Dual-probe Heat-pulse Research Articles

Open-source electronics Arduino platform for soil water measurement with the dual-probe heat-pulse (DPHP) method

Soil moisture content, which is important in agriculture, has large spatiotemporal variability, making inexpensive multi-site monitoring ideal. The dual-probe heat-pulse (DPHP) method is attractive for estimating soil volumetric water content from soil thermal properties, as the method is physically based and relatively straightforward. In this manuscript, we report on implementation of this method using a custom-made inexpensive open-source electronic hardware measurement system, based on the Arduino Uno platform. We compare the measurement and control performance of the Arduino system to those of a high-end commercial data acquisition system. By conducting the measurements with both systems, sequentially, with the sensors remaining in place, variations that may arise from the sensor installation and sample preparation are minimal, thereby allowing for a direct comparison between the data acquisition systems. The measurements are conducted with four rigid-DPHP (RDPHP) sensors in three media: immobilized water, dry quartz sand, and saturated quartz sand. The identical cylindrical perfect conductors (ICPC) model is fit to the data through parameter optimization, for estimating the unknown properties: effective probe spacing and thermal properties. The volumetric water content is calculated from the volumetric heat capacity through a linear relationship. The measurement results indicate that the open hardware Arduino system has similar accuracy to that of the commercial acquisition system. The results show that the proposed Arduino system can be successfully implemented for the DPHP method, making it an attractive option for a low-cost system, alternative to the available high-end commercial system.

Optimized measurement of frozen soil thermal properties using a heat-pulse sensor

The measurement of thermal properties in partially frozen soil is crucial for global climate models. As the primary method for soil thermal property measurement, the Dual-Probe Heat-Pulse (DPHP) sensor experiences significant errors when used in partially frozen soils. The application of heat pulses leads to ice melting, and the existing DPHP theory, which does not consider phase changes, fails to accurately determine thermal conductivity (k) and heat capacity (C) in the temperature range of − 5 °C to 0 °C. There is a lack of DPHP theory specifically designed for use in partially frozen soils. The existing DPHP theory employs the Infinite Linear Source (ILS) approximation, which does not account for the melting process and the moving ice-liquid interface in transient heat conduction theory. Currently, there is no effective and reliable method to assess DPHP errors associated with measuring the thermal properties of partially frozen soil. To accurately determine k and C in frozen soil, it is essential to perform an error analysis regarding ice melting caused by a DPHP heat pulse input. In this study, (1) we considered the latent heat of ice melting and a moving ice-liquid interface in finite element simulations and compared the results with an analytical solution presented by Paterson (1952); (2) we performed simulations of DPHP measurements based on the temperature-dependent ice content, liquid water content, and melting latent heat of three frozen soils used by Watanabe and Wake (2009); (3) based on COMSOL simulation results for the three frozen soils, we determined the optimal heating parameter combinations (heating duration, t0, and accumulated heating energy, ΔQ) to minimize DPHP measurement errors in frozen soil. The results showed that (1) Simulations that included the melting phase changes were in agreement with the analytical solution by Paterson (1952). (2) Including the ice-liquid moving interface significantly altered the spatial distribution characteristics of temperature, which were not captured by the ILS model in terms of the temperature distribution near the interface. (3) Below a temperature of − 5.5 °C, the simulations that included phase changes were consistent with measured results; (4) For partially frozen soils with initial temperatures ranging from − 0.5 °C to 0 °C, the relative errors in thermal conductivity fitted by the ILS model exceeded 100 %. To mitigate the influence of ice melting, we recommend using 200 < ΔQ < 400 J m−1 for sand, loam, and silt loam soils with 8 s < t0 < 60 s. For a loam soil with t0 = 60 s, ΔQ = 800 J m−1 should be used. For a silt loam soil with t0 = 60 s, we recommend using ΔQ = 600 J m−1. If the ILS model is chosen to calculate soil thermal properties, we recommend using 200 <ΔQ < 600 J m−1 and 30 s < t0 < 60 s for sandy and silt loam soils. For a loam soil, we recommend using 400 < ΔQ < 800 J m−1 and 30 s < t0 < 60 s.

