Table of contents

Progress in the instrumentation required to measure surfaces effectively has, in the past, been severely restricted because of a number of problems. One has been a true understanding of surface function and another has been a limitation in data processing power and a third has been due to instrumental limitations. In the last few years these three factors have been, to a large extent, brought together by using random process analysis, digital methods and both contacting and noncontacting methods. The author explores these separate themes and shows how they are likely to merge today and in the future.

The principles of operation of potentiometric, amperometric, coulometric and impedance-based oxygen gas sensors are reviewed. Factors influencing the speed of response are considered in detail and related to the information obtainable from impedance measurements. The response of a sensor to gaseous mixtures not in equilibrium is discussed, particularly in relation to measurements in gases generated by the combustion of fossil fuels.

A method is presented by which a absolute pressure gauge may be calibrated using a single piston gauge in a pressure range more than two decades lower than its usual working range. The calibration pressure is referenced to the weight of an additional mass and to the cross section of the piston. The auxiliary instrumentation necessary for this method is an uncalibrated reference pressure gauge and a mechanism for removing the additional mass from the piston. A measurement uncertainty of +or-0.6 Pa down to a pressure of 30 Pa has been achieved. From a test measurement it is inferred that the absence of a finite bell jar pressure in a piston gauge does not influence its operation.

Two primary standard piston gauges of 2 and 5 MPa capacity used for gas pressure measurement in gauge conditions have been modified and improved. The performance of each was checked, particularly in connection with piston rotational frequency and fall rates, with three different gases (nitrogen, helium and carbon dioxide), and with regard to the possible influence of surface magnetisation and electrostatic charges. The two systems have been directly cross-floated, in order to test gauge sensitivity and the reproducibility of pressure measurements under different conditions of use. The overall uncertainty in the gauge pressure measurements with the two piston gauges in the 0.1 to 5 MPa range was always better than +or-14 PPM, with nitrogen as the pressurising gas.

Spindle compensating high-precision roundness techniques have the resolution required for measuring modern precision miniature workpieces but their accuracy has been considered only at radii of tens of millimetres. As the performance of such instruments decreases with decreasing radius, it is not clear that their precision is adequately maintained. An investigation of the sources of systematic error shows that current designs are usable, in this context, down to radii of around 1 mm. An implementation using a calliperlike stylus on a turntable roundness instrument is described and evaluated on balls of this size.

An inexpensive multichannel scaler is described which incorporates a dedicated 6502 microprocessor. The instrument may be interfaced to any computer with an RS232 serial port, and is intended to relieve powerful mini- and microcomputers of the mundane tasks associated with multichannel scaling. The new instrument allows the supporting software to control all the parameters associated with the acquisition of data. It is also capable of performing functions that have previously been impossible with conventional multichannel scalers. The instrument has been successfully applied to a number of high resolution electron impact spectrometers, principally to investigate the excited states of gaseous atoms and molecules.

The description of an experimental set-up for the determination of the thermodiffusion factor alpha _T of binary mixtures with a thermodiffusion column is given working in the temperature range from 300 K to 500 K and at pressures operating in the interval from 10 to 1000 kPa. The construction of the thermodiffusion column allows the investigation and optimisation of the thermodynamic parameters and column geometry for separating particular gas mixtures.

A time-varying surface temperature may be measured by a thin film of encapsulated thermochromic liquid crystal applied to the surface. The work reported in this paper has assessed, for engineering purposes, the response of such a film to a rapidly increasing surface temperature. Rates of increase in temperature of greater than 2000 degrees C s^-1 were employed in these tests. The experiments show that the delay between the time at which the surfaces reaches the steady-state colour display temperature and the occurrence of the colour display is no more than a few milliseconds. The particular type of film used employed a micro-encapsulated chiral nematic thermochromic liquid crystal which was supplied by BDH and applied to the surface by a screen printing technique to a thickness of approximately 10 mu m. The conduction of heat into the film is analysed, but this would not appear to explain the observed response time of the thermometer.

A new type of temperature probe employing a novel feedback technique and making use of the temperature-dependent property of a commercial silicon 2N 2222 n-p-n transistor has been developed and successfully flight tested in meteorological balloons. The feedback technique employed not only overcomes the problems of DC drift but also enables handling of large signals. Its ability to measure absolute temperature and fast response ( approximately 0.3 s) make this probe suitable for atmospheric studies and meteorological applications.

A low-cost fluorescence temperature sensor using the decay-time principle has been constructed, using a millimetre size probe element, a dedicated microprocessor and simple logic circuitry with LED excitation of the material. Results in the region from -50 to +200 degrees C have been reported with a approximately +or-2 degrees C accuracy using Q100 neodymium glass and comparisons made with commercially available sensors. Extension of the work to higher temperature measurement is discussed.

