Table of contents

The following topics are dealt with: historic overview; user needs and markets; molecular recognition; signal transduction; potentiometric measurement; amperometric measurements; conductance-capacitance measurements; calorimetric measurements; optical measurements; mass sensitive measurements; and biosensor applications including clinical diagnostics, industrial process control, environmental applications and food industry. Future trends are also indicated.

Consideration is given to a number of aspects concerning the design and implementation of a simultaneous multi-point laser Doppler anemometry (LDA) system. The difficulties of data collection and processing for many channels is seen to be the principle limiting factor to system development, and a practical route making use of highspeed components for fast LDA signal sampling and storage, followed by fast Fourier transform (FFT) signal analysis in software is explored. A demonstration system is constructed consisting of ten optical channels arranged in a linear array, a single LDA sampling card and a controlling microcomputer, and the arrangement is applied to a closed water circuit. Received signals are compared with software-generated equivalents and the effect of various signal-to-noise ratios on the accuracy of a reconstructed velocity distribution is examined. The overall configuration is shown to provide a modular approach enabling a piecewise increase in the number of processing channels according to requirements.

Total electron yield (TEY) in He gas at atmospheric pressure is a data acquisition technique particularly well suited for X-ray spectroscopy measurements performed on coatings. We have developed a TEY detector which can operate close to liquid nitrogen temperature. The novelty of the design consists of a dissociation between the cooling body and the electronic measurement system: the electrically biased collector and the grounded sample are both mounted on a sample holder which is introduced under a permanent He gas flow in the cryostat. Measurements performed on standard pure materials allow investigation of the EXAFs amplitude improvements corresponding to the lowering of the temperature. We also report signal variations observed on thin nanocrystalline films and amorphous alloys.

An automated constant-volume dilatometer based on a null-measurement approach to the measurement of reaction extent, and hence reaction rate, in chemical systems is reported and described. In this device, liquid is added to or removed from the dilatometer in order to maintain constant volume within the dilatometer. The sensitivity of the device is between 0.1 and 1% of the volume change that would accompany the reaction in a constant-mass, constant-temperature system. The device is readily adapted to remote measurement. We suggest that it has potential for widespread use in chemical kinetics although the applications described here relate to polymerization from solution.

Optical tomography using interferometric data is applied in this study to measure three-dimensional flow and heat transfer phenomena in a differentially heated cubic enclosure. The interferometric recording and reconstruction system, associated fringe readout and tomographic reconstruction procedures, and the results from this analysis are described, highlighting the specific challenges facing application of the technique to enclosure flows. The features unique to the investigation of enclosure flows using interferometric tomography include limited view angle, refraction effects and the lack of a visible reference fringe. The experimental results are verified by comparison with independent temperature measurements. The comparisons indicate that this experimental technique can yield three-dimensional perspectives of complex flows having sufficient spatial resolution to verify detailed three-dimensional direct numerical simulations.

A digital synchronous filter for rejecting periodic noise such as that arising from the mains power supply is described. With a stationary noise signal the filter achieves a rejection ratio of 127 dB. With the somewhat variable mains, rejection ratios obtained in practice are typically 60 dB or better. The rejection notches are very narrow and the passband response is flat, leaving the signal of interest essentially undistorted. The performance of a hardware implementation of the filter is examined in detail and alternative implementations are discussed.

A new type of high-sensitivity vibrating-sample magnetometer (VSM) useful for measurements of soft and hard magnetic materials is described. This instrument differs from the Foner type in the vibration system, which is a bar vibrating at its resonance frequency. Additionally, the electronic control preserves a constant vibration amplitude and versions with parallel and/or perpendicular motion can be designed. Magnetic moments as small as 5*10^-8 Am² have been detected at room temperature and the overall accuracy is better than 0.5% using the simple prototype presented here. An instrument with this system is easy to construct, relatively inexpensive and, due to its versatility, it could be a useful tool for research, industry and teaching purposes.

Simple experiments are described which can be interpreted in such a way as to allow the identification of wavelength, frequency and head-to-tape interface effects in the replay process in digital magnetic recording. Increased tape speeds have been shown to result in increased head-to-tape spacings on replay in a way which directly relates to output voltage levels.