Improvement and Performance Evaluation of a Dual-Probe Heat Pulse Distributed Temperature Sensing Method Used for Soil Moisture Estimation.

Large-scale measurements of soil moisture play a critical role in many fields, such as agriculture, hydrology, and engineering. The distributed temperature sensing (DTS) technology, based on a dual-probe heat pulse (DPHP), is a novel approach to realizing large-scale soil moisture estimation. However, the application of the method is limited by the complex optical cable layout, calculation algorithm, and lack of standardized heating strategy. In this paper, an improved DPHP-DTS method considering the soil bulk density was proposed. The measurement accuracy of the DPHP-DTS method under different heating strategies was studied in laboratory experiments, and its long-term stability in regard to the monitoring of soil moisture during natural evaporation in different soils was tested. The results show that the improved DPHP-DTS method can accurately measure the soil moisture, and the fitting algorithm can reduce the error caused by the accuracy of the DTS temperature measurement under the low-power heating strategy. Its measurement accuracy increases with the increase in the heating strength and duration. In addition, the improved DPHP-DTS method can describe soil evaporation in both sand and loess with good reliability and stability.

Thermal properties of soft clayey soils from the former Lake Texcoco in Mexico

Thermal properties of soils are essential for the design of many engineering projects, such as underground cable systems, thermal ground improvement techniques, and energy geostructures. Although the thermal properties of typical soils have been widely studied, little research has focused on the thermal characterization of soils with unusual properties, such as the soft clays from the former Lake Texcoco in Mexico. These sediments are known for their exceptionally high water content, plasticity index, and porosities. The present study describes a systematic evaluation of the thermal conductivity, specific heat capacity, and thermal diffusivity of Texcoco clays and their inherent variability. Forty laboratory samples from eleven different locations were studied using the dual-probe heat pulse method, X-ray diffractometry, Scanning Electron Microscopy, and standard soil classification tests. The results showed that the studied samples have lower thermal conductivities and higher specific heat capacities than those previously reported for saturated clays. These differences were mainly attributed to the high porosities of the peculiar clays from the former Lake Texcoco. The thermal conductivity did not show a specific trend with depth. From statistical analyses, it was established that the shifted lognormal distribution and the normal distribution accurately describe the natural variability of the thermal conductivity and specific heat capacity, respectively. Finally, the efficacy of eight selected prediction models for the thermal conductivity of saturated clays was assessed. The geometric mean and the weighted series-parallel heat flow (WSP) models delivered the best fit to the experimental data. This study deepens our understanding of ground thermal properties by providing information about soil with unusual characteristics. Furthermore, it gives an initial database for the thermal properties of Mexican soils that can serve as a reference for the development and design of some of the geotechnical applications previously described.

High‐Resolution Field Measurement of Soil Heat Capacity and Changes in Soil Moisture Using a Dual‐Probe Heat‐Pulse Distributed Temperature Sensing Approach

AbstractIncreasing interest in studying the variability of soil water content and its spatial scale dependency necessitates the development of new techniques to accurately monitor soil water over a wide range of spatial scales. Distributed Temperature Sensing (DTS) techniques offer unprecedented opportunities to measure temperature with a spatial resolution of a few centimeters over several kilometers, which can be used to measure soil moisture. This study is the first of its kind that investigates under field conditions the feasibility of combining the Dual‐Probe Heat‐Pulse (DPHP) technique with the DTS technology to measure soil thermal properties and variation in soil moisture. A field experiment was conducted over a 30 m transect in the Lake Wheeler Field laboratory in Raleigh, NC. Three different DPHP sensors were constructed from combinations of different fiber optic cables and heating elements and were tested to assess their performance and the effect of their construction characteristics on their accuracy. Measurements were taken over different soil moisture conditions and the system performance was compared against independent soil water content sensors. The system was able to track changes in soil water content with a mean RMSE of 0.02 m3 m−3 using the optimal DPHP sensor. The key advantage of the tested system is that it does not need any site‐specific calibrations typically required for other DTS‐based systems. The findings of this study provide some practical information and measures that need to be taken for successful DTS‐DPHP construction and application under field conditions.

