Showing papers in "Measurement Science and Technology in 1996"

Hot-Wire Anemometry: Principles and Signal Analysis

Abstract: Hot-wire anemometry (HWA) is one of the basic measuring techniques used by research scientists and engineers working in fluid mechanics. It is applicable to a wide variety of flows from studies of atmospheric phenomena to investigations of supersonic flows. HWA is an indirect measuring technique based on the heat transfer from a sensing element and for this reason is very sensitive to ambient variations in the temperature. The use of HWA is therefore not usually recommended for the measurement of mean flow properties. However, owing to the fast response and good spatial resolution of the technique, it is irreplaceable for investigations of rapidly varying flows and especially turbulence. Different modes of operation of the hot-wires permit measurements of velocity, temperature and concentration at a modest price and effort which makes the technique attractive and profitable in many situations. In spite of tremendous developments of other measuring techniques over the past two decades, particularly optical techniques such as laser-Doppler anemometry (LDA) or particle image velocimetry (PIV), HWA retains a number of distinct advantages which ensure its present and future use. Hans Bruun has written a comprehensive book which provides a state-of-the-art survey on developments and use of HWA. In many details the book complements the classical monographs of Corrsin (1963), Melnik and Weske (1967) and the more recent contributions to the subject from Perry (1982) and Lomas (1986). The author approaches the subject in an easy to read, straightforward manner, starting with the basic principles of HWA. He avoids putting much effort into describing the electronic arrangements or compensation networks, concentrating on the heat transfer considerations and the time/space resolution capabilities of HWA for turbulence measurements. The author proceeds with a general introduction to the velocity measurements. The material is informative and introduces topics such as the response equations used for HWA signal interpretation and methods for data acquisition, processing and presentation. Separate chapters are devoted to one-, two- and three-component velocity measurements. The author continues at considerable length with all necessary information about HWA for conducting measurements, including probe design and manufacture, aerodynamic effects of prongs and probe support, calibration techniques and signal interpretation. The emphasis is placed on methods which have been successfully used in turbulence research in the past. Temperature effects and measurements of the temperature fluctuations are presented in chapter 7. A good account is given of the methods for correcting hot-wire readings for drift in the ambient fluid temperature. The text also covers the resistance-wire method for measurements of the instantaneous temperature fluctuations. The author dedicates a separate chapter to hot-wire techniques used for investigating reverse flows and near-wall flows. The pulsed and flying hot-wire techniques as well as the split film probes are discussed in detail. A brief account is also given of other interesting attempts to extend the applicability of HWA to flows of high turbulence intensity. This chapter ends with an extensive analysis of near-wall hot-wire measurements and the determination of the wall shear stress. The author summarizes in a separate chapter successful extensions of the HWA technique beyond conventional applications. The material covered includes the determination of the void-fraction and turbulence properties in two-phase flows, evaluation of concentration fluctuations in gas mixtures and turbulence measurements in compressible flows. Vorticity measurements are discussed in chapter 10. These are of special importance to turbulence modelling since the vorticity is directly related to the turbulent dissipation rate. Modelling of the transport equation for this quantity seems to be the weakest link in the existing turbulence models used for engineering applications (CFD). This is good discussion of instantaneous velocity gradient measurements in turbulent flows and a good description of basic and more recent complex hot-wire configurations used for the vorticity measurements. The last two chapters deal with conditional sampling and time series analysis of turbulence fluctuations. There is plenty of interesting material related to processing and averaging signals measured in periodic flows, intermittency measurements and identification of coherent structures in turbulent flows. The closing chapter provides a useful survey of conventional statistical analysis of random signals in amplitude and phase domains. The list of the references at the end provides a complete overview of virtually all important studies related to HWA published to date. This is an excellent book written in a clear and readable style suitable for students, research scientists and engineers. The text is well supported by mathematical analysis and, whenever necessary, with adequate illustrations and diagrams. I highly recommend the book to everyone working in the field of experimental fluid mechanics. It should be obligatory reading for students involved in turbulence measurements. J Jovanovic References Corrsin S. 1963 Turbulence: experimental methods Handbook of Physics vol 8.2 (Berlin: Springer) pp 523 - 90 Melnik W L and Weske J R 1967 Advances in hot-wire anemometry Proc. Int. Symp. on Hot Wire Anemometry (Department of Aerospace Engineering, University of Maryland) Perry A E 1982 Hot-Wire Anemometry (Oxford: Clarendon Press) Lomas C G 1986 Fundamentals of Hot-Wire Anemometry (Cambridge: Cambridge University Press)

