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.

By reconsidering the basic principles of capacitance, and also clarifying some of the misconceptions about the principles of guarding, which were correctly described by Kelvin more than a century ago, it is shown how guarded multi terminal capacitor geometries can be designed, analytically precalculated and correctly measured using simplified transformer bridge systems with phase-sensitive detection. An overview is given of the archetypal multi terminal capacitor configurations as well as ways to modify them for practical sensor designs. Detrimental side-effects are discussed and rules for practical designs are given. The possibilities for combining bridge circuits with simultaneous and sequential measurements of complex sensors are described. Expectations for future developments, partly based on silicon microfabrication techniques, are discussed and a comparison of the major aspects of capacitive sensors with resistive strain gauges and inductive sensors is given.

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.

A balance for measuring magnetic susceptibilities at room temperature is described. Two small permanent magnets are supported by a suspension strip. The force that the sample exerts on one of the magnets is balanced by a current through a coil placed between the poles of the other magnet. The apparatus can be used in either the Gouy or the Faraday mode, and a force measurement takes approximately 15 s For diamagnetic liquids an accuracy of approximately 1% is attained (80 mu l required) while for 5-10mg of paramagnetic solid the accuracy is about 2%.

Describes the characteristics of three types of sensors currently in wide use: platinum resistance thermometers, thermistors and thermocouples. It then explains how the transfer characteristics of each sensor can be established using readily available temperature standards. The article then shows how these characteristics can be used to design signal conditioning circuits which convert the sensor output into a convenient voltage signal. Bridge circuits are necessary for the resistance thermometer and thermistor. It is possible to design a bridge with a nonlinear transfer characteristic which almost compensates for that of the thermistor. The design of an automatic reference junction circuit for a thermocouple is discussed. The article concludes by describing how a microcomputer can be incorporated into these systems as a signal processing element to improve accuracy. In the case of the thermocouple, the computer solves a quartic equation to arrive at an improved estimate of temperature.

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 review of attempts which have been made to produce in vivo images of the spatial distribution of tissue resistivity is given. The collection systems for in vivo images and the methods for the reconstruction of such images are discussed. Some of the problems that remain to be solved for applied tomography are identified.

In determinations of radiative lifetimes in which time intervals are measured up to the first detected photon, an error is introduced when subsequent photons are neglected. An accurate correction for this effect is described, having the advantages of general applicability and allowing the use of high photon detection rates. It is also shown that the error introduced by the use of this correction is negligible under all practical experimental conditions.

Control systems are becoming increasingly dependent on digital processing and so require sensors able to provide direct digital inputs. Resonator sensors, configured to have a mechanical resonance frequency or relative phase of oscillation dependent on the measured parameter, are a subject of considerable practical interest. The author reviews the wide range of devices which have been proposed and developed, including sensors for liquid or gas density and viscosity, liquid level, mass and mechanical force, and fluid flow rates. Techniques cover the frequency range from audio, for sensors based on vibrating vanes or tubes, to 100 MHz or more for surface acoustic wave devices. Resonator sensors based on single crystal materials such as quartz and silicon are of particular interest because they combine high accuracy and repeatability and low cost of manufacture and potentially very low power consumption.

A mechanism for the 'charging' artifacts which obscure detail on SEM images of high resistivity specimens is described. An interaction equation, relating the properties of specimen and environment, predicts negative charge build-up and large potential fields. Charging effects are traced to the specular deflection of beam electrons in these fields, and experiments with uncoated polymer fibres are described.

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.

We review the imaging properties and electrical applications o f scanning optical microscopes. We show that the choice of a scanning approach allows the modification of the optical system to give differential phase contrast imaging and confocal imaging. The latter has unique properties which permit the high resolution imaging and metrology of thick device structures. We also discuss the optical beam-induced current method of device, VLSI circuit and material testing and consider the factors affecting its resolution.