Operative Calibration Methodology of a TDR Sensor for Soil Moisture Monitoring under Irrigated Crops

Time domain reflectometry (TDR) is more and more frequently used for soil water content measurements in replacement of other techniques as neutron probe. Such observations that can be continuously collected on dataloggers are convenient for monitoring water fluxes under irrigated crops. Though relationships to calculate volumetric water content from analysis of TDR signal and collected data are published in literature or given by manufacturers for different commercialized devices, the results are not satisfactory for many soils and specific calibrations are required. In replacement of the traditional calibration method, two rapid operative computer assisted methods are proposed. The first one is based on drip moistening of a cylindrical sample of disturbed dry soil in which a TDR sensor is vertically embedded. The second one uses the same cylindrical sample wetted close to saturation in which a TDR sensor is inserted step by step during measurements. The data from the two methods were fitted using second degree models. These results are in good agreement with conventional calibration method or gravimetric field measurements of soil water content. Utilization of short time step TDR measurements for monitoring soil water storage under a furrow irrigated corn shows consistent observations with water applications or uptakes by crop. Field comparison with neutron probe and gravimetric measurements corroborate the need of a specific calibration for the soil studied in this paper though its clay content is about 20%.     

Measuring and Modeling of Direct Ground Wave Depth Penetration Under Transient Soil Moisture Conditions

This study represents the first attempt to investigate the GPR direct ground wave sampling depth by comparing GPR estimated soil moisture contents with data from horizontally and vertically installed time domain reflectometry (TDR) probes at different depths. The GPR direct ground wave method (200 MHz centre frequency) was used to estimate the temporal soil moisture dependence during uniform irrigation and drainage. Uniform irrigation and drainage experiments were conducted in an experimental pit (2.5 × 1.0 × 0.8 m) filled with repacked sandy loam soil. The GPR moisture contents measurements were more consistent with the moisture contents from vertically installed TDR than horizontally installed TDR. An analytical solution for one-dimensional drainage of water was used to estimate the change in GPR ground wave sampling depth during drainage. The analytical solution was first fit to vertical TDR data to obtain an estimate of the soil hydraulic parameters and the GPR sampling depth was then estimated by fitting the drainage solution to the measured GPR data. The GPR direct ground wave sampling depth using the analytical solution during drainage varied from −20 cm at high moisture content to −50 cm at the lowest moisture content.     

Computational models of a nano probe tip for static behaviors

It is difficult to predict the measurement bias arising from the compliance of the atomic force microscope (AFM) probe. The issue becomes particularly important in this situation where nanometer uncertainties are sought for measurements with dimensional probes composed of flexible carbon nanotubes mounted on AFM cantilevers. We have developed a finite element model for simulating the mechanical behavior of AFM cantilevers with carbon nanotubes attached. Spring constants of both the nanotube and cantilever in two directions are calculated using the finite element method with known Young's moduli of both silicon and multiwall nanotube as input data. Compliance of the nanotube-attached AFM probe tip may be calculated from the set of spring constants. This paper presents static models that together provide a basis to estimate uncertainties in linewidth measurement using nanotubes. In particular, the interaction between a multiwall nanotube tip and a silicon sample is modeled using the Lennard-Jones theory. Snap-in and snap-out of the probe tip in a scanning mode are calculated by integrating the compliance of the probe and the sample-tip interacting force model. Cantilever and probe tip deflections and points of contact are derived for both horizontal scanning of a plateau and vertically scanning of a wall. The finite element method and the Lennard-Jones model provide a means to analyze the interaction of the probe and sample and measurement uncertainty, including actual deflection and the gap between the probe tip and the measured sample surface.     

