Showing papers in "Journal of Physics E: Scientific Instruments in 1982"

An ultra-low-load penetration hardness tester

Abstract: An ultra-low-load penetration hardness tester which operates at loads of up to 3 mN with a resolution of 10 mu N is described. It is capable of producing a continuous record of the penetration depth of the indenter and the resolution of the depth measurement is estimated to be 5 nm. A hardness value is deduced from the applied load and the penetration depth, which may be less than 0.1 mu m. The ability of the machine to obtain information on on-load hardness, off-load hardness, elasticity, adhesion and load-dependent effects is demonstrated. The authors describe a study of the properties of a soft metal (indium) and thereby show how the types of measurement obtained are related to conventional microhardness measurements.

Fabry-Perot and open resonators at microwave and millimetre wave frequencies, 2-300 GHz

Abstract: The design and metrological applications of Fabry-Perot resonators in the microwave and millimetre regions of the electromagnetic spectrum are reviewed. The review opens with introductory discussions on open resonator principles and configurations, continues with an analysis of developments in theoretical and empirical resonator characterisation and concludes with a broad survey of recent microwave Fabry-Perot applications. These include dielectric, anisotropy, magnetic resonance, scattering and atmospheric measurements and gas refractometry and spectroscopy. Other applications discussed include quasi-optical filters and diplexers.

Principles and theory of wattmeters operating on the basis of regularly spaced sample pairs

Abstract: The contribution to the errors of a regularly sampling wattmeter caused by using n distinct instantaneous samples of the input signals taken at equal intervals of time is evaluated and discussed. It is shown that although most of the possible harmonic products are handled correctly, a significant number of harmonic autoproducts and particularly of harmonic cross products are not, namely all the components of the nth, 2nth, 3nth, ... product harmonics. Sinusoidal waveforms are handled correctly, and the errors with near-sinusoidal waveforms are generally negligible. Two particular classes of non-sinusoidal waveforms are examined, those controlled by phase-angle switching and those associated with discharge lamps.

Advances in lock-in amplifiers

Abstract: The key specification points of lock-in amplifier systems for signal recovery and signal characteristics are reviewed and it is shown how these can be improved and modified by more advanced system design. The configurations of several commercially available systems are described and the facilities available in computer-controlled lock-in systems are discussed briefly, together with some new application areas.

Noise reduction techniques for CCD image sensors

Abstract: The properties of several widely used CCD signal-processing schemes are discussed in detail. A common feature is that correlated double sampling techniques are used to attenuate low frequency noise. The relative merits of each scheme depend on the exact nature of the noise sources. Formulae are derived which enable assessment of the best scheme to use for a particular device once its noise spectrum is konwn; conversely, the relationship between output noise and such parameters as signal integration time can be used to determine the noise spectrum. The analysis also applies to charge injection devices and voltage sampling diode arrays. For each scheme the formulae are used to predict the readout noise for a system employing a cooled GEC MA328 CCD.

Materials testing at high constant strain rates

Abstract: An improved version of the split-Hopkinson pressure bar system for materials testing at high rates of loading is described. This new system has a preloading bar together with a pulse-shaping dummy specimen of the same material as the test specimen. Using the transmitted pulse through the pulse shaper as a tailored incident pulse for loading the test specimen enables testing to be carried out at virtually constant rates of strain, from about 100 s-1 to 300 s -1. This represents a significant improvement over the normal method where there can be considerable variation in strain rate throughout a test with a consequent uncertainty in the interpretation of the results.

Magnetic field gradient coils for NMR imaging

Abstract: A new magnetic field gradient coil system is described for use in whole body NMR imaging experiments. The coil assembly has a low inductance and can therefore be used in structures where switching speed is important. Curves showing the performance of the coil system are given and an NMR image of a phantom object obtained with the assembly is presented.

The charge-density method of solving electrostatic problems with and without the inclusion of space-charge

Abstract: Discusses the singularities and approximations that occur in the charge-density method of solving electrostatic problems in which space-charge is absent (Laplace's equation) or present (Poisson's equation). Describes the application of the method to electrostatic lenses in general and to the particular case of a high-current triode gun.

