Table of contents

The various factors involved in making measurements of the polarization of weak atomic and molecular emissions in the vacuum ultraviolet are discussed. Materials in current use are evaluated with particular emphasis on the most important sources of systematic error. For measurements of circular polarization these involve possible misalignment of the retarder as well as uncertainty in the phase shift itself. The circular polarization of Lyman alpha is considered as a particular example. For measurements of the linear polarization fraction, the need for selection of devices with high polarizance is stressed, together with care in choice of materials and operating environment to minimize the risk of temporal drift of device characteristics. Errors relating to the actual detection of optical signals are also considered.

A diaphragm-type pressure transducer with a sputtered platinum film strain gauge (sensing film) has been designed and fabricated. The various steps followed to prepare thin film strain gauges on the diaphragm are described. M-bond 450 adhesive (Measurements Group, USA) has been employed as the insulating layer. A detailed procedure to cure this layer is given. A d.c. sputtering method is employed to prepare the platinum films. This paper also includes details of the strain gauge pattern and its location on the diaphragm. A description of the output characteristics and overall behaviour of the platinum thin film pressure transducer is reported.

A new algorithm is introduced to analyse multiexponential transient signals optionally including a non-zero baseline. It requires a digitized transient and allows one to determine time constants and amplitudes of discrete components which are supposed to compose the signal. The algorithm is based on a fit of the signal against its (for more than one component multiple) integral(s). It is tested using simulated transients with white or discretization noise. The results are found to be quite accurate even for noisy signals, and they are compared with those obtained from other fitting methods.

This paper reports on the modification of a stylus instrument for studying dynamic effects in surface finish measurements. A feedback system allows control of the force from freely suspended to 10 mN and of the damping ratio from 0.02 to larger than one. Specimens of mild steel and copper have been traced repeatedly with damping ratios of 0.02 and 0.59 over the traverse speeds of 0.05-1 mm s^-1. From these profiles it is observed that more consistent profile shapes and surface finish parameters are obtained within an optimal damping range. The performance of the system is investigated over a large range of damping ratios by freely suspending the system while applying a force proportional to a previously measured profile. The closest correspondence between the stylus output and the original profile, taken to represent optimal fidelity, occurs with damping ratios in the range of 0.5-0.7. Dynamic forces at the stylus are evaluated by direct measurement using a load cell and by indirect estimation through friction measurements. To compensate the additional mass due to the force actuator and damping sensor, it is demonstrated that the state feedback control can be used to vary the natural resonant frequency and the damping ratio independently.

A thermomechanical analyser with a simple and original detection system is described. The same device is used at the same time to exert a well determined traction on the sample and to measure its lengthening. A weight is hung at a end of a thin wire the other end of which is hooked at a end of the sample. The wire drives a pulley which bears a mirror. The rotation of a reflected laser beam is measured at 1/1000 degrees so that relative length variations of the sample lower than 10^-6 may be detected. The sample is set in a thermally regulated chamber the temperature of which may vary in a controlled way from -180 to 200 degrees C. The performances of the apparatus (sensitivity, accuracy and reproducibility) are presented and discussed for some characteristic examples, in particular on amorphous polymers for the study of which this device is essentially devoted.

Details of a circuit and application of a new simple cold-wire thermometer employing a DC bridge are described. Very high stability and low noise level are special features of the present design. The test current injection method has been improved to obtain more robust operation under strong 60 Hz line noise. The 29.5 kHz bandwidth of the circuit makes it possible to measure temperature fluctuations up to 13.5 kHz, which is the highest frequency obtainable with the smallest cold-wire currently available without signal distortion. The static and dynamic characteristics of the thermometer have been verified in the axisymmetric heated jet experiment.

A Tian-Calvet heat-flux microcalorimeter system is described for measurement of heats of adsorption and reaction on solid surfaces. Heat-flux signals are measured in transducer assemblies consisting of several hundred Seebeck-effect thermoelements connected in series and arranged in a thimble configuration. These transducers surround quartz calorimeter cells connected to a high-vacuum, volumetric adsorption system. The sensitivity of the calorimeter can be calibrated with a Joule-effect device or by measurement of heats of adsorption for known processes. Microcalorimetric measurements have been conducted at temperatures from 300 to 473 K, and operation at temperatures from about 200 to 573 K is possible. Calorimeter cells are described that facilitate sample handling under controlled atmosphere conditions, allowing studies of freshly treated materials and samples that have been exposed to catalytic reaction conditions.