Effect of tip shape on capacitance determination accuracy in scanning capacitance microscopy

We present an analysis of the measurement error caused by the stray field of scanning capacitance microscope probes of various shapes. Cylindrical islands and wells of varying radius and height or depth, in both conducting surfaces and structures containing dielectric films, were used as test features for modelling. The results show that high accuracy and good contrast of small details are contradictory requirements. Probes with small radius of curvature of the tip apex yield smaller errors on features with small diameter but larger ones on features with large diameter than tips with large radius of curvature. The stray electrostatic field causes large errors, which are exceptionally severe with microfabricated probes. Contrary to general belief, differential measurements, based on modulation of the probe/sample separation or of the width of depletion layer in semiconductors, do not reduce the effect of the stray field significantly. For best results, the probe should be shielded as close to the tip apex as possible. In the case of microfabricated probes, at least the side of the cantilever facing the sample should be shielded.     

Development of dual-probe atomic force microscopy system using optical beam deflection sensors with obliquely incident laser beams

We developed a dual-probe (DP) atomic force microscopy (AFM) system that has two independently controlled probes. The deflection of each cantilever is measured by the optical beam deflection (OBD) method. In order to keep a large space over the two probes for an objective lens with a large numerical aperture, we employed the OBD sensors with obliquely incident laser beams. In this paper, we describe the details of our developed DP-AFM system, including analysis of the sensitivity of the OBD sensor for detection of the cantilever deflection. We also describe a method to eliminate the crosstalk caused by the vertical translation of the cantilever. In addition, we demonstrate simultaneous topographic imaging of a test sample by the two probes and surface potential measurement on an α-sexithiophene (α-6T) thin film by one probe while electrical charges were injected by the other probe.     

Parallel Plates Compared with Conventional Rods as TDR Waveguides for Sensing Soil Moisture

Dielectric methods of estimating the water content of soils, especially TDR, have become accepted as routine measurement techniques. Basic to the TDR technique is the waveguide, which is inserted into the soil for obtaining measurements of the effective soil permittivity, from which water content is estimated. In this study we compare the use of flat stainless steel plates with cylindrical stainless steel rods. We suggest that the use of plates gives a more even distribution of electromagnetic energy within the soil volume sampled and reduces the so called ``skin effect' where the electromagnetic energy is concentrated close to the surface of the electrodes. Plates will not be suitable for all measurement purposes but wherever they can be applied they should result in more representative measurements from the medium under investigation. Calculations of electrical field intensity are presented to compare the relative energy density between the electrodes. Field and laboratory studies were alsoconducted considering some of the practicalities of using the alternative waveguides.     

High-accuracy roundness measurement by a new error separation method

This paper presents a new error separation method for accurate roundness measurement called the orthogonal mixed method. This method uses the information of one displacement probe and one angle probe to separate roundness error from spindle error. This method was developed from the mixed method, which uses the information of two displacement probes and one angle probe to carry out the error separation. In the present paper, the relationship between the characteristics of the mixed method and the probe arrangement is analyzed. Well-balanced harmonic response of the mixed method is verified to be obtainable for the case where the angular distance between the displacement probe and the angle probe is set at 90°. This orthogonal mixed method also had the simplest probe arrangement, because it requires only one displacement probe and one angle probe to realize the error separation. Optical probes were used to construct an experimental measurement system that employs the orthogonal mixed method. The displacement probe and the angle probe both use the principle of the critical angle method of total reflection, and they have stabilities of 1 nm and 0.01 in., respectively. The measurement results show that roundness measurement can be performed with a repeatability on the order of several nanometers.     

Conductive transparent fiber probes for shear-force atomic force microscopes

New conductive transparent (CT) probes that can inject currents into nanometer-sized regions and collect light from them have been developed for shear-force atomic force microscopy (SF-AFM) of partially isolative regions. The CT probe consists of a straight elastic silica fiber with one end tapered to a point. The taper is coated with an indium-tin-oxide film as a transparent electrode, and the probe apex has a nanometer-scale radius. The essential feature of the CT probes is coaxial nickel plating on the shaft of the isolative silica fiber, which is adjusted to obtain suitable elasticity for smooth shear-force feedback as well as for supplying currents to the transparent electrode. Experimental results clarified that nickel thickness between 0.5 and 15 microm on 20 mm-long fibers makes resistance low enough for supplying current to the probe apex and also makes the Q curves smooth enough for shear-force feedback. Clear SF-AFM and current images were successfully obtained for a sample containing both conductive and isolative regions. The CT probes for SF-AFM can expand applications of probe-current-induced luminescence measurements to samples that contain highly resistive and isolative regions, for which scanning tunneling microscopy cannot be applied.     