A prototype noncontact high-bandwidth optoelectronic probe for surface topology and laser materials processing stand-off control is presented. Radiation from a low-power laser beam is focused onto a workpiece surface; the spatial variation in the reflected irradiance is modelled. A Gaussian beam and radiometric system model are investigated and compared with experimental results. The experimental results lie between the Gaussian and radiometric simulations which appear to be limiting cases. The system facilitates a measuring range of +or-6 mm. A predicted performance is reported for the device. A set of experiments are performed, results are analysed and compared with the mathematical models. The approximate range of instrument linearity is +or-1 mm for the 75 mm focal length main lens and +or-2 mm for the 120 mm lens; in this linear range the optimal accuracy resolution is 1 mu m.

A novel type of interferometric fibre optic sensor is described, based on a curved section of a buffered single-mode optical fibre. Interference occurs between the core-guided mode and the 'whispering gallery' mode guided within the fibre buffer. The fibre buffer coating thus serves as the sensing element. The technique is illustrated by its application to temperature measurement. A linear output is produced by using a two-wavelength phase-stepping technique.

A new version of the linear critical-angle refractometer for liquids is described: a 1728 element linear CCD array is used to scan the intensity profile of the output image, within which the position of a cut-off edge is a linear function of refractive index. The precision in locating this cut-off edge is greatly increased, especially for milky liquids, by differentiating the intensity profile numerically. Further information about complex particulate liquids is found to be given by the shape of the differentiated profile, and alternative internal and external illumination modes are provided which may give more information. The present instrument covers the refractive index range 1.3000 to 1.4700, with an accuracy of +or-0.0001 for transparent liquids. The light source is an LED at 635 nm. The intensity profile, the differentiated intensity profile, and the refractive index value are all computed, displayed and printed by a desk-top computer.

A novel near ultraviolet (UV) gas-filled detector which is operated in the prebreakdown regime is described. The detection mechanism is photoionization of the excited state. Modulated continuous UV light was used to study experimentally the detector responsivity and the spectral response for typical electrode materials such as Mo, Ti, W and Ta at around 200 nm wavelength. Of the materials investigated, molybdenum yielded the highest relative responsivity. This is attributed to the secondary electron emission and, in particular, to the initial electron energy after secondary emission, which contributes to internal signal gain and to improved quantum efficiency through increased excitation. For molybdenum electrodes, the detector performance was studied with various electrode geometries. Of the electrode configurations considered, two straight parallel wires which are located parallel to the incoming UV photon direction yields the best responsivity. This is attributed to maximum electric field nonuniformity and photon interaction depth between the electrodes. Effective quantum efficiency, including detector internal gain, is found to be about 5570% per photon at 200 nm wavelength. This leads to an effective yield of 9 A W^-1, including internal gain, at 200 nm wavelength. The noise equivalent power NEP is estimated to be approximately 10^-15 W Hz^-12/.

A pulsed low-energy positron system for positron-lifetime spectroscopy has been up-graded and modified with regard to its three main components. A new moderator preparation chamber has been added, the radiofrequency pulsing concept has been modified using a pre-buncher and a pre-chopper in front of the existing chopper-buncher section. Furthermore, a new target station allowing measurements at variable temperatures has been incorporated. The lifetime spectra now reveal a strongly increased ratio of peak to background and of peak to satellite peak.

The first results using a scanning X-ray microscope, with zone plate optics, specifically designed for use with a laser-plasma X-ray source are reported. Unlike previous microscopes of this type, in which the specimens were mechanically scanned, in the present arrangement the specimen was kept stationary and the source was moved, the zone plate demagnifying this movement on the specimen. Images of test specimens show an edge resolution of 0.35+or-0.14 mu m, apparently limited by noise.

An inexpensive, high repetition rate (1 to 100 kHz), thyratron driver has been developed which employs a single solid state switch. The driver delivers an unloaded 1.5 kV positive trigger pulse superimposed onto a negative 300 V recovery voltage to the thyratron trigger grid. The voltage rate of rise (>10 kV mu s^-1) results in low-trigger jitter (less than the 5 ns resolution of the diagnostic apparatus). The driver has been used to trigger a thyratron in the excitation circuit of a copper hybrid laser at a pulse recurrence frequency of 70 kHz.

