provide new opportunities to improve the understanding of soil properties and processes. One such sensor is the thermo-TDR sensor, which combines the functions of heat pulse probes and time domain reflectometry probes. Recent advancements in fine-scale measurements of soil thermal, hydraulic, and electrical properties with the thermo-TDR sensor enable measuring soil state variables (temperature, water content, and ice content), thermal and electrical properties (thermal diffusivity, heat capacity, thermal conductivity, and bulk electrical conductivity), structural parameters (bulk density and air-filled porosity) and fluxes (heat, water, and vapor) simultaneously. This chapter describes the theory, methodology, and potential applications of the thermo-TDR technique.

The comparative analysis of thermal effects in elastomers modified with MCNT at constant DC voltage

The author carried out the comparative analysis of elastomers – polyurethane (NPC) and silicone compound (NCOC) modified with carbon nanotubes (MCNT) with a mass content of 1 to 9 %. MCNTs were synthetically produced by the CVD technology using Co-Mo/Al2O3-MgO (MCNT1) and Fe-Co/2,1Al2O3 (MCNT2) catalysts. The analysis of experimental study results showed that the lowest specific bulk electrical conductivity (5×10-10 Cm×cm-1) was typical for polyurethane elastomer (1 mass. % MCNT synthetically produced using Fe-Co/2,1Al2O3 catalyst). For the silicone elastomer modified with 9 mass. % MCNT1, the specific bulk electrical conductivity was 4×10-1 Cm×cm-1. The author identified the parameters of percolation of electrical conductivity model for NPC, NCOC with MCNT1 and MCNT2, taking into account the MCNT packing factor and electrical conductivity critical index. The maximum temperature field uniformity is typical for silicone elastomer with 7 mass. % MCNT2. Nonuniform temperature field in modified polyurethane-based elastomers can be caused by the local MCNT entanglement manifested in the creation of agglomerates or more dense electrically-conductive circuit packing, which, in its turn, results in the decrease in heat power. The heating temperature of nanomodified composites produced from NCOC 1 and NCOC 2 can vary from 32.9 to 102 °С. The author studied the modes of nanomodified elastomers heat generation in the range of 6 to 30 V, compared heat generation in the elastomer-based and ceramics-based samples. The study allowed identifying the best combination of the polymeric matrix and MCNT type. For the electric heater, it is the most efficient to apply silicone compound at the 7 % MCNT concentration and, depending on the feeding voltage level of 12 or 24 V, to use MCNT1 or MCNT2.

Spatial and Temporal Variability of Soil Redox Potential, pH and Electrical Conductivity across a Toposequence in the Savanna of West Africa

Soil redox potential is an important factor affecting soil functioning. Yet, very few agronomy studies included soil redox potential in relation to soil processes. The objective of this study was to evaluate the spatial and temporal variation in soil redox potential and to determine the soil parameters affecting its variation. Soil redox potential, soil moisture, soil temperature, pH and bulk electrical conductivity were measured in upland rice fields during two growing seasons at six positions along an upland–lowland continuum, including two positions at the upland, two at the fringe and two at the lowlands in central Côte d’Ivoire (West Africa). The measurements were made at the following soil depths: 3, 8, 20 and 35 cm. Soil redox potential varied between 500 and 700 mV at the upland positions, 400 and 700 mV at the fringe positions and 100 and 750 mV at the lowland positions, and increased with soil depth. Variations in soil redox potential were driven by soil moisture, bulk electrical conductivity and soil organic carbon. We concluded that for proper interpretation of soil redox potential, sampling protocols should systematically include soil pH, moisture and bulk electrical conductivity measurements.

Field Comparison of Electrical Resistance, Electromagnetic Induction, and Frequency Domain Reflectometry for Soil Salinity Appraisal

By using different physical foundations and technologies, many probes have been developed for on-site soil salinity appraisal in the last forty years. In order to better understand their respective technical and practical advantages and constraints, comparisons among probes are needed. In this study, three different probes, based on electrical resistance (ER), electromagnetic induction (EMI), and frequency domain reflectometry (FDR), were compared during a field survey carried out in a large salt-threatened agricultural area. Information about the soil bulk electrical conductivity (σb) at different depths was obtained with each of the probes and, additionally, other soil properties were also measured depending on the specifications of each instrument and, moreover, determined in samples. On average, the EMI and FDR techniques could be regarded as equivalent for σb measurement, whereas ER gave higher σb values. Whatever the case, EMI, and also ER, had to be supplemented with information about soil clay, organic matter, and water mass fractions to attain, despite this effort, poor soil salinity estimations by means of multiple linear regression models (R2 < 0.5). On the contrary, FDR needed only probe data to achieve R2 of 0.7, though root mean standard error (RMSE) was still 1.5 dS m−1. The extra measurements and calculations that modern electrical conductivity contact probes integrate, specifically, those based on FDR, remarkably increase their ability for soil salinity appraisal, although there is still room for improvement.