A New Model for the Response of TDR to Heterogeneous Dielectrics

The use of time domain reflectometry (TDR) techniques for measuring the moisture content of composite materials is a mature art but usually assumes homogeneity of the material in the transverse plane. As the basis of a forward solution to TDR imaging, we describe an integral equation approach to model the response of the TDR system to a lossless heterogeneous dielectric body. Then, in conjunction with a suitable dielectric model of the composite material, the TDR response to moisture content distribution may be quantified.Several methods for integrating the field values between the transmission line rods were compared and a new method that combines a priori information with linear interpolation provided the most consistent integration for three different permittivity distributions. A self-consistency approach was used to compare the modeled propagation velocity with that expected from transmission line theory.     

鲜猪肉水分近红外光谱在线检测方法研究

本研究目的在于应用近红外光谱技术对冷鲜猪肉深层水分含量进行在线检测方法研究。根据稳态空间分辨光谱技术设计了具有多个检测器的传感器,以750nm、805nm、850nm和970nm的LED作为光源,检测器距离为30-48mm之间,用于检测肉块样品一定深度和范围内的水分含量,从而为无损、快速地给出鲜猪肉水分含量提供一种可靠方法。实验中针对猪背最长肌进行检测,获得184个样品,其水分含量在71%-78%之间,采用多元校正预测模型计算肉块水分含量。所建模型相关系数（r）为0.783,呈显著相关,建模集标准残差（RMSEP）为0.988,验证集标准残差（RMSEP）为1.037。由此得出结论,在短波近红外范围内,利用稳态空间分辨光谱技术能够对猪肉水分含量进行在线检测。     

Analysis of simulated scanning of atomic-scale silicon surface by atomic force microscopy

This study constructs a contact-mode atomic force microscopy (AFM) simulation measurement model with constant force mode to simulate and analyze the outline scanning measurement by AFM. The simulation method is that when the probe passes the surface of sample, the action force of the atom of sample received by the atom of the probe can be calculated by using Morse potential. Through calculation, the equivalent force on the cantilever of probe can be acquired. By using the deflection angle equation for the cantilever of probe developed and inferred by this study, the deflection angle of receiving action force can be calculated. On the measurement point, as the deflection angle reaches a fixed deflection angle, the scan height of this simulation model can be acquired. By scanning in the right order, the scan curve of the simulation model can be obtained. By using this simulation measurement model, this study simulates and analyzes the scanning of atomic-scale surface outline. Meanwhile, focusing on the tip radii of different probes, the concept of sensitivity analysis is employed to investigate the effects of the tip radius of probe on the atomic-scale surface outline. As a result, it is found from the simulation on the atomic-scale surface that within the simulation scope of this study, when the tip radius of probe is greater than 12 nm, the effects of single atom on the scan curve of AFM can be better decreased or eliminated.     