An instrument for the highly accurate and simultaneous determination of thermal conductivity and thermal diffusivity at elevated temperatures and pressures was built and tested. The temperature range of the instrument is 273-473 K, and the pressure range 0-16 MPa. The instrument is based on the classic transient hot-wire technique. New data are presented in the range 288-460 K and at 10 MPa for toluene and heptane, which are the proposed standards for thermal conductivity data.

A manganin pressure transducer has been calibrated for use with a computer-controlled data gathering system. A single polynomial for R(p, t) reproduces the measurements at pressures up to 350 MPa and temperatures from 0 to 40 degrees C.

The design philosophy of an electronic circuit for a single-element liquid-crystal touch switch is described. The resulting device achieves excellent touch sensitivity and retains good noise rejection and long-term stability. Successful circuit operation does not rely on critical selection of components.

A phase measuring system is described which can be used to obtain the cosine and sine of the phase difference between two input signals independent of input amplitude over a range of more than 60 dB. This approach, in which analogue multipliers are used as phase detectors, allows the user to measure both the magnitude and sign of phase without some of the disadvantages of conventional phase measuring systems based on digital phase detectors and the necessity of eliminating the amplitude information from the output of a system based on analogue multipliers. Test results on a system built for use in plasma wave experiments are presented.

The performance of a highly directional electrostatic energy analyser, using a glass microchannel plate as a collimator, can severely deteriorate through accumulation of electric charge on the channel surfaces, especially at high-charged-particle densities. It is shown that wet-silvering these surfaces eliminates such deterioration.

A magnetic resonance spectrometer is described which operates between 500 MHz and 3 GHz and uses an NE72089 GaAs MESFET operating at liquid helium temperatures as the high frequency active device. Although the sensitivity is not remarkable, it compares favourably with such other techniques as are available in this frequency range and it combines small size with ease of use.

The essential design features of a vertical field 0.08 T, 3.4 MHz NMR proton imager are described. The machine was constructed in-house following successful clinical trials with a prototype 1.7 MHz imager. Based on a four-coil air-cored resistive electromagnet, the machine acquires single-slice 128*128 pixel images in about 4 min using an interleaved saturation recovery, inversion recovery sequence, where spin echoes are formed by gradient reversal. The image formation technique is spin warp. Since its installation in the Aberdeen Royal Infirmary in early 1983, over 4000 patients have have scanned. Images are presented to illustrate its performance.

Measurements of sizes and concentrations of coal particles in densely laden media are carried out by means of a visible/infrared double extinction technique and the underlying theory is explained. It is shown that simultaneous sizing and concentration measurements can be obtained by recording the transmittances of the medium under study at two well chosen wavelengths simultaneously (for instance in the visible and in the far infrared ranges). Experimental results for coal particles sieved between 20 and 80 mu m are described and discussed. The size of particles given by the described technique agrees with the one given by optical microscopy and Malvern diffractometry. The technique is shown to be very suitable for investigating densely laden flows up to 5*10⁹ m^-3 (1 kg m^-3), for example in the case of 60 mu m coal particles within a 0.25 m length medium.

By means of a simple analogue computer the signals of a multichannel far-infrared interferometer/polarimeter can be converted into suitable profile shape parameters which provide instantaneous information on the time evolution of the electron and current density distributions in Tokamak plasmas. For circular cross sections formulas approximating the profiles are presented. The mathematical dependences of the profile shape parameters u and w on the measured signals are derived, and suitable computational circuits are presented. The performance is illustrated by some experimental results from the TEXTOR Tokamak.

A Michelson-type fringe-counting wavemeter is described. A low noise direct carriage drive was constructed that allows interpolation to ¹/₁₂₈ of a wavelength. A total accuracy of some parts in 10^-8 is achieved within a scan of only 165 mm. Any erroneous wavelength measurement is suppressed by an appropriate control unit. Small dimensions and automatic error detection allow easy application under various experimental conditions.

A far-infrared interferometer, illuminated with an optically pumped formic acid vapour laser, which uses a rotating mirror assembly spinning at high speed to scan the probing beam radially across a tokamak plasma, is described. This interferometer has been used on the University of Sydney's TORTUS tokamak, where the data recorded during each scan has been processed to yield radial electron density profiles of the 200 mm diameter plasma. The beam sweeps across the plasma in 1.0 ms, in which time the plasma electron density does not change significantly. Scans are obtained every 8.3 ms throughout the lifetime of the plasma. The phase shift due to the plasma is measured with an accuracy of +or-¹/₂₀th wavelength fringe shift, corresponding to an uncertainty in the electron density of +or-1*10¹⁸ m^-3 and a spatial resolution of 20 mm for the 433 mu m wavelength used.