Passive and active seismic isolation for gravitational radiation detectors and other instruments

Abstract: Some new passive and active methods for reducing the effects of seismic disturbances on suspended masses are described, with special reference to gravitational radiation detectors in which differential horizontal motions of two or more suspended test masses are monitored. In these methods it is important to be able to determine horizontal seismic accelerations independent of tilts of the ground. Measurement of changes in inclination of the suspension wire of a test mass, relative to a direction defined by a reference arm of long period of oscillation, makes it possible to carry this out over the frequency range of interest for earth-based gravitational radiation detectors. The signal obtained can then be used to compensate for the effects of seismic disturbances on the test mass if necessary. Alternatively the signal corresponding to horizontal acceleration can be used to move the point from which the test mass is suspended in such a way as to reduce the effect of the seismic disturbance and also damp pendulum motions of the suspended test mass. Experimental work with an active anti-seismic system of this type is described.

Field-ion microscopy-a review of basic principles and selected applications

Abstract: The following topics related to field-ion microscopy (FIM) are dealt with: image magnification; imaging capabilities; field ionisation; field-ion energy distributions; the hopping gas model; image resolution; field evaporation; the field-desorption microscope; microchannel plate image intensification; the atom-probe FIM; surface diffusion in the FIM; and biomolecular imaging.

Non-intrusive methods of velocity measurement in pneumatic conveying

Abstract: Velocity measurements in pneumatic conveying systems is becoming increasingly important for energy saving. Doppler techniques using lasers, microwave and ultrasonic sensors are discussed; the laser techniques appear most suitable for use as research and development aids in system design. Injected tracers are difficult to use, but can give an adequate measure of bulk velocity for system development purposes. Cross correlation of natural tracers in the flow uses robust sensors and shows promise of being a cheap method for on-line process control purposes.

Compact spherical coil for fluxgate magnetometer vector feedback

Abstract: A novel design of a magnetic field feedback compensation coil system for triaxial fluxgate magnetometers is presented. The coils are wound on three concentric spherical shells of slightly different diameters with the fluxgate sensors placed in the 60 mm diameter internal field-free space. Measurements of the temperature coefficient and of the internal field homogeneity are given, and the noise performance is evaluated for a magnetometer incorporating the coil. The total outer diameter of the system is 90 mm and the weight is 350 g including the sensors. The main advantage of a vector feedback system is the exact definition of the magnetic axes. These are determined exclusively by the coil axes, and they are independent of even large angular displacements of the individual magnetic axes of the fluxgate elements.

Multi-terminal capacitor sensors

Abstract: A survey is given of the theoretical background for designing multi-terminal capacitor sensors of several types, suitable for control of every physical quantity that can be related with the change in position of two carriers, covered with electrodes. Previously published investigations of capacitor sensors, are briefly mentioned, and a description of the capacitance bridge used and the results of investigations of a multi-terminal capacitor test model are given. The main conclusion is that the capacitor sensors described can be used in cases where reliability and sensitivity of laser interferometry and strain gauge measurements fail.

Frequency response of fine wire thermocouple

Abstract: A theoretical and experimental study of the frequency response of fine wire thermocouples is presented. Influence of sensor geometry and flow parameters on the behaviour of transfer functions is given. Comparison between time constants obtained from internal heating technique (IHT) and external heating technique (EHT) shows significant differences due to the nonhomogeneous heating of the wire by both Joule and Peltier effects when IHT is used.

Instrumentation for NMR spin-warp imaging

Abstract: Implementation of a new NMR imaging technique has lead to the introduction of new instrumentation in imaging machines. The authors describe magnetic field gradient drivers and microprocessor control circuits which have increased the versatility of the system. The performance of the imaging method and improved resolution whole body images are discussed.

Reconstructing NMR images under magnetic fields with large inhomogeneities

Abstract: A novel technique is presented for reconstructing accurate NMR images from projections under inhomogeneous main magnetic field. The mathematical basis and the point-spread functions of such a curvilinear reconstruction for two- and three-dimensional imagings are fully described. The proper use and the effectiveness of this technique are demonstrated by computer simulations. The promising results presented could strongly influence practices in the design, manufacture and use of the main magnet for NMR imaging by the projections method.

