Journal of Physics E: Scientific Instruments

The rigorous expression for the transmission of a thin absorbing film on a transparent substrate is manipulated to yield formulae in closed form for the refractive index and absorption coefficient. A procedure is presented to calculate the thickness to an accuracy of better than 1% with similar accuracies in the values of n. A method to correct for errors due to slit width is also given. Various formulae to calculate the absorption coefficient accurately over almost three orders of magnitude are discussed. Only data from the transmission spectrum are used and the procedure is simple, fast and very accurate. All formulae are in closed form and can be used on a programmable pocket calculator.

Phase sensitive detection is formalized in order that its operation can be understood and its practical use optimized. The theory explains how narrow bandwidth amplification is achieved in order to reduce the noise content of a measurand and how the process can be considered simply as a multiplier followed by a low-pass filter. The responses to various excitation and measurand forms are derived for signals ranging from the simple sinusoids to the more complicated signals produced when modulating interference fringes and stabilized laser cavities.

Presents a review of fluxgate magnetometry based on a survey of the literature and on the work of the author and colleagues. The theory is outlined and an evaluation of the merit of the different types of sensors is made based on the sensor geometry. The design of a second-harmonic feedback magnetometer is discussed with respect to sensor configuration, frequency response, noise, offset, temperature dependence and long-term stability. Examples of the parameters are compiled from the literature and some new results on the long-term stability are presented.

A new calculation following traditional methods is proposed for deducing optical constants and thickness from the fringe pattern of the transmission spectrum of a thin transparent dielectric film surrounded by non-absorbing media. The particular interest of this method, apart from its easiness, is that it makes a directly programmable calculation possible; the accuracy is of the same order as for the iteration method.

The technique of acoustic emission (AE) uses one or more sensors to 'listen' to a wide range of events that may take place inside a solid material. Depending on the source of this high frequency sound, there are broadly three application areas: structural testing and surveillance, process monitoring and control, and materials characterisation. In the first case the source is probably a defect which radiates elastic waves as it grows. Provided these waves are detectable, AE can be used in conjunction with other NDT techniques to assess structural integrity. Advances in deterministic and statistical analysis methods now enable data to be interpreted in greater detail and with more confidence than before. In the second area the acoustic signature of processes is monitored. In the third area, AE is used as an additional diagnostic technique for the study of, for instance, fracture, because it gives unique dynamic information on defect growth.

Dichroism is usually measured using two separate polarisation spectra. However, when the difference in the transmission of oppositely polarised light is very small, the superior method of modulating the polarisation of a single light beam should be used. The techniques involved in this method are reviewed.

The theory of optimal filtering and smoothing of noisy data is presented. Implementation of this theory is made on the AS/7000 computer at Daresbury Laboratory. The Fortran code and examples of application on 'typical' spectroscopic data are given. The routine size is 35 kbytes and the CPU time 0.11 s for 1024 points.

Previous researchers have observed turbulent transport of scalars in liquid jets by fluorescence. However, the technique has not been thoroughly investigated. This work examines a fluorescence technique in detail. Fluorescein sodium in a buffer solution was used in bench tests and in an experimental jet flow. Instrumentation and properties of the dye were considered. The author measured the variation of fluorescence intensity with temperature, pH, concentration and laser intensity. Concentration of reaction product in an acid-base reaction by laser-induced fluorescence is also discussed. The distinction between requirements for qualitative flow visualisation and for quantitative concentration measurements was clarified.

A capacitance micrometer system is described which produces an output voltage linearly proportional to displacement within 1% over a range of 0.35 mm, with a resolution of 4 nm. The electronics employ a linear synchronous detector of new design.

An homogeneous stirred reactor designed for kinetic studies of hydrocarbon oxidation in the intermediate temperature range is described. The originality of this reactor lies in its ability to operate under pressure up to 10 atm ( approximately 1 MPa). The design of the injectors makes it possible to move a thermocouple and a sampling probe throughout a whole diameter of the reactor.

A new method of surface roughness measurement was developed for use in a production environment. This method employs a microcomputer-based vision system to analyse the pattern of scattered light from the surface to derive a roughness parameter. The roughness parameters were obtained for a number of tool-steel samples which were ground to different roughnesses. A correlation curve was established by plotting the roughness parameters against the corresponding average surface roughness readings obtained from a stylus instrument. Similar correlation curves were produced for different materials such as brass and copper. Surface roughness measurement was also performed for specimens immersed in oil, a condition similar to that of a production environment. Some observable trends were found. The proposed method provides a fast and accurate means for measuring surface roughness. Its repeatability and versatility compares favourably with other methods.

A model has been developed, and tested experimentally, to account for the apparent dependence of the effective cross-sectional area of pistons in gas-piston-gauge pressure standards on the particular gas with which the gauge is operated, in the absolute mode. The model treats a pressure drop or loss that depends on the pressure difference across the gauge (P₁-P₂), the fall rate of the piston, the density of the gas and the viscosity of the gas. The model was tested using helium, neon, argon, nitrogen and krypton, and several values of (P₁-P₂). An algorithm was developed for calculating the piston cross-sectional area. The experimental results confirmed the model and resulted in a reduction in measurement uncertainty arising from this effect by approximately one order of magnitude.

The feasibility of using silicon diodes as the temperature sensors for thermoelectric power measurements below room temperature has been established. The advantages and disadvantages of diodes as compared to thermocouples for this purpose are discussed.

The propagation of thermal waves in thin needle-shaped samples is described. The waves are generated by Joule power oscillation at one end of the sample. The amplitude and phase of the temperature oscillation are dependent upon thermal diffusivity. At low frequencies no phase shift appeared and only thermal conductivity was involved in the measurement. Specific heat was evaluated on the basis of thermal diffusivity and thermal conductivity.

A method is described for making LiF constructions involving LiF-LiF and LiF-metal seals which are vacuum tight over a wide temperature range. As an application, rectangular LiF (001) surfaces have been assembled to form a flat channel which is connected to a copper vacuum flange. LiF machining was done using an ultrasonic technique. Mixtures of LiF and PbF₂ were used as sealing agents throughout. The entire construction has been proved vacuum tight in the temperature range 78 K to 700 K.

The author reviews the basic characteristics and progress in transducer properties of the different types of piezoelectric ceramics. The application of these materials in practical ultrasonic transducers is also reported.