Sensors: A Comprehensive Survey

Abstract: This book can be divided into two parts. The first, consisting of sensor-relevant micro and nanotechnological aspects, deals with materials in nanotechnology and discusses future tendencies in this field. The development in this sensor field is highly influenced by the rapid progress in solid-state physics, new transducer technologies, materials research and the technological application of quantum effects. The book gives an account of this trend. It begins with non-IC and IC technologies, proceeds with acoustic-wave devices, high-temperature microsensors, integrated optical sensors and optical microsensors and ends with molecular sensors and nanotechnology. These selected aspects of sensor science describe the state of the art and, in part, the future trends in this field. Without doubt microsensors or nanosensors play, and will continue to play, an important role in many areas, e.g. in medicine, microreactors used in pharmacy and automotive applications, but as yet they represent only a relatively small part of the whole sensor family. Nevertheless the information on new technologies, new materials and new sensors is important for every researcher, engineer in industry and sensor user. The value of the first part of this book lies in the concentrated summary of this new sensor field. The second part of the book, consisting of chapters 12 to 18, presents information on the sensor market. It covers areas such as aerospace, process control, medicine, environmental monitoring, automotive control and manufacturing technology. Such information is not only meaningful for industry managers, but also for scientists dealing with sensor development and considering new sensor applications. This section of the book gives a different approach to the field - it does not deal only with micro and nanosensors and it is not written in a scientific manner. Presumably the reader of part one is not very often the reader of part two, and vice versa, but such an approach is welcome in the fast developing field of sensor research and application. This book, volume 8 of the Sensors series, is the continuation of a good tradition, namely the published volumes 1 to 7 of Sensors, which have been devoted to different types of sensors from the viewpoint of known principles, techniques and well tested materials. This volume goes beyond this frontier. It is warmly recommended to everyone working, or interested in, sensor research and applications.

Electrical resistance tomography for process applications

Abstract: Measurement of electrical resistivity via four probes is widely used in a variety of applications ranging from geophysical prospecting to silicon wafer manufacture. Electrical resistivity tomography is an extension of this approach and is performed via an array of equi-spaced electrodes mounted into the periphery of a process vessel to map non-intrusively the spatial distribution of resistivities within. The digitized boundary data are 'inverted' by an image reconstruction algorithm to produce a map of the internal resistivity distribution. This paper focuses on the design and operation of flexible electrical resistivity tomography instrumentation for use on laboratory- and plant-scale process equipment.

A method for determining the spring constant of cantilevers for atomic force microscopy

Abstract: Cantilevers fabricated by means of micromachining techniques are usually used for atomic force microscopy. In this paper, the spring constant of an atomic force microscope (AFM) cantilever is determined by using a large-scale cantilever. Since the spring constant of the large-scale cantilever is calibrated accurately, the spring constant of the AFM cantilever is determined precisely by measuring the deflections of both cantilevers simultaneously using heterodyne interferometry. The slope of the force curve gives the spring constant of the AFM cantilever. It is not necessary to measure the dimensions of the AFM cantilever in the proposed method. Although this method is simple, the spring constant of the AFM cantilever is obtained accurately.

Recent progress in fibre optic low-coherence interferometry

Abstract: A review of recent progress in signal processing, sensor developments and multiplexing techniques for fibre optic low-coherence interferometry is presented, with an emphasis on the new developments in this field which have the potential to be exploited for practical applications.