A Dual-Probe Heat Pulse Approach using Heated Fiber-Optic Temperature Sensing

The distributed actively heated fiber-optic method is widely used to measure soil thermal conductivity and water content. However, the significant limitations of this technique are that prior soil specific calibration is required, and the measurement of soil volumetric heat capacity, a critical soil thermal parameter, is restricted. This study investigates a novel fiber-optic dual-probe heat pulse method, which utilizes fiber-optics to monitor the temperature response at a certain distance from the heat source to obtain the thermal conductivity and the volumetric heat capacity. Subsequently, the water content can be estimated from its linear relationship with soil volumetric heat capacity. The fiber-optic dual-probe heat pulse method is aimed to overcome the spatial-scale measurement limitations considering the distributed sensing capability of fiber-optic. This requires the sensor to be more robust than the traditional dual-probe method, hence a completely different ratio of the probe diameter to the distance between adjacent cables. Therefore, the applicability of the existing theory needs further verification, and the heating strategies need careful consideration. This study uses a 6 mm diameter fiber Bragg grating (FBG) sensor encapsulated by the alundum tube to conduct laboratory feasibility tests. Two adjacent sensors are fixed in the soil at a distance of 14.60 mm. Different heating powers (30 W/m, 40 W/m, and 50 W/m) and heating durations (10 s, 30 s, 50 s, and 120 s) are implemented. The results showed that the novel approach allows accurate soil volumetric heat capacity and water content estimation with appropriate heating strategies and data interpretation methods. The heating duration below 30 s is a poor fit because of the low-temperature rise, requiring a longer heating duration. For the 120 s heating duration, the measurement error in soil volumetric capacity is less than 2%. On the other hand, the instantaneous line heat source theory overestimates the soil volumetric heat capacity by 5%, while the short-duration line heat source theory performs well in the method.

Integral form of the cylindrical perfect conductors solution for the dual‐probe heat‐pulse method

AbstractThe dual‐probe heat‐pulse (DPHP) method is useful for measuring soil thermal properties. Measurements are made with a sensor that has two parallel rods: one for introducing a pulse of heat and one for measuring change in temperature. Thermal properties are obtained by fitting a solution to the temperature data. The cylindrical perfect conductors solution of Knight et al. (https://doi.org/10.2136/vzj2011.0112) allows for accurate determination of those properties because it accounts for the finite radius and finite heat capacity of the rods. In this note, we show how their solution can be written in integral form. Specifically, we rearrange their Laplace‐domain solution and perform contour integration in the complex plane to express their solution as the integral of a real function over the positive real axis. This form of their solution is easier to evaluate and thus improves its accessibility for routine use in determining thermal properties with the DPHP method.

A dual-probe heat pulse-based sensor that simultaneously determines soil thermal properties, soil water content and soil water matric potential

A dual-probe heat pulse (DPHP) sensor is a robust tool that can measure soil thermal properties and associated soil physical properties and processes. A new DPHP based sensor was developed to simultaneously measure soil thermal properties, soil water content and soil water matric potential. A series of experiments were performed to evaluate the sensor performance. The new sensor was able to measure each soil property with acceptable accuracy. The accuracy of the matric potential determinations was approximately 10% in log scale. The effective range for ψ measurements was −1000 to −2.5 m of water, and the accuracy of the ψ determinations was best between −350 and −2.5 m of water, which included values of field capacity and wilting point for most soils and crops. The sensor design included a single heater wire to provide simultaneous heat inputs to two different materials, i.e., the soil and the sensor porous medium. The single heater wire did not influence sensor accuracy as long as the soil and the sensor porous medium thermal conductivity values were not significantly different. The new DPHP based sensor effectively measured various soil properties, including soil water matric potential, soil water content, and soil thermal property values.