Microwave Dielectric Study on Water Structure and Physical Properties of Aqueous Systems Using Time Domain Reflectometry with Flat-End Cells

Microwave dielectric measurements were performed in the frequency range from 1 mHz up to 30 GHz using a time domain reflectometry (TDR) method for emulsions and gels. Flat-end sample cells have been used in the TDR measurement to contact a small spot of the surface of those viscoelastic and solid samples without any destruction. Relaxation processes due to various water structures were observed for these aqueous systems. Relaxation parameters thus obtained offer information about these water structures and amounts. The relaxation strength obtained from the high frequency process due to free water can be an adequate measure of water content in spite of some ambiguities for different water structures in some materials. Comparisons of actual water contents in emulsion with those estimated from the relaxation strength indicate that water structure is affected by the interaction between water and micelle. Unfreezable water observed in DNA gel under the freezing point consists of bound water and a fraction of free water. Bound water molecules are still unfreezable to keep the double helical structure of DNA, when the fraction of free water is frozen at lower temperatures. These water structures determine physical properties of moist materials. TDR measuring technique with the flat-end cell is effective to investigate water structures in viscoelastic moist materials and to evaluate physical properties and structures of complex molecular systems.     

On the many advantages of local-electrode atom probes

Local extraction electrodes offer several crucial advantages for operation of atom probes. Because of the proximity of the local extraction electrode to the specimen, the electric field produced at the specimen apex by a given voltage is enhanced and the voltage required for field evaporation is reduced. In a voltage-pulsed atom probe, the absolute magnitude of the energy uncertainty is correspondingly reduced. High mass resolution (m/deltam > 1000) may therefore be obtained by accelerating the evaporated ions to a greater total potential after the local extraction electrode. The low extraction voltage may also be pulsed rapidly (100 ps rise time) and at high repetition rates (up to 10(5) pulses per second) using currently available solid-state pulsers. Furthermore, a local electrode and intermediate electrodes may be used as optical elements to control the image magnification. All of these benefits may be applied to any type of atom probe. Local-electrode atom probes (LEAP) should be especially advantageous for developing three-dimensional atom probes with high mass resolution and a large field of view. A sample has been developed that consists of many microtips formed on a planar sample using ion beam mask etching. Microtip samples are especially suited to LEAP. Analysis of electrically insulating samples may also be possible with microtip samples in a LEAP. This combination of features suggests flexible, high speed, high mass resolution atom probes that can work with either conventional needle-shaped specimens or the new style of planar microtip specimens.     

Near-field scanning optical microscope probe analysis

In this article results of a comparison of two NSOM probe characterization methods are presented. Scanning electron microscopy analysis combined with electromagnetic field modeling using the finite difference in time domain method are compared with measured far-field radiation diagrams of NSOM probes. It is shown that measurement of far-field radiation diagrams can be an efficient tool for daily checking of the NSOM probes quality. Moreover, it is shown that the inner probe geometry has large influence on the directional radiation of an NSOM probe and the far-field radiation diagram can be used as a simple method to distinguish between different probe geometries.     

Developments of scanning probe microscopy with stress/strain fields

An innovative stress/strain fields scanning probe microscopy in ultra high vacuum (UHV) environments is developed for the first time. This system includes scanning tunneling microscope (STM) and noncontact atomic force microscope (NC-AFM). Two piezo-resistive AFM cantilever probes and STM probes used in this system can move freely in XYZ directions. The nonoptical frequency shift detection of the AFM probe makes the system compact enough to be set in the UHV chambers. The samples can be bent by an anvil driven by a step motor to induce stress and strain on their surface. With a direct current (dc) power source, the sample can be observed at room and high temperatures. A long focus microscope and a monitor are used to observe the samples and the operation of STM and AFM. Silicon(111) surface in room temperature and silicon(001) surface in high temperature with stress were investigated to check the performance of the scanning probe microscope.     

Effects of prong-wire interferences in dual hot-wire probes on the measurements of unsteady flows and turbulence in low-speed axial fans

