Showing papers in "Measurement Science and Technology in 1990"

Optimization of particle image velocimeters. I, Double pulsed systems

Abstract: The spatial resolution, detection rate, accuracy and reliability of a particle image velocimeter (PIV) depend critically upon the careful selection of a number of parameters of the PIV system and the fluid motion. An analytical model and a Monte Carlo computer simulation have been developed to analyse the effects of experimental parameters and to optimize the system parameters. A set of six nondimensional parameters that are the most significant in optimizing PIV performance are identified. They are the data validation criterion, the particle image density, the relative in-plane image displacement, the relative out-of-plane displacement, a velocity gradient parameter, and the ratio of the mean image diameter to the interrogation spot diameter. These parameters are studied for the case of interrogation by autocorrelation analysis. By a single transformation, these results can be applied to interrogation by two-dimensional Fourier transform analysis of the Young's fringes.

The manufacture of microlenses by melting photoresist

Abstract: It has been shown that it is possible to to generate very small lenses by melting 'islands' of photoresist on a glass substrate. The authors have made lenses with diameters ranging from 5 mu m to 750 mu m in the form of spheres, cylinders and crossed cylinders and have studied their optical properties. Lenses with numerical apertures between 0.5 and 0.2 may be made close to diffraction limited but those of lower numerical aperture can only be achieved using more complex techniques.

Application of artificial neural networks to an electronic olfactory system

Abstract: The human sense of smell is the faculty upon which many industries rely to monitor items such as beverages, food and perfumes. Previous work has been carried out to construct an instrument that mimics the remarkable capabilities of the human olfactory system. The instrument or electronic nose consists of a computer-controlled multi-sensor array which exhibits a differential response to a range of vapours and odours. The authors report on a novel application of artificial neural networks (ANNS) to the processing of data gathered from the integrated sensor array or electronic nose. This technique offers several advantages, such as adaptability, fault tolerance, and potential for hardware implementation over conventional data processing techniques. Results of the classification of the signal spectra measured from several alcohols are reported and they show considerable promise for the future application of ANNS within the field of sensor array processing.

Design of capacitance electrodes for concentration measurement of two-phase flow

Abstract: Capacitance electrode systems for the measurement of the volumetric concentration of two-phase flows have inherently non-uniform sensitivity distributions over the pipe cross section and therefore have different responses to different flow regimes (eg core, annular and stratified flows) In processes with varying flow regimes, this can lead to significant measurement errors An electrode system which has a uniform sensitivity distribution and consequently a response independent of flow regime is therefore desirable The system performance parameters of surface electrode systems used with stray-immune measurement transducers have been characterised quantitatively using a two-dimensional finite-element model The electrode system design parameters have also been identified and their effects on system performance parameters have been investigated As a result, optimum electrode designs are proposed, with responses to different flow regimes having a minimum deviation from linearity

An electronic speckle pattern interferometer for complete in-plane displacement measurement

Abstract: Measurement of plane-strain surface displacement with ESPI requires at least two in-plane illumination geometries. For static loading conditions it is acceptable to record these two interferograms sequentially. However, for time-dependent strain fields, it is necessary to use both illumination geometries simultaneously so that a recording is made with identical strain conditions existing for both. The authors describe a new interferometer that has been devised to measure two in-plane interferograms at the same time. The determination of the method of operation and experimental verification of the technique are given.

Silicon thermal flow sensor with a two-dimensional direction sensitivity

Abstract: The design and testing of a new type of silicon sensor to measure the magnitude and direction of a fluid flow is described. The measurement principle is based on the detection of flow-induced temperature differences on a hot sensing surface. By measuring temperature differences in two directions perpendicular to each other, a full 360 degrees sensitivity can be obtained. The sensor is made of a silicon IC, on which the flow-induced temperature differences are measured with integrated thermopiles. The use of an IC sensor allows the realisation of a compact multicomponent sensor, which can easily be combined with electronic circuitry. Calibration measurements are presented for air flow.