Field test of two background temperature correction methods of dual probe heat pulse method

Dual probe heat pulse (DPHP) method is a useful method to measure many soil physical properties in-situ, which play important roles in soil science, meteorology, hydrology and so on. It is essential to remove the effect of background temperature fluctuations for DPHP estimations. We compared two background temperature correction methods of DPHP method, linear trend method (LTM) and reference temperature method (RTM) in field experiments. We found that misused LTM lead to large errors in DPHP estimations with shallow burial depth. When the burial depth of temperature probe (d) of DPHP sensor is <14 mm, background temperature changes are related to environmental factors such as solar radiation and wind, and RTM can decrease the relative error (RE) in thermal conductivity (λ) from 667% to 68%. Besides, our results also showed during the period from 9:00 to 16:00, the RTM method is valuable for getting accurate DPHP estimations with the 2 mm burial depth. After replacing LTM by RTM, REs were decreased to (6 ± 28)% from (17 ± 87) % in λ, and were decreased to (9 ± 17)% from (10 ± 18)% in volumetric heat capacity (Cv). But RTM is also affected by other factors such as the spatially heterogeneous temperature distribution of tested sample and should be taken into account in the further studies.

Analysis of heat paths in dual-probe-heat-pulse soil-moisture sensors for improved performance

We analyse heat paths of a packaged Dual-Probe-Heat-Pulse (DPHP) soil-moisture sensor to improve the sensor’s performance at higher levels of volumetric water content. A DPHP sensor consists of heater and temperature probes protruding out into the soil from a casing. The two probes help estimate the soil moisture content by assessing the heat conduction through the soil in the intervening space between them. At lower soil-moisture conditions, the heat conduction through soil dominates whereas at higher soil-moisture contents heat flow through soil decreases due to the increase in volumetric heat capacity and specific heat. Inappropriate placement of the heater element could lead to erroneous measurement due to the heat conduction to the temperature probe through the casing material and the epoxy adhesive inside the casing. To study the heat flow in DPHP sensors, and to develop an improved design, thermal paths in the DPHP sensors having different heater element positions were first analysed through Finite Element Analysis. A Lumped Element Model was also developed to further examine the heat paths in the DPHP sensor and suggest design modifications. The trend exhibited by the lumped model corroborates the results of Finite Element Analysis. Laboratory experiments were performed with DPHP sensors having different heater element positions, results of which validated the modeling outcomes. The insights gained in this study would serve as the design guidelines for manufacturing DPHP soil-moisture sensors with improved accuracy at minimal power consumption.

Wind effects on soil thermal properties measured by the dual‐probe heat pulse method

AbstractWhen a dual‐probe heat pulse (DPHP) sensor is installed in soil near the soil–atmosphere interface, the basic assumptions of the infinite line heat source (ILS) model and its improvement, the infinite line heat source model with an adiabatic boundary condition (ILS‐ABC) might not be satisfied because of wind. This study aims to explore the effects of wind on DPHP measurements and to compare the performance of the ILS and ILS‐ABC models at different values of wind velocity (υ) and burial depth (d). Our study shows that the results of laboratory experiments, COMSOL simulations, and field experiments are consistent with each other. For dry sand with d ≤ 4 mm, the effects of wind is non‐negligible at υ ≥ 3.5 m s−1 and the DPHP method does not provide accurate estimations whether the ILS model or the ILS‐ABC model is used. Field experiments are prone to large background temperature fluctuations, which can cause a commonly used linear detrending method to perform poorly. In general, at υ &lt; 2.7 m s−1 in field and d = 5 mm, the ILS‐ABC model provides more accurate estimations with relative errors of &lt;15% in thermal conductivity and relative errors of &lt;9% in heat capacity.

Measurement and prediction of thermal properties of pantoa during deep-fat frying

Thermal properties of pantoa were determined during deep-fat frying in sunflower oil at 125-145°C. Thermal conductivity (k: 0.357 to 0.199 Wm -1 K -1 ), thermal diffusivity (α: 0.105x10 -6 to 0.140x10 -6 m 2 s -1 ) and volumetric specific heat (C p : 3.275 to 1.401 MJm -3 K -1 ) were measured simultaneously using dual probe heat pulse technique. Thermal conductivity and volumetric specific heat of pantoa decreased as the moisture content decreased, while thermal diffusivity increased with the decrease in moisture content. Empirical models were also developed as a function of moisture content, fat content and temperature of frying of pantoa . It was found that moisture content was inversely related to fat content during the progress of frying. The thermal properties and the developed models would be useful in the mathematical modeling of heat transfer in pantoa during DFF.