Support needles of Dual Hot Wire (DHW) anemometers induce significant inaccuracies for flow angle and turbulence measurements in the case of X-array probes with prongs perpendicular to the flow plane. At certain angular ranges of the incident flow, a wake interference is established between the sensors which leads to a practical limitation of the device. In the case of turbomachinery environments, this is even more critical due to the inherent unsteadiness of the flow direction rotor downstream.In the present work, the measurement deviation caused by hot-wire probes operated under interference effects has been studied and evaluated, in both steady and unsteady conditions, especially for turbomachinery flows. New designs of DHW probes without prong-wire interference effects in their operative angular ranges were developed for validation. In particular, both V-type and Z-type interference-free probes are compared to a classic X-type probe susceptible for prong-wire interferences. Firstly, a steady calibration is performed to show the baseline deviation of the X-array probe in the measurement of the velocity magnitude, the flow angle and the turbulence intensity. Typical errors up to 10–13% in velocity, 5.5–7 deg in angle and 1.5–2.5 points overestimation in turbulence levels are observed. Also, unacceptable inaccuracies are found in the turbulence spectra of the measurements.Following, the impact of the interference for unsteady flow measurements is highlighted comparing the performance of the three probes within the single stage of a low-speed axial fan. The unsteady measurements of the X-array probe have revealed similar averaged discrepancies to those observed in the steady performance, but the instantaneous deviations can be as high as a 20% in velocity and 16–18 deg in flow angle in those regions (rotor wakes) with large unsteady velocity gradients and turbulence generation. Turbulence intensity measured in the rotor wakes is also excessively higher.     

用于工业三维点测量的接触式光学探针

为了快速、准确地获得大尺寸工业产品或带有深槽孔工件的关键点三维坐标,本文基于工业近景摄影测量理论、立体视觉技术等,研究并实现了两种工业便携式、接触式光学探针测量系统。研究了测量系统涉及的探针设计、探针标定以及三维点解算等关键技术,设计了点阵式和手持相机式两种适用于不同工业场合的工业探针。针对点阵式探针的测量,提出了一种用于解算探针坐标系与世界坐标系相对关系的点云匹配方法。此外,采用拟合虚拟球的方法准确标定了两种探针的内部参数。最后,通过对比标准球与三坐标测量机的测量结果,得到系统的测量精度可达0.1 mm/m。该精度满足一般大、中型工件的三维点测量精度标准。     

A critical review of soil moisture measurement

Soil moisture content has paramount importance in dictating engineering, agronomic, geological, ecological, biological and hydrological characteristics of the soil mass. Though earlier researchers have employed various techniques of moisture content determination of soils, both in laboratory and in situ conditions, ascertaining the applicability of these techniques to soils of entirely different characteristics and the ‘types of moisture content’, which they can measure, is still a point of debate. As such, a critical review of all the established and emerging soil moisture measurement techniques with respect to their merits and demerits becomes necessary. With this in view, efforts have been made in this paper to critically evaluate all the soil moisture measurement techniques, limitations associated with them and the influence of various soil-specific parameters (viz., mineralogy, salinity, porosity, ambient temperature, presence of the organic matter and matrix structure of the soil) on the measured soil moisture content. This paper also highlights the importance of various innovations based on Micro Electro Mechanical Systems (MEMS) and nano-sensors that are emerging in this context.     

Harmonic demodulation and minimum enhancement factors in field‐enhanced near‐field optical microscopy

Field‐enhanced scanning optical microscopy relies on the design and fabrication of plasmonic probes which had to provide optical and chemical contrast at the nanoscale. In order to do so, the scattering containing the near‐field information recorded in a field‐enhanced scanning optical microscopy experiment, has to surpass the background light, always present due to multiple interferences between the macroscopic probe and sample. In this work, we show that when the probe–sample distance is modulated with very low amplitude, the higher the harmonic demodulation is, the better the ratio between the near‐field signal and the interferometric background results. The choice of working at a given n harmonic is dictated by the experiment when the signal at the n + 1 harmonic goes below the experimental noise. We demonstrate that the optical contrast comes from the nth derivative of the near‐field scattering, amplified by the interferometric background. By modelling the far and near field we calculate the probe–sample approach curves, which fit very well the experimental ones. After taking a great amount of experimental data for different probes and samples, we conclude with a table of the minimum enhancement factors needed to have optical contrast with field‐enhanced scanning optical microscopy.