The authors have investigated the effects of Freon-22 (range 0-5%) on CO (range 0-0.3%) analysis and observed that Freon-22 seriously interferes with the analysis of CO. This is due to overlapping peaks on the CO and Freon-22 IR spectra. They conclude that, to analyse CO using an IR gas analyser with Freon-22 present, the gain of the analyser should be decreased, the analyser should be calibrated specifically for a CO/Freon-22 mixture, and a correction equations must be applied to correct the CO concentration for the concentration of Freon-22 present.

A proportional chamber (MWPC) with 96 channels-each supported by a separate electronic readout-and 1 mm wire spacing is described. Construction features, chamber tests and performance data of a MWPC used with a Johann spectrometer on a tokamak are presented.

An imaging XUV (6.2-31.0 nm) telescope (the wide-field camera (WFC)) is to be flow on the German X-ray astronomy satellite ROSAT. The calibration system uses a series of fused silica lenses to project the magnified image of a rectangular array of 20 mu m diameter illuminated spots on to the XUV detector (a microchannel plate), so that any misalignment of the XUV mirror system or distortion in the image plane can be readily recognised.

A two-axis X-ray diffractometer run by a microprocessor and equipped with a climbing detector to cover the upper hemisphere was provided with the option of exchanging the point detector with flat or cylindrical film cassettes. By synchronisation of the specimen rotation and the film motion, Weissenberg-type and other exposures are obtained. Various choices of the relative motions may be used to focus the attention on selected parts of the reciprocal space, which may be magnified and reshaped. The importance and convenience of being able to perform film and point detector studies on the same instrument and at any temperature are pointed out.

Two simple methods are described by means of which broad-band excimer laser pulses of 2 to 3 ns duration can be generated. The first of these uses gain depletion, and the second a surface plasma switch.

The unsteady fluctuations due to turbulent flow which occur at points on the image plane of a shadowgraph visualisation system are measured using a small photodetector. The characteristics of the output signal have been investigated to determine when it has linear response to the turbulent fluctuations, and to verify that it has the expected spectral composition. It has been found that the signals are related in magnitude to the difference in density between the mixing air jet flow and the ambient air surrounding the turbulent mixing flow. Distributions of the optical shadowgraph fluctuations have been transformed by an Abel integral method to yield radial distributions of the turbulent density fluctuations within the mixing region.

The effect of four metal and four non-metal wall materials on the hot-wire signal has been examined using three different hot-wire probe orientations under no-flow condition. The results indicate that the effect of the wall on the hot-wire signal at a given distance from the wall increases with wall thermal conductivity up to a relative thermal conductivity, k/k₀, of 100, and thereafter the signal due to wall proximity remains at this asymptotic value. It has been further observed that the wall effect becomes negligible for distances greater than 0.5 mm from the wall.

A small differential laser Doppler velocimeter which uses a semiconductor laser and a small number of optical components is described. In this device the light from the laser diode is split into coherent beams by means of a diffraction grating. The two first-order beams are crossed in a probe volume with a lens. In a test experiment the velocity of water containing 0.9 mu m polystyrene spheres was determined from the fluctuation of the scattered light. This velocity agreed with the velocity calculated from flux measurements.

The design of an inexpensive but flexible transient digitiser is described. The device is intended for installation in one of the peripheral expansion slots of a general purpose microcomputer, and obtains an eight-bit digitisation rate of 1 MHz with a unipolar input range of 0 to -5 V and a temporal acquisition window of 2 ms.

The author presents modifications to the original instructions, which make it possible to have a reliable high speed transfer from a CAMAC transient recorder to a computer by means of a Le Croy CAMAC-GPIB interface.

A device is described which provides an improved technique for the quantitative handling of volatile substances. Examples of its application are given.

A simple compact ultra-high-vacuum (UHV) electron-beam evaporator, to deposit ultra-thin layers of low vapour pressure, high melting point transition metals is described. It has an integral cryoshield and shutter which will fit a UHV system with a standard 70 mm flange.

Heterodyne laser Michelson interferometry is widely used in precision length measurement but the accuracy is often limited by the unknown size of the periodic non-linearity due to imperfect separation of the two optical frequencies. A method for measuring this non-linearity with an accuracy of about 0.15 nm using pressure scanning of the interferometer is described and results are given for three configurations of a commercial measuring system.

A method has been developed to separate superconducting particles from non-superconducting ones during the preparation of high-temperature superconducting samples.