Theory and Solutions of Heat Pulse Method for Determining Soil Thermal Properties

Information on thermal properties of soil is of paramount importance for environmental and earth science, and engineering. The heat pulse (HP) method has become the key technology for accurate determination of soil thermal properties and a variety of other physical properties (e.g., water content, bulk density, and water flux) in both laboratory and field environments. The HP method is a transient method that is commonly based on the analytical solutions to the radial heat flow equation when a line-heat source is applied. Over the past few decades, great endeavors have been devoted to advance the HP method. For example, the evolution and development in probe design, data logging equipment, data interpretation and computing capability has remarkably improved the accuracy and ease of use for determining soil thermal properties. However, there is a lack of study collating and synthesizing the development of the theory/solutions to obtain thermal properties of soil using the HP method. In this paper, we review the fundamental theories and solutions of the HP method, including differences and similarities of theories and applications between instantaneous line heat source (ILHS) and short-duration line heat-source (SLHS), between dual-probe heat-pulse (DPHP) and single-probe heat-pulse (SPHP) methods, and between the non-linear model fit (NMF) method and single point (SPM) method for data interpretation. In addition, the numerical solutions and semi-analytical solutions are also presented to provide heat pulse users information for selecting the best-fit method to meet their goals.

Towards Improved Field Application of Using Distributed Temperature Sensing for Soil Moisture Estimation: A Laboratory Experiment.

The “dual probe heat pulse” (DPHP) method using actively heated fiber optic (AHFO) cables combined with distributed temperate sensing (DTS) technology has been developed for monitoring thermal properties and soil water content at the field scale. Field scale application, however, requires the use of robust and thicker fiber optic cables, corroborating the assumption of an infinite thin heat source in the evaluation process. We therefore included a semi-analytical solution of the heat transport equation into the evaluation procedure in order to consider the finite thermal properties of the heating cable without a calibration procedure to estimate effective thermal properties of the soil. To test this new evaluation procedure, we conducted a laboratory experiment and tested different heating scenarios to infer soil moisture from volumetric heat capacity. Estimates were made by analyzing the shift of the temperature amplitude at the sensing cable and the characteristics of the response heating curve. The results were compared with results from the calibrated infinite line source solution and in situ water content point measurements and showed a good approximation of thermal properties for strong and short heat pulses. Volumetric water content estimates are similarly accurate to the results of the calibrated infinite line source solution. Problems arose with the cable spacing and the resettlement process after burying the cable.

Thermal conductivity of sands treated with microbially induced calcite precipitation (MICP) and model prediction

Thermal resistance between underground energy geo-structures and surrounding soils limits heat exchange efficiency particularly when soils are at low saturation degrees or nearly dry conditions. Microbially induced calcite precipitation (MICP) technique has great potential to enhance soil thermal properties thus increasing the possibility to expand geothermal applications to arid environments. This study investigates the thermal conductivity (k) of MICP treated sands in drying process by using a newly developed dual probe heat pulse (DPHP) sensor. The results show that thermal conductivity of sands was significantly improved after MICP treatment, and k value increased as the number of treatment cycles increased. The improvement in k is attributed to the MICP induced CaCO3 crystals functioning as “thermal bridges” among sand grains, which provides more highly effective heat transfer path and increases surface contact area in heat exchange process. Based on the normalized thermal conductivity (kr) concept and the experimental data obtained in this study, a modified thermal conductivity predictive model was proposed for MICP treated sands. The model prediction matched the experimental results very well, and the predicted values of k agreed with the measured values with only 10% deviation in the entire saturation range.