Assessment of the Combined Effect of Temperature and Salinity on the Outputs of Soil Dielectric Sensors in Coconut Fiber

Dielectric sensors are useful instruments for measuring soil moisture and salinity. The soil moisture is determined by measuring the dielectric permittivity, while bulk electrical conductivity (EC) is measured directly. However, permittivity and bulk EC can be altered by many variables such as measurement frequency, soil texture, salinity, or temperature. Soil temperature variation is a crucial factor as there is much evidence showing that global warming is taking place. This work aims to assess how variations in the temperature and salinity of coconut fiber affect the output of EC5 (voltage) and GS3 (permittivity and bulk EC) Decagon sensors. The results showed that the effect of temperature and salinity on the output of the sensors can lead to substantial errors in moisture estimations. At low salinity values, permittivity readings decreased as temperature increased, while voltage readings were not affected, regardless of substrate moisture. The GS3 sensor underestimated the bulk EC when it is measured below 25 °C. The temperature dependence of the voltage of EC5 was not significant up to 10 dS m−1, and the permittivity of the GS3 was more affected by the interaction between temperature and salinity. The effect that salinity has on the permittivity of the GS3 sensor can be reduced if a permittivity–moisture calibration is performed with saline solutions, while the effect resulting from the interaction between temperature and salinity can be minimized using a regression model that considers such an interaction.

Assessment of Concrete Curing Duration using Bulk Electrical Conductivity and Porosity

Controlling permeability of concrete is essential for enhancing durability and, thus, the service life. Concrete permeability is affected by the total volume of permeable voids and the continuity of the capillary pore structure. Even though concrete strength is the typical performance parameter used to define a wet curing duration, it is important to maintain wet curing until concrete develops a discontinuous capillary pore structure with a minimum volume of total permeable voids to assure durability. Therefore, the required wet curing period can be defined as the longest duration out from the (i) time to achieve the specified strength, (ii) time to develop a discontinuous capillary pore structure, and (iii) time to develop a minimum volume of total permeable voids. Since concrete strength evaluation methods are well developed, there is a need for developing procedures to evaluate concrete pore structure characteristics to decide on the wet curing duration. This study investigated the use of bulk electrical conductivity and porosity test methods described in ASTM C1760 and C642 to evaluate the (i) time to develop a discontinuous capillary pore structure and (ii) time to develop a minimum volume of total permeable voids, respectively. The suggested procedure of wet curing duration assessment is demonstrated for two concrete mixes: one with only Type I cement and the other with Type I cement and slag.

Time-lapse cross-hole electrical resistivity tomography (CHERT) for monitoring seawater intrusion dynamics in a Mediterranean aquifer

Surface electrical resistivity tomography (ERT) is a widely used tool to study seawater intrusion (SWI). It is noninvasive and offers a high spatial coverage at a low cost, but its imaging capabilities are strongly affected by decreasing resolution with depth. We conjecture that the use of CHERT (cross-hole ERT) can partly overcome these resolution limitations since the electrodes are placed at depth, which implies that the model resolution does not decrease at the depths of interest. The objective of this study is to test the CHERT for imaging the SWI and monitoring its dynamics at the Argentona site, a well-instrumented field site of a coastal alluvial aquifer located 40 km NE of Barcelona. To do so, we installed permanent electrodes around boreholes attached to the PVC pipes to perform time-lapse monitoring of the SWI on a transect perpendicular to the coastline. After 2 years of monitoring, we observe variability of SWI at different timescales: (1) natural seasonal variations and aquifer salinization that we attribute to long-term drought and (2) short-term fluctuations due to sea storms or flooding in the nearby stream during heavy rain events. The spatial imaging of bulk electrical conductivity allows us to explain non-monotonic salinity profiles in open boreholes (step-wise profiles really reflect the presence of freshwater at depth). By comparing CHERT results with traditional in situ measurements such as electrical conductivity of water samples and bulk electrical conductivity from induction logs, we conclude that CHERT is a reliable and cost-effective imaging tool for monitoring SWI dynamics.

A wireless system for volumetric water content measurement by TDR

&lt;p&gt;Time Domain Reflectometry (TDR) is an accurate and widely used technique for real time estimation of soil volumetric water content (&amp;#952;), and the bulk electrical conductivity (&amp;#963;). Although there are multiple software that allow monitoring &amp;#952; and &amp;#963; by connecting the TDR device to a PC, this system used under field conditions can be in many cases awkward. This paper presents a wireless, portable, unexpansive, simple, and versatile system to measure &amp;#952; and the &amp;#963; by connecting the TDR device to a smart phone. The system consists on a M5Stack processing unit that integrates a Wifi connectivity. The UART port of the M5Stack is connected to the TDR device through RS232-ttl adapter. The hardware is programmed in micropython language that allows the M5Stack acts as a server between the user and the TDR device through a web page read with a smart phone. The software, which is compatible with Campbell TDR100 and 1502C Tektronix devices, allows creating different project where the TDR waveforms are stored. A simple &amp;#952; and the &amp;#963; measurement is also allowed. Since the objective of the portable system is to ease and makes &amp;#952; and &amp;#963; samplings faster, a complementary web page for subsequent and more accurate estimates of &amp;#952; and &amp;#963; was also developed.&lt;/p&gt;