Table of contents

A torsion balance has been modified by clamping the fibre, under tension, at both the top and the bottom of the instrument housing. Operating with the centre of mass of the boom displaced from the near vertical fibre, the instrument is influenced by the Earth's gravitational field as well as by the shear modulus of the fibre. This causes the restoring constant of the balance to depend on the angle of tilt between the fibre direction and local vertical. The contribution from the Earth's field either adds to or subtracts from that of the shear modulus, according to the direction of tilt. The result is an instrument of variable sensitivity when used as a force-measuring device. It can also be configured to function as a highly sensitive tilt meter.

The behaviour of an electromagnetic wall velocimeter for non-intrusive measurement of thin film flows is studied both theoretically and experimentally. A solution for the electromagnetic flowmeter equation is presented and evaluated numerically for particular magnetic fields and thin film velocity profiles. Overall transducer behaviour is examined by a parametric study. An experimental apparatus was designed and built to confirm the calculated transducer behaviour. Flow velocity measurements using a multiple electrode prototype transducer were made over a range of Reynolds numbers. The experimental results are in good quantitative agreement with the theory. The wall velocimeter can be designed to provide a robust, non-intrusive measurement of the mean film velocity.

A backface penetration control system has been developed for DC pulsed tungsten inert gas (TIG) welding. The system is based on a binary vision system built around the BBC microcomputer. The system utilizes a coherent optical bundle to transmit the images of the backface bead to a b/w vidicon camera. The penetration control system regulates the bead size by controlling the pulse duration of the TIG process. Comparisons are made between the required and actual bead sizes in real time, the arc current being returned to the background level when the required penetration is reached. The system has been tested extensively on type 304 stainless steel, using various plate thicknesses for both bead on plate and butt weld tests. The system has also been tested on tapered plates from 0.5 mm to 4 mm. The backface penetration control system performs well over the complete range of tested plate thicknesses, in addition to the tapered plate tests. The image of the weld bead does not necessarily have to be in the centre of the screen to produce good welds; as long as the whole bead appears within the field of view of the lens, the bead size may be controlled. This allows the system to be easily set up and operated.

The 'core-ring ratio' method for surface roughness measurement using a laser beam is generalized to one using extended non-monochromatic light. The relationship between the core-ring ratio and the surface roughness is derived. It is concluded that fast measurement of surface roughness using incoherent light is of practical use.

A split film sensor is preferred over an X-wire probe for the measurement of reverse flow fields. However, the prevailing calibration techniques for the split film sensor are reliable for small pitch angles only. In this study, Stock's method for pitch angle response is improved to provide a more reliable calibration for a wide range of velocity magnitude and pitch angle. To show its usefulness, the proposed method is applied to a turbulent reattaching flow behind a rearward-facing step. The measured data compare favourably with those measured using conventional hot-wire anemometry.

The dielectric and magnetic properties of some ferrite materials have been measured by four widely different methods. These are measurements in waveguide at 14 GHz using a slotted line approach; measurements in waveguide using S parameter data; the open resonator at 24 GHz (the solution of the equations is presented); and an oblique reflection method at 35 GHz. A successful form of apparatus is described. The results from the methods are contrasted. The open resonator method appears to be a good technique for frequencies above about 30 GHz, but it requires very thin specimens, which are difficult to make. The oblique reflection method is promising and has advantages for lossy materials.

A new approach for the linear conversion of temperature to time period using standard thermistors is described. The major advantages of the proposed converters are high sensitivity, simplicity and very good linearity. Analytical expressions for system performance are given and verified experimentally. A nonlinearity error of less than +or-0.002 K over a range of 10 K is obtained.

A simple experimental set-up for the measurement of the microwave surface resistance in the temperature range 20-300 K is described. The microwave setup consists of a Gunn generator, a set of standard Hewlett Packard microwave components and a cylindrical resonant cavity. The measurement technique is presented and applied to the study of the resistive transition of high-temperature superconductors.

A technique for the determination of the abundance and isotopic composition of nitrogen within organic compounds is described. It is intended for use with high-sensitivity static vacuum mass spectrometers. The accuracy of the abundance determination is +or-5% by weight and replicate delta ¹⁵N determinations of nitrogen for five organic compounds were all within +or-8%. The compounds selected contained nitrogen within a variety of organic structures and with different types of nitrogen bonding including -NO₂, -NH₂, C identical to N, C=N, and C-N. In no case were detectable amounts of nitrogen oxides formed during oxidation. Blank analyses suggest that meaningful N abundance and delta ¹⁵N determinations can be performed on materials containing greater than 20 ppm nitrogen by weight.