The stability of eleven standard-type rhodium-iron resistance thermometers of Chinese manufacture has been studied while they were exposed to 30 thermal cycles between 293 and 4.2 K. Over these cycles, seven of the eleven thermometers remained stable at 24.5 K within the 0.1 mK precision of the measurements. Two of the thermometers exhibited unstable behaviour at the 10 mK level. The characteristic resistance versus temperature behaviour of, and the self-heating effect in, the other nine thermometers, together with that of two other thermometers of the same type, have been studied in the temperature range 1.3-27 K. The Chinese thermometers appear to be similar in behaviour to the more familiar rhodium-iron thermometers manufactured by H. Tinsley and Co. Ltd.

Maxwell's equations for cylindrical loop-gap resonators used for EPR spectroscopy have been solved with the assumption of the existence of the magnetic z-component only. Resonant structures of the loop-gap resonators with one, two, and four gaps have been investigated. The formulae obtained were used to find the electric and magnetic field distributions numerically. Simple analytical formulae for the magnetic field inhomogeneity were also found and are in good agreement with the experimental observations that the magnetic field homogeneity is improved when the number of gaps is increased. Using the obtained E field distributions a quality factor and signal amplitude for the resonator containing a capillary filled with water were calculated numerically. In spite of neglecting the fringing electric field, the results obtained are in agreement with experiments.

New results on an active compensation technique for improving the performance of a mu -metal magnetically shielded room are presented. This technique is based on measuring the magnetic field inside the room by a SQUID magnetometer. The output of this sensor is used to feed a current through a coil surrounding the room, thus compensating the field detected inside the room. After previous experiments on a single walled room, we applied the technique to a room with two mu -metal walls. At low frequencies a shielding improvement of more than 40 dB was obtained. Furthermore, the contribution of the intrinsic noise of the compensation electronics to the noise present in the room was considered. Also the possibility of using a normal conducting coil outside the room as the sensor in the compensation loop was tested. In the paper, the compensation set-ups are described and experiments are presented and discussed.

To obtain undistorted deuteron solid-state NMR spectra requires extremely short RF pulses which are difficult (sometimes impossible) to achieve. Ways out are composite and shaped pulses or using pulses shorter than 90 degrees. We focus our attention on the latter and show a simple method to avoid phase and amplitude distortions that arise in this case. Our method does not require additional spectrometer hardware, pulse sequences or additional data manipulation. It works well with samples with constant quadrupole coupling constants and seems particularly useful in cases of inhomogeneous RF fields, i.e. when large sample volumes exceed the homogeneity region of the RF coil.

A custom pulse discriminator designed to reduce interchannel crosstalk effects in an inverse photoemission spectrograph is presented. The design and performances are reported. The circuit is based on an IC TTL voltage comparator, the sensitivity of which is extended to nanosecond input pulses by means of a simple additional input stage.

A temperature jump apparatus is described which is based mainly on commercially available components. Temperature jumps of about 2 degrees C in about 150 mu l of aqueous solutions can be produced within 10 mu s by absorption of the infrared radiation of a pulsed atomic iodine laser. The time range available for kinetic experiments is about 30 mu s to 1s. The necessary modifications of the commercial iodine laser are discussed. A prominent feature of the apparatus is the very high sensitivity of the spectrophotometric detection system in the visible spectral range. Signal-to-noise ratios of up to 2*10⁴ are reached at a time resolution of 1 mu s. A method is described to determine the size of the temperature jump of each individual experiment. Thus not only the relaxation times, which are independent of the size of small temperature jumps, can be evaluated but also the corresponding relaxation amplitudes can be determined in a precise manner. Furthermore, an effective method to recycle the laser medium i-C₃F₇I is described.

A new approach to densitometric quantification is described which uses a single photon transmission profile to provide tomographic data in axially symmetric objects. Using phantom tests, the specially developed computer algorithm is shown to perform data transformation with a high degree of accuracy. The main advantage of the novel application compared to the complete tomographic scanning is in its speed. The single profile absorptiometry technique allows the quantification of voidage variations in particulate flows over time resolutions of the order of a few seconds, thus making it especially suitable for investigation of flowing bed porosity in the immediate vicinity of a hopper outlet where the particles are known to accelerate significantly accompanied by significant dilation of the bed voidage. Application to powder flows in hoppers is discussed and a comparison is drawn with complete tomographic scanning.

A personal computer-based system for measuring the distribution of time intervals between pulses is described. The system is distinguished by its simplicity, using only one counter in a configuration that counts and writes 16 time intervals to the memory. The dead time between channels is given by two clock periods (limited by the circuits used to 80 ns) and the maximum number of channels is 4095. Results are presented for the case of recognition of a periodic signal in noise.

A conventional molecular gas far-infrared laser pumped by a low-pressure Q-switched CO₂ laser has been cavity-dumped. The dumping of far-infrared radiation was accomplished by means of an optical semiconductor switch activated by the pulsed output of either a Q-switched or an actively mode-locked Q-switched Nd:YAG laser. As a result, 10 ns FIR pulses have been generated. Dumping efficiency turns out to depend on the speed of the cavity-dump optical switch.