Thermal properties of sandy and peat soils under unfrozen and frozen conditions

In situ measurements of soil thermal properties are frequently required when studying coupled heat and water transfer in soils. While these values are widely available for mineral soils, but not for organic-rich peat soils. This study aimed at measuring the thermal properties of six peat:mineral mixtures collected from an oil sands mine reclamation study site using a dual-probe heat-pulse (DPHP) method. The soil mixes were tested across a range of water contents (θ) and bulk densities (ρ). Generally, compared with the thermal conductivity (λ) of sand soil, λ of peat soil subsequently decreased with increasing peat:mineral ratios and increased slightly with the increase of θ, indicating the significant impact of organic matter. Similar to λ, the heat capacity (C) decreased with the increase in soil organic matter content, and the decrease in θ and ρ, but differences were relatively small between the different soil mixtures. With increasing θ, λ increased exponentially, but C increased linearly. At all temperatures but between −2 °C and 0 °C for sandy soils and −4 °C and 0 °C for peat soils due to ice melting, the de Vries thermal conductivity model described the measurements very well and the DPHP method is suitable to measure the real soil thermal properties. The measured thermal properties for the peat soil can be used in modeling heat conduction in a high latitude cold area, and is a prerequisite for designing peat soil cover in reclamation soil.

Thin Film Dual Probe Heat Pulse (DPHP) Micro Heater Network for Soil Moisture Content Estimation in Smart Agriculture

We report a thin film Dual Probe Heat Pulse (DPHP) micro heater network for long-term data collection of soil moisture. Microheaters nodes were deployed over a large agricultural area and the entire infrastructure was monitored with high spatial and temporal resolution. FTO and ITO coated microheaters were built on a glass substrate. Sensor nodes were powered by a single 5V DC, supported by a central Atmega328 microcontroller for bidirectional communication as well as a database connection. Our network can be extended to monitoring other soil parameters like soil nutrient or pH. The maximum temperature recorded for FTO and ITO heaters for sensor nodes were 71.85°C and 45.45°C respectively while the maximum change of temperature(ΔTM) were 39.12°C and 13.19°C. Thermal cycling experiments were performed to compare the efficiency of the heaters.

Simplified Computational Approach for Determining In Situ Probe Spacing of a Dual Probe Heat Pulse Sensor

Core Ideas Probe spacing corrections greatly improve soil thermal property determination. The linear model and the non‐linear model are unified into a single algorithm. The new model simplifies significantly the spacing calculation procedure. For the dual probe heat pulse (DPHP) method, the probe spacing r is very important for the calculation of volumetric heat capacity and thermal diffusivity. Separate models for probe spacing correcting resulting from linear and nonlinear deflections have been reported earlier. In this paper, the linear model and the nonlinear model are unified into a single algorithm. Compared with the earlier models, the new model simplifies the procedure. To use the new model, it is not necessary to calculate a deflected angle, and also there is no longer a need to initially identify whether probe deflection is inward or outward. The new r correcting approach provides an easy way to use relationship that accounts for linear and nonlinear probe deflections.

Summary of Advances in the Heat‐Pulse Technique: Improvements in Measuring Soil Thermal Properties

Core Ideas The dual probe heat‐pulse (DPHP) method is used to measure soil thermal properties. Special care is needed to obtain accurate DPHP measurements in field soil. A review of theory, instrumentation and procedures needed is presented for DPHP. This essay provides a summary of “Advances in the Heat‐Pulse Technique: Improvements in Measuring Soil Thermal Properties” recently appearing in Methods of Soil Analysis. Series.

Effect of spatial variations and desiccation cracks on the DPHP and MPHP sensors

In sensing soil-moisture, there are inherent field related issues such as uncertainty and unpredictable distribution of minute amounts of trapped water and morphological changes of soil such as lumps and cracks due to desiccation. This poses problems for measurements done using a dual-probe heat-pulse (DPHP) technique that uses a narrow cylindrical sampling zone between heater and temperature probes. Thus, DPHP sensors are susceptible to errors. Hence, we studied and compared single-point and multi-point soil-moisture measurements using the heat-pulse technique. For this, we designed and developed a multi-point heat-pulse (MPHP) sensor with three temperature probes that enhanced the sampling zone to a volume of 840 mm3, which is three times that of the DPHP sensor. Sequentially deployed DPHP and MPHP sensors with spatial variations revealed that the maximum difference between the measured percentage of moisture from the DPHP sensor and standard oven-dried instrument was more than 10%, whereas for the MPHP sensor the discrepancy was only around 3%. Furthermore, when cracks appeared in the soil mass, it was observed that the discrepancy of DPHP sensor is around 16% while that for the MPHP sensor was still only 3%.