The quartz crystal microbalance; radial/polar dependence of mass sensitivity both on and off the electrodes

Abstract: The response of AT-cut quartz crystal microbalances is analysed theoretically to show how mass sensitivity varies over the electroded and bare quartz areas Analytical solutions are presented for the radial and polar dependence of mass sensitivity as a function of the electrode areal density for both planar and plano-convex crystals In many surface science applications it is desirable to minimise the off-electrode sensitivity of the QCM, so as to make the fractional on-electrode response as close to unity as possible This fractional response is shown to increase with the electrode mass, but to be in the range 090 to 094 for the planar crystals in common use On the other hand, for plano-convex crystals this response readily exceeds 099 To check the accuracy of the calculations a simple method has been devised to deposit small dots of well defined mass at points on the crystal surface These measurements show that the derived expressions for mass sensitivity are correct to within the measurement accuracy The dots are deposited using a permanent marker pen giving a mass per dot of 021 mu g After measurement, the dots are readily removed by solvent washing leaving the crystal calibrated and ready for other studies

An inductive method for estimating the composition and size of metal particles

Abstract: A method for estimating the composition and size of metal particles is described. An inductive coil sensor is used which, on the introduction of a metal particle, changes impedance. The nature of the impedance change, for a given coil, depends on the composition and size of the particle which can be detected by incorporating the coil into a marginal oscillator circuit and effecting amplitude and frequency demodulation. Experimental results are given for various materials and an application of the method to lubricant wear-debris condition monitoring is described.

Dielectric measurements on reference liquids using automatic network analysers and calculable geometries

Abstract: Knowledge of the in vivo dielectric properties of human tissues is essential to the development of biomedical techniques which make use of radiofrequency and microwave radiation The sensors used for measurement of tissue properties can be conveniently calibrated using dielectric reference liquids, but this in turn requires that the dielectric properties of such liquids be accurately known Many complex permittivity measurements on suitable liquids, such as the primary alcohols and saline solutions, have been reported in the literature, but unfortunately there are in some cases significant discrepancies between reported values The authors describe a program of work undertaken to resolve some of these anomalies A number of techniques which make use of 14 mm coaxial cells and sensors coupled to an automatic network analyser are under investigation Comparison of the different techniques should facilitate evaluation of systematic uncertainties The liquid measurement geometries and the associated computational methods are described Details are given of the uncertainty analysis, and examples of mappings from the complex permittivity plane on to the scattering coefficient plane are shown Some preliminary results are presented, which highlight the value of the comparative approach that has been adopted

A high-resolution chirp transform spectrometer for microwave measurements

Abstract: A microwave heterodyne spectrometer consists of a radiometer front-end and a real-time spectrum analyser back-end. Common spectrometer types are filterbanks, autocorrelators and acousto-optic spectrometers (AOS). A high-resolution chirp transform spectrometer using reflective array compressor (RAC) filters with a time bandwidth product of 6400 is presented. The spectrometer has 1600 channels within a 40 MHz input bandwidth and a linear dynamic range of 63 dB for a CW input. It seems to be very promising for future space applications.

An efficient stacked-Blumlein HV pulse generator

Abstract: A 500 kV fast pulse generator has been constructed using coaxial cable in a stacked-Blumlein arrangement. The generator design is relatively simple, being easy to construct and inexpensive. The output pulse from the generator rises in less than ten nanoseconds and is suitable for low power applications such as HV testing of small-capacitance loads, trigger generators with low jitter and X-ray generator power supplies.

Measurement of Young's modulus and internal stress in silicon microresonators using a resonant frequency technique

Abstract: The values of Young's modulus and internal stress in boron doped silicon microresonators fabricated by anisotropic etching techniques are reported. An array of resonators has been fabricated on a (100) orientation substrate and their resonant frequencies measured. Using the measured data, and equations relating the resonant frequencies to mechanical properties, the Young's modulus of the boron doped silicon is calculated to be 1.33*1011 Pa whilst the built-in stress is calculated to be 1.12*108 Pa.

Measuring microwave fields directly with an optically modulated scatterer

Abstract: The modulated scatterer technique allows one to measure fine structure in electromagnetic fields with the minimum of perturbation. The authors present a general theory for the monostatic use of a modulated scatterer and show that in certain circumstances, for example in the reactive near-field of an antenna, the modulated scatterer may be used monostatically where conventional techniques would fail. They have verified the technique experimentally by constructing a broadbanded homodyne detector, working over the frequency range 2-18 GHz, and using it to measure scattering from an optically modulated electric dipole in the near-field of an open-ended waveguide. The modulating waveform is conveyed to the scatterer via a flexible plastic optical fibre, this provides a constant level of modulation as the scatterer is moved around, without introducing any conducting material into the neighbourhood of the scatterer.

A computer-controlled system for the measurement of complete admittance spectra of piezoelectric resonators

Abstract: A measurement system for the characterisation of the electrical properties of piezoelectric resonators is presented. Admittance of the resonator is measured at successive frequencies controlled by an algorithm which uses small frequency steps when a resonance is found, and large steps between the resonances. During the measurement, characteristic frequencies are marked for each resonance. Further evaluation of the data is done by a least-squares fit, which yields the parameters of the equivalent circuit for each resonance in closed form.