A finite element model of the wire of a hot-wire probe has been used to examine the effect of transverse structural vibrations of the probe wire on the probe response. For the range of Reynolds numbers of interest in laboratory flows, fluctuating velocity spectra correspond to fluctuating excitation force spectra. Hence, the first natural frequency of the wire must be chosen to be outside the range of frequencies of interest in the flow. If this condition cannot be met, then the first and second natural frequencies of the probe wire must be close to suppress insensitivity in the probe response due to excitation of the first mode. Therefore, for small scale turbulence measurements, the selection of probe wire dimensions depends on both the diameter and length of the sensitive wire, and not just on the sensitive length-to-diameter ratio. A recommended selection procedure is given.

A third-order correlation system has been developed, which enables the contrast ratio of high-intensity ultra-short laser pulses to be measured on a single shot, with a dynamic range of better than 10⁸.

We report measurements of dynamic range, gain profile and absolute gain of three second-generation image intensifiers under pulsed illumination. One electrostatically focused (Mullard 1330) and two proximity focused (Varo 9732 and Photek microchannel plate 140) image intensifiers have been characterized. The dynamic ranges were found to be 240, 560, and 400 for the Mullard, Varo, and Photek tubes, respectively, for all gain settings. At a microchannel plate bias voltage of 900 V the radiant gain for these tubes is 4*10³, 3*10⁴, and 2*10³, respectively. The sensitivity and dynamic range of the charge-coupled device camera recording system were found to be superior to that of conventional highspeed film (Kodak TMAX).

A simple frequency modulated continuous wave (FMCW) optical sensor using intensity modulated light, for use in non-contact ranging applications, is discussed. Results over a range of several metres were obtained with an accuracy of a few centimetres.

The application of measuring methods with continuous waves for study of the propagation velocity of fast ultrasonic waves in a liquid-saturated porous material is considered. The technique has recourse to (i) detection of the amplitude of interfered waves and (ii) detection of the phase of the transmitted wave, while the frequency varies linearly with time and the thickness of porous samples is constant. The two procedures permit convenient determination of the presence of dispersion and the velocity of the dispersionless waves. Relatively large amplitudes of excited continuous waves and high degree of accuracy of the methods recommend them as useful tools for study of dynamic properties of saturated porous materials.

This paper describes an iterative numerical calibration method for an orthogonal triple-hot-wire probe and the associated non-iterative data reduction procedure. The performance of the triple-hot-wire probe has been assessed by various verification tests, including comparison tests with an X-wire probe in two-dimensional flow measurements. In order to assess the capability of the triple-hot-wire probe in three-dimensional flow measurements, the angular response of the probe on the mean-flow and turbulence quantity measurements have also been investigated.

A new method using an indigenously developed electronic circuit for identifying all the flow regime transitions in a vertical upward gas-liquid two-phase flow is reported. The circuit basically converts the void fraction signal of the two-phase mixture into a series of square pulses of varying width. The probability density function of the pulse width spectra is analysed for detection of bubble-to-slug flow transition. Slug-to-churn flow transition is detected on the basis of liquid slug length measurements. Churn to annular flow transition is identified on the basis of the number of liquid bridging. Experimental analysis shows that, in bubble flow through narrow tubes, bubbles bigger than a critical size cannot exist without forming Taylor bubbles. The technique has potential to minimize substantially the adverse effects of electrode polarization, stray impedance and varying conductance of the liquid medium. The sampling time and sampling frequency can be varied independently. The circuit can be used for measurement of both conducting and dielectric fluids.

In this work a new reconstruction algorithm for use with oil/gas pipe flow imaging has been developed. Accurate images of representative oil/gas distributions (that is, flow regimes) occurring in flow pipes has been obtained. A capacitance sensor system has been used. In the algorithm the oil/gas distribution has been represented by a set of parameters describing the oil/gas interface. A finite-element-based mathematical simulator of the multi-electrode capacitance sensor system has been developed. The simulator is capable of calculating the capacitances for a set of input parameters (namely for a given oil/gas distribution). The reconstruction algorithm calculates the image by finding the parameters that give fewest discrepancies between calculated and measured capacitances, using an optimization routine. All regimes tested in this work have been successfully reconstructed with the new algorithm, and the reconstructions are compared with images produced by the well-known linear back projection algorithm. The new algorithm has been tested using data from a capacitance imaging system; however, it can in principle be used with other imaging techniques.

In time-of-flight ultrasonic gas flowmeters, the reflection of ultrasonic energy causes significant measurement uncertainties if a fast response to changing gas flow rates is required. The reflections can be attenuated by using a sufficiently high ultrasonic frequency. The direct signal is, however, also attenuated and this also makes accurate measurement difficult. The selection of the best frequency range to be used for various pipe diameters is given and it is shown that, for a large pipe diameter, the choice is very restricted.