Miniature imaging photon detectors. III. An assessment of the performance of the resistive anode IPD

Abstract: For pt.II see ibid., vol.13, p.763 (1980). The imaging performance of a proximity-focused imaging photon detector using microchannel plate (MCP) intensifiers, and a resistive anode is determined by four major factors. These are: the electron optics of the biplanar lens formed by the photocathode and MCP, with a gap of between 300 mu m and 1 mm; the random walk process of intensification within a triple-MCP (Z-plate) intensifier; the noise factor of the anode and the preamplifiers, and the errors of the main signal processing electronics. A device with a 1.0 mm photocathode to MCP gap and a potential difference of about 250V across this gap, and a 'linear' resistive anode read-out system, shows a point spread function with a full width half maximum (FWHM) of 86 mu m. Consideration of the potential improvements which would materially affect resolution indicate that about 30 mu m FWHM is achievable, corresponding to a limiting resolution of 34 line pairs per mm, at 3% modulation.

Acoustic measurement of the arc voltage applicable to arc welding and arc furnaces

Abstract: A new technique is proposed for determining the time evolution of the voltage in the column of an electric arc. It is based on measurement of the sound wave produced by the arc, which is shown to depend on the rate of change of the electrical power fed to the arc column. There is no electrical connection to the arc circuit so that the errors usually caused by the voltages induced by the arc DI/dt's and ohmic drops in the arc electrodes are avoided. The technique is of interest for controlling the voltage or length of the arc and offers applications in arc welding and the operation of arc furnaces.

Fibre-optic laser Doppler anemometer with Bragg frequency shift utilising polarisation-preserving single-mode fibre

Abstract: A fibre-optic laser Doppler anemometer (LDA) utilising a single-mode polarisation-preserving fibre as a (transmitting) link is discussed. Since these fibres are bimodal in the sense that they can sustain two modes with orthogonal polarisations, they can be used to transmit the two beams of a normal LDA in which one beam is frequency shifted. By rotating the polarisation of one beam 90 degrees before coupling it to the fibre, the two beams propagate independently through the fibre and can be separated again at the fibre exit. The performance of this type of fibre in a laser Doppler set-up is investigated and experimental results are given. A miniature optical measuring head using this principle and operating in backscatter is described. The backscattered light is collected by a conventional multi-mode fibre.

Conditions necessary to get meaningful measurements from the Kelvin method

Abstract: In this work, the authors emphasise the difficulties encountered in determining the contact potential difference (CPD) using the Kelvin method with a small area reference electrode. A new theoretical analysis of this problem based on a plane capacitors scheme has been tentatively worked out. It gives a rather good explanation of the various experimentally observed phenomena. They also show that it is misleading to attempt to measure a CPD corresponding to a perfectly defined surface without setting a guard shield around the reference electrode, and that experiments carried out with a large sample guard spacing (>0.2 mm) could not give significant results.

Central data collection facility for protein crystallography, small angle diffraction and scattering at the Daresbury Laboratory Synchrotron Radiation Source (SRS), England

Abstract: An experimental workstation for protein crystallography using synchrotron X-radiation is described. Different modes of the single, bent, triangular monochromator are discussed for both the rapid collection of high Bragg resolution native crystal data and high spectral resolution (( delta lambda / lambda )

High-precision measurements with a bellows volumometer

Abstract: A metal bellows volumometer operating to 250 MPa is described which uses a linear differential transformer in a novel, indirect way to establish the position of the moveable end of the bellows with a precision of about 1 in 60000. Calibration with both water and n-heptane has shown that the effective cross-sectional area of the bellows depends on both pressure and the extent of movement of the bellows. Comparison of excess volumes for acetonitrile-water mixtures calculated from the bellows volumometer data with values determined directly by other authors indicates that the device provides densities accurate to +or-0.02%.

Gamma-ray scattering studies of composition variations in alloys

Abstract: The physical principles underlying the use of gamma-ray scattering to detect composition variations in bulk alloy samples are described and the sensitivity of the method, which is an alternative to conventional transmission radiography, is evaluated for titanium alloys. The elastic (Rayleigh) and inelastic (Compton) scattered beams are recorded separately and their intensities used to monitor composition changes collectively and individually. The sensitivity of the method is illustrated by measurements on titanium alloys containing aluminium and zirconium.