A description is given of the measurement of the frequency response of two avalanche photodiodes and their associated amplifier over the frequency range 1 kHz and 25 MHz by an opto-mechanical frequency response calibrator. The photodiodes are part of a laser interferometer which is used in the calibration of ultrasonic hydrophones. The accuracy of the calibrator has been estimated to be better than 3% for measurements made at frequencies below 20 MHz.

A compact multigas capacitive transfer transversely excited (TE) gas laser is described. The laser produces a high-intensity beam ( approximately=20 MW cm^-2) operating with Co₂ mixtures at atmospheric pressure. A pulse of 20.5 mJ is obtained in the second positive (UV) N₂ band and 5 mJ in the first positive (IR) band. The laser is suitable to work with other gases and is useful as a pumping source for dye lasers.

The laser beam shaping system described is used for high-power pulse lasers of high spatial and moderate temporal coherence. It provides an exact flat-top intensity profile with a maximum of a 1*1 cm² rectangular cross section, by means of a two-step splitting of the beam into mutually incoherent parts and by superimposing decorrelated speckle patterns in a diffusing/light-guiding system. After clipping the edges of the intensity distribution, the profile is projected onto the target by means of a double-telecentric optical system. Using the radiation of a Q-switched Nd-glass laser, a flatness of the macroscopic intensity profile with maximum +or-1% deviation from the mean intensity and a nearly speckle-free microscopic intensity distribution could be achieved that exhibited a contrast ratio of less than 0.03 at 2-3 mu m lateral dimension of the residual inhomogeneities.

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 authors describe a novel instrumentation system capable of measuring the particle volume fraction and solids flux of optically opaque suspensions undergoing sedimentation. Two independent but complementary measurement techniques-capacitance and pressure-are integrated into the system.

The relationship between capacitive signal and variable gap size between the capacitor plates of the cell can be obtained by heating a copper disc which transmits its thermal expansion to the sample without exchanging heat with the sample itself. The method allows the determination of thermal diffusivity with a repeatability of about 3% for low conducting materials with thermal expansion coefficients greater than 0.5*10^-5 K^-1.

Electrocapillary methods for excitation of capillary-gravitational waves at liquid surfaces have been modified for application to liquid-liquid interfaces. The necessary experimental modifications are described. Typical results are presented for a water-hexadecane interface. The observed propagation of the generated waves accords with theoretical expectation.

The Mueller matrices of light scattering by the thin film edge of a SiO_x film on a silicon substrate were experimentally determined. The method for Mueller matrix measurement is discussed in detail. The normalized matrix elements S*₁₂ and S*₃₄ were found to be most sensitive to edge height variation at the scatter angle close to the specular direction. An experimental means for determination of edge height, i.e. film thickness, using this phenomenon is proposed. The error, sensitivity and precision of the proposed method are discussed.

An apparatus is presented which allows in situ high pressure dosing of gases at surfaces and subsequent characterization of the surface in UHV by established surface sensitive tools. A sample mounting has been constructed that allows dosing of corrosive gases at elevated temperatures and that allows pick-up by a sample transfer rod for transfer to an ion scattering apparatus.

An inexpensive electronic system has been designed and constructed to generate the field gradients necessary for magnetic resonance imaging applications. This system consists of a three-channel digital-to-analogue converter interface followed by a power amplifier. The digital-to-analogue converter interface sits in one of the expansion slots of the IBM compatible microcomputer and converts the digital input codes into the corresponding analogue output voltages. The power amplifier converts these voltages into currents, to generate the X, Y and Z magnetic field gradients. The software, in the form of an assembly language programme, is also presented for this system to control the Z-gradient coils.

The authors have studied the copper K absorption discontinuity in the metal and its oxides using a Cauchois-type bent crystal X-ray spectrograph equipped with a silicon crystal, along with a conventional X-ray source. The performance of the silicon crystal, which has been used for the first time in the Cauchois geometry in this study, is discussed.

A pulse-by-pulse analysis system provides a simple and direct method for measuring the pulse-to-pulse frequency jitter and the spectral width of nanosecond pulses from single-mode lasers, up to repetition rates of a few kHz. To test the method, the jitter (<or approximately=1 MHz) and bandwidth ( approximately 80 MHz) of a pulsed laser system at lambda =540 nm, was measured.

Solid state LEDs are used as light sources in a photoacoustic continuous wave experimental set-up. It is shown that their typical relatively high power and narrow emission spectrum in the VIS-near IR range, together with the low cost of such devices, yield performances comparable to those obtained with commercial low-power lasers. Since the modulation of light pulses is accomplished by simply feeding the LED with a square wave, a wide range of chopping frequencies is obtained. Test measurements are reported, showing the good results obtained.

The author discusses the evolution of the International System of Units (SI) since it was adopted for global use in science and technology. This evolution includes revised definitions of the base units, and additional base and derived units and prefixes.