The opportunities for temperature measurement by a high-speed high-spatial-resolution pyrometer system in a wide range of laser applications are illustrated by heat treatment, welding, cladding and alloying. Continuous CO₂ and pulsed Nd:YAG laser machining are considered. The influence of such factors as beam traverse speed, laser power (or pulse energy), slab thickness, melt drop expulsion and distance from the seam centre on temperature evolution is analysed. The experimental results are analysed by numerical simulation.

A mathematical model has been developed to predict the void fraction in a stratified gas-liquid system from the resistance value measured using a conductance probe. A finite-difference technique has been used for solving the electrostatic field equations. A simple probe configuration and measuring circuit have been designed to verify the performance of the model experimentally.

A test set-up was constructed for Hall measurements at temperatures up to 1100 degrees C in freely selectable gas atmospheres to characterize gas-sensitive semiconducting metal oxides. A technique based on conventional specimen geometry is used. The specimen current is low-frequency AC, that allows the transverse voltage to be measured with low noise by the lock-in technique. A magnetic field is controlled between 0 T and +or-1 T or alternates between these values and the change in transverse voltage in consonance with the magnetic field change is evaluated. A cylindrical miniaturized high-temperature measurement cell with an external diameter of 5 cm was constructed to satisfy the measuring conditions. Temperatures up to 1100 degrees C are generated by an electric heating coil, a constant temperature being ensured by a water cooler acting as a defined heat sink. The air tightness of the measurement chamber is achieved via a quartz glass cell. Initial test measurements were performed with Pt thin films and with Ga₂O₃ and SrTiO₃ monocrystals.

A new spectroscopic technique is presented which combines infrared Fourier transform spectroscopy and contactless microwave conductivity measurements. Its performance is demonstrated by observing shallow donor transitions in silicon. The experimental set-up uses a TE₁₁₁ cavity with two hole-coupled waveguides. The effect of the infrared radiation (input through a third hole) is to change the conductivity of the sample causing a change in Q of the cavity and therefore of the transmitted microwave power.

This article describes a new instrument to image the local capacitance (or permittivity) and conductance (or conductivity) of colloidal particles and membranes in an aqueous environment. Capacitance and conductance are measured by the three-terminal (guarded electrode) method with a coaxial probe electrode, which is laterally scanned over samples on a plate electrode. The images of capacitance and conductance are obtained at frequencies between 1 kHz and 10 MHz, which enables the study of dielectric relaxation of individual particles and local areas of membranes.

A novel multichannel integrating nephelometer (MIN) is described which monitors the aerosol scattering coefficient in real-time for different aerosol size fractions. A May cascade impactor (1975) fractionates the aerosol into eight specific size ranges, of which the latter five are monitored by five helium-neon laser beams in the geometry of a reciprocal integrating nephelometer. The May impactor stage fractions are more limited than the MIN size ranges which are accumulative. However, after subtraction, the MIN scattering coefficients and the aerosol mass on the impactor stages are for similar size fractions and should therefore be directly correlated. Scattering coefficients in the range 2*10^-6-1*10^-4 m^-1 may be determined by a primary calibration while values up to 1*10^-2 m^-1 may be determined by a secondary calibration using a NaCl aerosol.

The production of a fluid jet with a thickness variable in the range of 5-20 mu m is described. In the investigated arrangement a liquid jet flowing out of a cylindrical nozzle hits a plexiglass plate near its edge and spreads forming a very thin sheet. The thickness can be easily varied by changing the angle between the nozzle and the plate. Using a dye solution in this set-up, a short-cavity dye laser of length 15 mu m has been constructed.

An angular and acceleration measurement system using the piezoelectric polymer poly (vinylidene fluoride) is described, furthermore a compensation of the polymer's pyroelectric effects has been achieved. The advantages of this sensor are that there are no moving parts and that it has a very low cost compared with other sensors. The measured sensitivity is 3.7 mV rad^-1 s².

A magneto-optical tachometer specially designed for an automotive anti-lock braking system is described. It employs a plastic optical fibre for the bidirectional data transmission and its transducing mechanism is magneto-optical. It uses a Bi-substituted iron garnet as sensor element and a ferromagnetic tooth gear, which produces 46 pulses per turn. Owing to its compact design and its constitutive materials it is suitable for withstanding all the environmental hazards present in the car. The tachometer can measure car speeds of 0-400 km h^-1 with a signal variation less than 0.6 dB throughout the whole range.