Displacement transducers based on reactive sensors in transformer ratio bridge circuits

Abstract: Displacement transducers which employ differential reactive sensors in transformer ratio bridge circuits are considered. Archetype models of inductive and capacitive sensors are developed and the performance of bridge circuits containing them is analysed. Aspects of performance which are examined include linearity, discrimination, excitation forces, power dissipation and the effects of stray impedances. Practical design problems relating to long-term stability and environmental influences are also discussed. It is concluded that capacitive transducers are most suitable in applications requiring high levels of accuracy, stability and discrimination and low power dissipation and excitation forces. In situations where these requirements are not so stringent both inductive and capacitive transducers are suitable.

A calibration system for calorimetric mass flow devices

Abstract: Describes a simple gas flows calibration system which consists of the combination of a secondary standard mass flowmeter and a primary standard volume displacement method. The system has been built for the routine calibration of calorimetric mass flow devices as well as for changing their flow range. Reviews the literature on thermal mass flow instruments and discuss the operations of a commercial shunt type mass flow controller. In the appendix the authors derive the pertinent equations for the different calorimetric mass flow devices in detail. They review a few of the more common calibration methods used in industry and give the corresponding equation and flow corrections required.

A tensile technique for materials testing at high strain rates

Abstract: A method of tensile testing of materials under dynamic conditions has been developed involving little modification to the normal compressive split Hopkinson bar system. The specimens are of tapered shape enabling testing to be carried out at stresses in excess of the yield stress of the pressure bars, while the easy access to the specimens allows testing over a wide range of temperature. Virtually constant strain rates of up to 1500 s-1 have been achieved for stainless steel specimens. A constant effective specimen gauge length has been shown to apply for strain rates from quasi-static to dynamic.

Survey of pipeline flowmeters

Abstract: This article is a critical review of the major types of pipe-mounted flowmeters in current use. It treats the basic principles of each type, considers applications, rangeabilities and accuracies as well as pointing out advantages and pitfalls for the user. Variants of the differential pressure meter, still a popular choice by and large, are examined in greater detail. More modern techniques come to the fore in such examples as electromagnetic, vortex-shedding, acoustic and cross-correlation forms. Comparative costing of different flowmeter types is touched on and the authors put forward suggestions for further reading for those who want fuller information on the material reviewed.

Fabry-Perot interferometers for use at submillimetre wavelengths

Abstract: Reviews the use of Fabry-Perot interferometers at submillimetre wavelengths. The theory of these interferometers is discussed, and practical limitations on the performance are considered. The authors discuss the development of practical instruments, and the applications in which such instruments have been used for submillimetre wave spectroscopy. They identify some areas in which instrumental improvements are desirable, and suggest some possible developments.

An accurate digital instrument for the measurement of circular polarisation of luminescence

Abstract: An apparatus to detect circular polarisation of luminescence (CPL) is described. The instrument, involving a digital synchronous sampling photon counting system and an effective square-wave modulation, is absolute and has a high sensitivity. Its baseline is flat and stable, enable prolonged measurements to detect small degrees of circular polarisation, to study poorly fluorescing compounds or to detect circular polarisation in chemiluminescence. An additional advantage is that the instrument can be simply switched to measure linear polarisation.

Air-core induction-coil magnetometer design

Abstract: This paper discusses the theory of optimising an induction-coil magnetometer and the realisation problem. The optimisation is based on a simple magnetic model of the source, an induction coil and a current-to-voltage converter. The electric models and characteristic equations needed for the optimisation procedure have been derived from prevailing theories. The equations have been modified for computer-aided design. This procedure maximises the signal-to-noise ratio of the magnetometer. The results have been verified by constructing and measuring the optimised magnetometer and its parameters. The theory has also been used to construct a differential magnetometer which improves the single-coil magnetometer performance considerably in magnetically noisy environments. Some magnetocardiographic (MCG) measurements have been carried out in the laboratory environment.