Mass flow measurement of bulk solids in pneumatic pipelines

Abstract: Many types of techniques for metering the mass flow rate of bulk solids in a pneumatic pipeline have been proposed and developed during the past 20 years. This paper presents a detailed and comprehensive review of the techniques and the current state of knowledge and experience. The techniques are classified under three main categories: direct measurement of solids mass flow rates, measurement of volumetric concentrations of solids and measurement of solids velocity. Future developments and possible trends in this field are also included.

A review of reconstruction techniques for capacitance tomography

Abstract: Capacitance tomography has been used to image several processes, such as liquid/gas pipe flow, oil/water/gas gravity separation, pneumatic conveying, fluidized beds and flame combustion. The nature of the capacitance sensors is such that reconstruction algorithms well developed for medical tomography are not applicable. The main problem is that the relationship between the measured quantity (capacitance) and the parameter of interest (distribution of the dielectric constant) is nonlinear. Furthermore, it is impossible to establish an explicit expression which relates the dielectric constant distribution to the measured capacitance. Also it should be pointed out that the number of measurements in capacitance tomography is small (typically less than 100) compared to medical tomography. For these reasons the first tested algorithm in capacitance tomography was based on the crude back projection algorithm. This algorithm has over the years been enhanced for use with a capacitance tomograph. In addition other techniques, such as various iterative methods, algorithms based on artificial neural networks and `look-up' tables have been developed and tested. This paper outlines the working principles for the different techniques and presents the main results.

Hardware design of electrical capacitance tomography systems

Abstract: Electrical capacitance tomography (ECT) was one of the techniques which were firstly developed for process tomography (PT) Two types of capacitance measuring circuits are the most suitable for the use in ECT systems - the charge/discharge circuit and the AC-based circuit - because of their immunity to stray capacitance Since the standing capacitances involved are relatively large and the capacitance changes to be measured are very small, the standing values need to be cancelled by DC offset compensation or AC feedback compensation Alternatively, the capacitances can be measured by high-accuracy self-balancing circuits without standing value compensation In this paper different designs of sensing electronics for ECT systems are presented and the advantages and disadvantages compared

Nonlinearity in measurements of length by optical interferometry

Abstract: The periodical nonlinearity of an interferometer usually ranges from sub-nanometre scale to several nanometres. This nonlinearity limits the usefulness of an interferometer to the sub-nanometre level of accuracy in length measurements. A theoretical analysis of length measurement nonlinearities both in heterodyne and in one-frequency interferometers is reported in this paper. The nonlinearity of a heterodyne interferometer arises mainly from the cross talk between different frequencies of two linearly polarized beams. Both one-cycle and two-cycles nonlinearities were observed as the difference in optical path length changed from 0 to 2. Unlike the heterodyne interferometer, the nonlinearity of the one-frequency interferometer arises mainly from the cross talk between two linearly orthogonal beams of the same frequency and only two cycles of periodical nonlinearity were observed as the difference in optical path length changed from 0 to 2.

New developments and applications of skin-friction measuring techniques

Abstract: This survey covers recent developments and applications of four skin-friction measurement techniques (oil-film interferometry, wall hot wire, surface fence and wall pulsed wire). Comparisons of the techniques with each other and with other methods are presented. Applications in attached and separated fully turbulent boundary layers and in highly accelerated laminar-like flows will be shown to demonstrate the application range and the limits of the various techniques.

An overview of electromagnetic inductance tomography: Description of three different systems

Abstract: This paper presents a general overview of electromagnetic inductance tomography (EMT). A general introduction is given together with a description of the theoretical background of the technique. Three examples of different EMT systems are discussed and images produced using several different image reconstruction techniques are presented. A discussion of the main features of the techniques is included and some potential applications are suggested.

Process tomography: a European innovation and its applications

Abstract: Recent developments in the design and application of tomographic sensors for measurements in industrial processes are reviewed. The principal sensing methods and their relative performance are summarized. Industrial scale applications, future prospects and limitations of the technology are discussed.

Recent developments and industrial/research applications of capacitance tomography

Abstract: In this paper the general principles of capacitance tomography are explained. Recent developments in the design of the primary sensor, the sensor electronics and the reconstruction are shown as well as future trends identified. Industrial and research applications of capacitance tomography are given.