A direct reading ionic discharge flowmeter capable of measuring rates of flow of gases from 0.30 mu g s^-1 to 0.20 mg s^-1 is described. The meter uses a differential current and voltage measurement arrangement. The meter functions best at a pressure of 4*10² Pa and has application for use with dry non-explosive gases. A semi-empirical model is devised to explain the theory of the instrument.

Details of a personal computer-based data acquisition system, which allows monitoring of large-diameter plasma source ion current profiles and Langmuir probe data, using an interchangeable rotatable, Faraday cup array and Langmuir probe array respectively are described. The arrays span the plasma diameter and can be rotated under computer control. Software-selectable monitor modes allow investigation of the angular and temporal dependence of ion current profiles, basic plasma parameters and their associated non-uniformities. Isolated aluminium plates can be fixed to the respective arrays to investigate the intrusive influence of a substrate holder and substrate bias on plasma parameters. Angular dependences is of ion current non-uniformity values of a large-diameter permanent magnet electron cyclotron resonance source are reported to demonstrate the benefits of the system over a single-probe/cup system.

The determination of the Fermi level in semiconductors is not always easy by the usual methods, especially when one is dealing with polycrystalline materials. A new method is presented based on a spectroscopic study of the current transients generated by the filling of the traps present in the material. Application of this technique has determined the Fermi level position in polycrystalline p-CdTe thin films deposited on n-CdS layers by the isothermal close-spaced vapour transport method. The result (E_v+0.15+or-0.03 eV) agrees well with the value estimated from C-V measurements.

An electronic nose is described, which consists of a gas sensor array combined with a pattern recognition routine. The sensor array used consists of ten metal-oxide-semiconductor field effect transistors with gates of catalytically active metals. It also contains four commercially available chemical sensors based on tin dioxide, so-called Taguchi sensors. In some studies, a carbon dioxide monitor based on infrared absorption is also used. Samples of ground beef and pork, stored in a refrigerator, have been studied. Gas samples from the meat were then led to the sensor array, and the resulting patterns of sensor signals were treated with pattern recognition software based on an artificial neural network as well as with an algorithm based on an abductory induction mechanism. When using all sensors for learning, the two nets could predict both type of meat and storage time quite well. Omitting the carbon dioxide monitor, both nets could predict type of meat, but storage time not so well. Finally, it is also shown how a net based on unsupervised training could be used to predict storage time for ground beef.

An optical method based on birefringence is described, which can be used to monitor the longitudinal variation in the diameter and nature of the cross section of uniaxial crystalline fibres produced by the laser-heated miniature pedestal growth process. Modification of the technique enables the crystallographic direction to be confirmed relative to the fibre's axis of symmetry.

This note describes the design of a programmable digital delay. An analogue signal (0-30 kHz) is digitized by a sample-and-hold plus 14-bit analogue-to-digital converter and the resulting digital data is stored in digital random access memory whose address is supplied by the parallel output of programmable binary down counters. The stored digital samples are reconstructed (after a programmable delay) in analogue form by a digital-to-analogue converter, so that the reconstructed signal is a time-delayed version of the original analogue input signal. The amount of memory used, as well as the sampling rate, determines the maximum possible delay.

High-temperature material processing is used for a wide variety of applications. These processes often require a specific temporal temperature profile. This design note describes an inexpensive programmable temperature profile controller. The proposed controller is based on an 8051 microprocessor.

A three-electrode cell system is described which incorporates a cell holder assembly, a lithium ribbon punch, an electrode insertion tool and a sealed container. The cell design is such that the current collectors for the lithium electrodes do not come into contact with the electrolyte. Use of the system enables cells for studying reversible cathodes to be reliably assembled in the glove box and then removed for extended cycle testing.

The design of a novel capacitance displacement transducer is described. A voltage-to-frequency converter is used as a voltage supply to a resonant series LC circuit. The in-phase component of the voltage across the capacitor is detected using a phase-sensitive detector and the output is fed back to the frequency-to-voltage converter so that resonance is maintained as the capacitance varies. The capacitance is then read out in terms of frequency by a computer. The system is simple, cheap and robust. We have achieved a dynamic range of three decades from 10 pF, frequency read-out resolution of 1 ppm and linearity of 0.2%.

Design equations are given for second-order space focusing in two-field time-of-flight mass spectrometers. Simulations and experimental tests confirm that, for diffuse sources and high-energy fragments, second-order designs give superior resolution and linearity of flight time deviations to initial ion velocities.

It is shown that the efficiency of metal-semiconductor-metal (MSM) detectors can be improved using a dielectric diffraction grating, positioned appropriately above the detector. The grating has the effect that the light is channelled into the grooves between the metallic electrodes. In this way the efficiency is increased relative to a conventional MSM detector.