Impacting particle temperature monitoring during plasma spray deposition

Abstract: Instrumentation for monitoring the thermal history of individual plasma sprayed particles as they impact on a substrate is described. A double-wavelength fibre optic temperature sensor is focused on a small spot on the substrate surface to record the cooling rate of particles impacting on this region. Discrimination against in-flight particles intersecting the pyrometer field of view is obtained by a second fibre optic sensor viewing the same spot at an angle and working in coincidence with the first sensor. Typical recorded thermograms are presented and interpreted with reference to a numerical thermal propagation model.

NMR imaging of solids using large oscillating field gradients

Abstract: A method of imaging short T2 materials is demonstrated that makes use of a large sinusoidally time-varying magnetic field gradient. The technique is experimentally verified by imaging both rubber and PTFE samples. A multiple pulse scheme is also described that is shown to provide a signal-to-noise improvement with no time penalty.

A quadrupolar array for measuring the complex permittivity of the ground - Application to earth prospection and planetary exploration

Abstract: Quadrupolar arrays have been used for mapping ground resistivity; low-frequency currents are injected into the Earth and the induced potential drop is simultaneously measured with electrodes necessarily in contact with the Earth's surface. A new version of the quadrupolar array is proposed; this instrument makes use of high-frequency signals, does not require any mechanical contact with the ground and measures the resistivity and permittivity of the subjacent medium. A practical design applicable to unmanned planetary missions is presented.

Refractometry of liquid samples with spectrally resolved white light interferometry

Abstract: An interferometric method is presented to measure the differential refractive index Delta n of a liquid solution. The procedure is based on the spectroscopic analysis of the phase of a light beam which travels through the sample. The interferogram is recorded by a linear array of CCD photodiodes. The analogue signal so obtained is automatically processed by a microcomputer in order to get the refractive index. Precision of 2*10-7 in Delta n is obtained. Experimental examples and alternative methods of data processing are presented.

Precision measurement aspects of ion traps

Abstract: The use of ion traps in precision measurements of various kinds has been growing rapidly in recent years. This review attempts to survey work in this area. It starts with a brief description of the two main types of ion traps and how they work, and discusses the different methods available for detection of ions in a trap and for reducing their kinetic energy. The main part of the review deals with measurements of the magnetic moments (g-factors) of electrons, positrons and ions in traps; precision mass determinations, especially for rare isotopes produced in small quantities; and measurements of microwave and optical transition frequencies in ions, especially with applications to frequency standards in mind. The review concludes with a very brief sketch of some of the other main uses of ion traps to date, touching on the study of quantum jumps and ion crystals, and the measurement of the lifetimes of excited electronic states of ions

Use of magnetic forces to control distance in a surface force apparatus

Abstract: A simple system that permits the surface separation to be varied smoothly and accurately in a surface force apparatus is described. It is based on the deflection of the force-measuring spring in a magnetic field gradient created by a current passing through two coils. The resulting displacement of the surface is highly linear with applied current and free of hysteresis over separations of 10 mu m or more. This allows the accurate measurement of the force between two surfaces by monitoring the difference between expected and observed spring deflection (using, for example, multiple-beam interferometry).

Sub-nanometre displacements calibration using X-ray interferometry

Abstract: The technology of X-ray interferometry has been applied to the calibration of micro displacement transducers at sub-nanometre levels Using the lattice spacing of high perfection silicon of semiconductor industry grade as a reference, the system provides a portable, absolute length standard with an accuracy of 1 part in 106 traceable to primary standards, a demonstrated resolution of 10 picometres and a double-beam phase-quadrature output for providing sensitivity to the direction of motion of the moving interferometer blade The system and its performance are discussed in detail, methodologies for the analysis of data are presented and an application to the calibration of Talystep transducers (both absolute and comparative between models) for overall accuracy and thermal drift is reported The design and performance of a portable instrument using a miniature X-ray source is also reported

Characteristic quantities of a cascade arc used as a light source for spectroscopic techniques

Abstract: The authors have determined the characteristic quantities (the electron temperature and the electron density) of an argon plasma in a cascade arc (diameter of 2 and 4 mm) for a pressure range of 1*105-8*105 Pa and a current range of 20-70 A. The absolute continuum intensity was also determined in the wavelength ranges from 250-320 nm and 380-800 nm for pressures of 2, 4 and 6 bar and 20, 40 and 60 A in the case of a 2 mm arc. The plasma is close to local thermal equilibrium (LTE). Using the mentioned quantities, prediction of the absolute intensity as a function of the wavelength is possible from 140 nm to the infrared within 10%. The use of the arc as a light source in photon induced chemical vapour deposition, spectroscopic ellipsometry and infrared absorption spectroscopy is discussed.