Broad-band ultrasonic measurement techniques for liquids

Abstract: Various methods for broad-band ultrasonic absorption and velocity measurements in liquids, using CW and pulsed signals, are reviewed. Several different ultrasonic spectrometric techniques are required to cover the wide range of liquid wavelength and attenuation values that exist in the kilohertz to gigahertz frequency range. Advantages and problems of various ultrasonic and related methods are discussed. Sonic cells for liquids and electronic equipment for spectral measurements are presented. Together with applications in relaxation spectrometry, examples of broad-band ultrasonic absorption spectra, utilizing different CW resonator and pulse-modulated travelling wave techniques, are presented.

Process tomography applied to multi-phase flow measurement

Abstract: This paper presents the state of the art in measuring multi-phase flows by using tomographic techniques. The results presented show a wide range of industrial applications of process tomography from the nuclear and chemical to the food industry. This is illustrated by examples of the application of various tomographic sensors to the measurement of geometric or kinematic parameters of multi-phase flows. An application of process tomography for the validation of computational fluid dynamic models and the possibility of constructing a flowmeter for multi-phase flow are addressed.

A dual sensor flow imaging tomographic system

Abstract: A dual sensor tomograph for three-component flow imaging has been built at the University of Bergen in cooperation with Christian Michelsen Research AS and Norsk Hydro AS. It utilizes an eight-electrode electrical capacitance tomograph and a -ray tomograph with five radiation sources and 85 compact detectors. Embedded transputers using memory-mapped I/O ensure high-speed data acquisition into an Alpha AXP-based on-line processing unit. The first results demonstrate that three-component flow regime identification is possible at rates of about 30 frames per second, provided that sufficient computing capability is available.

A portable fracture toughness tester for biological materials

Abstract: A portable mechanical tester is described which is both lightweight and cheap to produce. The machine is simple and convenient to operate and requires only a minimum of personnel training. It can be used to measure the fundamental mechanical properties of pliant solids, particularly toughness (in the sense of `work of fracture') using either scissors or wedge tests. This is achieved through a novel hardware integration technique. The circuits are described. The use of the machine does not require a chart recorder but it can be linked to a personal computer, either to show force - displacement relationships or for data storage. The design allows the use of any relatively `soft' mechanical test, i.e. tests in which the deformability of the frame of the machine and its load cell do not introduce significant errors into the results. Examples of its use in measuring the toughness of biomaterials by scissors (paper, wood) and wedges (mung bean starch gels) are given.

Porous silicon as a substrate material for potentiometric biosensors

Abstract: For the first time porous silicon has been investigated for the purpose of application as a substrate material for potentiometric biosensors operating in aqueous solutions. Porous silicon was prepared from differently doped silicon substrates by a standard anodic etching process. After oxidation, penicillinase, an enzyme sensitive to penicillin, was bound to the porous structure by physical adsorption. To characterize the electrochemical properties of the so build up penicillin biosensor, capacitance - voltage (C - V) measurements were performed on these field-effect structures.

Environmental applications of analytical biosensors

Abstract: A review of the fundamental aspects and environmental applications of biosensors is presented. The bases of different transducer principles such as electrochemical, optical and piezoelectric are discussed. Various examples are given of the applications of such principles to develop immunosensor devices to determine common environmental contaminants. Attention is also paid to catalytic biosensors, using enzymes as sensing elements. Biosensor devices based on the use of cholinesterase and various oxidase enzymes such as tyrosinase, laccase, peroxidase and aldehyde dehydrogenase are reported. Some examples are given of the applications of other biomolecules such as whole cells, DNA or proteins, to determine pollution. Validation studies are presented comparing biosensors with chromatographic techniques to determine organophosphorus pesticides and phenolic compounds in environmental samples.

ERT monitoring of environmental remediation processes

Abstract: The use of electrical resistance tomography (ERT) to monitor new environmental remediation processes is addressed. An overview of the ERT method, including design of surveys and interpretation, is given. Proper design and lay-out of boreholes and electrodes are important for successful results. Data are collected using an automated collection system and interpreted using a nonlinear least squares inversion algorithm. Case histories are given for three remediation technologies: Joule (ohmic) heating, in which clay layers are heated electrically; air sparging, the injection of air below the water table; and electrokinetic treatment, which moves ions by applying an electric current. For Joule heating, a case history is given for an experiment near Savannah River, Georgia, USA. The target for Joule heating was a clay layer of variable thickness. During the early stages of heating, ERT images show increases in conductivity due to the increased temperatures. Later, the conductivities decreased as the system became dehydrated. For air sparging, a case history from Florence, Oregon, USA is described. Air was injected into a sandy aquifer at the site of a former service station. Successive images clearly show the changes in shape of the region of air saturation with time. The monitoring of an electrokinetic laboratory test on core samples is shown. The electrokinetic treatment creates a large change in the core resistivity, decreasing near the anode and increasing near the cathode. Although remediation efforts were successful both at Savannah River and at Florence, in neither case did experiments progress entirely as predicted. At Savannah River, the effects of heating and venting were not uniform and at Florence the radius of air flow was smaller than expected. Most sites are not as well characterized as these two sites. Improving remediation methods requires an understanding of the movements of heat, air, fluids and ions in the sub-surface which ERT can provide. The Florence site provides an excellent example of using information from ERT to improve a remediation system design. At Florence, the injection well used too long a sand pack in the injection zone which decreased the injection depth and thus the zone of influence of the system. Though in retrospect this is obvious, it would not have been noticed without ERT.

Sub-surface imaging by scanning thermal microscopy

Abstract: Scanning probe thermal microscopy has been used to achieve sub-surface imaging of metallic particles embedded in a polymer matrix, using a probe which can act as both ohmic heater and thermometer. A lateral resolution of the order of a micron and a depth detection of a few microns were achieved. Together with the description of the technique and the experimental results obtained, a basic theoretical framework is presented which describes heat flow and temperature distributions within a sample consisting of inclusions buried within a bulk material. Computer models have been developed to give theoretical heat flows and temperature profiles: these are compared here with the experimental data. The theoretical lateral resolution was found to be in good agreement with the experimental observation. We show that theoretical modelling can be used to calibrate the instrument for specific investigations. For example, the technique could be used quantitatively to determine and map thermal conductivity variations across heterogeneous samples, or to determine the depth at which inclusions are located in the case where the thermal conductivities of both the inclusions and the enclosing material are known as well as the geometry of the inclusions.

Correction of nonlinearity in one-frequency optical interferometry

Abstract: The nonlinearity of one-frequency optical interferometry with quadrature fringe detection is the result of a number of factors including polarization mixing, unequal gain of detectors, imbalanced beams and lack of quadrature An elliptical least-squares fitting technique to correct the nonlinearity is presented in this paper Experimental results demonstrate that a nonlinearity-free condition can be achieved and the stability of the interferometer set-up in our laboratory is better than nm over 25 h

Process tomography using ultrasonic sensors

Abstract: The use of ultrasonic transducers for process tomography is in its infancy relative to many other techniques, in particular considering the widespread use of ultrasonic transducers in a variety of non-tomographic sensing applications. This review deals with the background sensing principles of ultrasound transducers employed in tomography, their associated advantages and their difficulties, including the critical time required for data collection, and implications for multiple projection tomography. The theoretical and basic practical implementation of tomography systems using ultrasonic transducers is then considered. Finally a review is included of research investigations into the use of ultrasonic-based process tomography systems.

Optical fibre based absolute extrinsic Fabry - Pérot interferometric sensing system

Abstract: The design and principle of operation of a novel fibre optic sensing system is presented. The absolute extrinsic Fabry - Perot interferometric (AEFPI) system is shown to possess high sensitivity, wide dynamic range and a real-time output signal. Applications of the AEFPI system to high-performance civil structures are suggested. Multiplexing of many such sensors is achieved by employing the path matching configuration.