Off-axis response for particles passing through long apertures in Coulter-type counters

Abstract: A novel modification of the resistive pulse technique (Coulter principle) has been used to investigate how the measured resistance pulse amplitude depends on the off-axis particle position in long pores By pressure drive, a particle enters a current-carrying pore and an increase in resistance proportional to the particle volume is detected When the particle exits the pore, the pressure is reversed such that the particle re-enters the pore and the same particle can thus be studied for a long time In Poiseuille flow, solid spheres migrate to an off-axis equilibrium position and this non-linear hydrodynamic effect has been utilised to study how the measured pulse amplitude from a single particle flowing back and forth through a pore increases when the particle migrates closer to the pore wall The increase in pulse amplitude corresponding to a radial particle displacement from the pore axis to the wall is found to be less than 10% for all particle and pore sizes studied This is considerably less than predicted by the off-axis upper-limit theory of Smythe

A new Golay code system for ultrasonic pulse echo measurements

Abstract: Many measurements involving ultrasonic pulse echo techniques are limited by poor signal-to-noise ratios. It is well known that random or pseudorandom codes in conjunction with cross correlation can enhance signal-to-noise ratio and it has been shown that a form of pseudo-random sequence known as a Golay code enables the signal-to-noise ratio improvement to be obtained whilst requiring only two sequences of the code to overcome problems of self noise. A significant parameter associated with a system employing this technique is the amplitude of the coded signal applied to the transducer. The authors describe the development and application of a system having a transmitter capable of delivering peak-to-peak signals of 100 V. Measurements show that this instrument provides considerable signal-to-noise improvements when compared with conventional equipment.

A Couette shear flow cell for small-angle neutron scattering studies

Abstract: Shear flow alignment is an established technique for aligning anisotropically shaped molecules and colloids in small-angle neutron scattering studies. The authors present details of the construction of a Couette shear flow cell designed for such studies. Examples of small-angle neutron scattering studies using this cell are included.

A vision system for automated crack detection in welds

Abstract: The authors describe the development of a vision-based system for automated inspection of surface cracks in materials. The magnetic particle inspection method is used to reveal cracks in ferromagnetic materials. The vision system consists of an IBM AT compatible computer, a framestore and a camera. Details of the preparation procedure and the software algorithms for crack recognition are discussed. Results are presented for transverse and longitudinal cracks in welds used to fabricate structures.

Miniature laser anemometer for 3D measurements

Abstract: The authors describe an extremely compact laser anemometer primarily intended for boundary layer measurements in centrifugal compressors, but retaining the features and performance of a conventional instrument. The instrument has a very high spatial resolution of 80 mu m in all three dimensions and uses a technique of oblique backscatter to allow sequential measurement of the three orthogonal velocity components. An optical head with a diameter of 20 mm is remotely connected using optical fibres to the laser, photodetector and signal processing equipment. The design of the instrument is discussed, together with the results of laboratory tests which confirm its satisfactory operation.

A fully compensated hot/cold wire anemometer system for unsteady flow velocity and temperature measurements

Abstract: Constant-current anemometers operating as resistance thermometers are ideally suited for measurement of stream temperature fluctuations In order to extend their limited frequency response due to the thermal inertia of the wire, open-loop compensation using a zero-pole network is often used This method becomes unsatisfactory for unsteady flows with large velocity changes as the time constant of the wire changes significantly from instant to instant An instantaneous velocity-dependent compensation method is described for use with a velocity-temperature probe Once set, the compensator time constant matches the wire time constant at all times Limits for good compensation are presented

Electron swarm experiments in fluids-signal waveform analysis

Abstract: The authors report on several methods developed in order to extract information on electron attachment frequency and on electronic and ionic mobility from the numerical analysis of the waveforms recorded during electron swarm experiments. These numerical methods, implemented on a personal computer, enable one to analyse the large number of data acquisitions made during a typical experimental run in a reasonably small time.

The performance of a longitudinal Zeeman-stabilised He-Ne laser (633 nm) with thermal modulation and control

Abstract: The frequency stability performance of a small internal-mirror stabilised laser is presented. The statistical frequency stability is 5*10-10 for one-second averages, and is white-noise limited for averaging times between 100 ms and 10 min. The long-term reproducibility has been investigated over four years. It is the sum of the day-to-day resettability (+or-5*10-10) and the gas clean-up effect, which gives rise to a drift rate of -1*10-11 per hour of operation.