Void fraction measurements in gas - liquid flows using capacitance sensors

Abstract: Void fraction measurements for vertical flow in a small diameter tube (9.53 mm) were taken using two non-intrusive capacitive void fraction sensors. The sensors were needed to measure the void fraction of water - air two-phase flow under normal gravity and microgravity conditions. Void fraction data were collected with: (1) a sensor having helical wound electrodes that was used to collect data under normal gravity and microgravity conditions, (2) a sensor having concave plate electrodes, used to collect data at normal gravity. This paper covers the calibration results for both sensors and some of the problems associated with the helical wound design. Nonlinearity in the helical sensor is addressed, with improvements shown in the concave plate sensor. Comparisons are made between the capacitive sensors, quick-closing valves and a gamma densitometer.

Flow pathways in porous media: electrical resistance tomography and dye staining image verification

Abstract: Electrical resistance tomography permits the determination of the spatial distribution of electrical resistivity by non-destructive means. The technique may be applied to the study of flow through porous media by following the changes in resistivity distribution within a soil or rock core as an electrolytic tracer is passed through the specimen. Image reconstruction of the electrical resistance tomography data may be achieved by robust regularized nonlinear inverse methods provided data errors are suitably characterized. The procedure is reported here for a natural (undisturbed) soil core using four planes of 16 electrodes placed around the boundary of the core. The experiment was performed with a dye tracer that provided, by destructive slicing, visual verification of the flow pathways observed with the tomographic imaging.

Applications of nuclear magnetic resonance imaging in process engineering

Abstract: During the past decade, the application of nuclear magnetic resonance (NMR) imaging techniques to problems of relevance to the process industries has been identified. The particular strengths of NMR techniques are their ability to distinguish between different chemical species and to yield information simultaneously on the structure, concentration distribution and flow processes occurring within a given process unit. In this paper, examples of specific applications in the areas of materials and food processing, transport in reactors and two-phase flow are discussed. One specific study, that of the internal structure of a packed column, is considered in detail. This example is reported to illustrate the extent of new, quantitative information of generic importance to many processing operations that can be obtained using NMR imaging in combination with image analysis.

Coaxial to circular waveguide transition as high-precision easy-to-handle measuring cell for the broad band dielectric spectrometry of liquids

Abstract: For an abrupt transition from a coaxial to circular waveguide with a dielectric interface in the plane of discontinuity, the input admittance has been calculated as a function of frequency and of relevant geometrical and electrical quantities. The calculations have been performed applying a mode matching technique. A simple lumped-element representation has been derived from the theoretical considerations which was found to provide an excellent description of the input admittance of the discontinuity over a wide range of values of the parameters of interest. Network parameters are discussed for transitions that are most suited as sample cells for dielectric measurements in both the time domain and the frequency domain. Results from test measurements are reported to show the agreement of our numerical predictions with precise capacitance values of other authors and also to demonstrate the sensitivity and accuracy attainable with such cells.

Single-photon timing detectors for fluorescence lifetime spectroscopy

Abstract: We review the principle of operation, performance and application of the different types of single-photon timing detectors presently employed in fluorescence lifetime determination using the time-correlated single-photon counting technique. The devices discussed include side-on and linear focused photomultiplier tubes, microchannel plate photomultipliers and avalanche photodiodes.

Rapid numerical estimation of soil thermal properties for a broad class of heat-pulse emitter geometries

Abstract: The recently developed dual-probe heat-pulse technology permits the rapid, automatic monitoring of soil volumetric heat capacity, thermal diffusivity, and conductivity. The method relies on parameter fits of equations describing received pulse shape. These, in turn, depend on the particular geometric model assumed to characterize the sensor system. This paper describes an estimation procedure applicable to a wide class of sensor geometries. Key attributes of the method are that: (1) it replaces multidimensional, parameter space searches with a single, univariate optimization; (2) no derivatives are required for point estimates; (3) except for a one-time-only table precalculation, computational requirements are independent of model complexity; and (4) it computes its own initial estimates from the data. A computer implementation of the method is presented along with fits of finite and infinite line source emitters to temperature data from laboratory dual-probe heat-pulse experiments. The results show the new method gives answers identical to the commonly used Marquardt algorithm.