Table of contents

Consideration is given to the concept of the 'intelligent transducer' with particular reference to the expected capabilities of an instrument incorporating local signal processing. The implications of devolving the tasks of instrument management and data transmission to separate processing elements are explored. The use of communication protocols within instrumentation systems is discussed both in the context of techniques currently in use and proposed future standards. A three-layer instrumentation protocol model consisting of physical link, transport mechanism and application data layers is defined together with a consideration of the individual layers. The networking of power and data within distributed instrumentation systems is considered with a view to the commercial implications of the multiplicity of techniques both existing and proposed.

The development of an automatic adapted Wheatstone bridge arrangement for the measurement of high value resistance standards between 10⁹ and 10¹⁴ Omega at voltages up to 1 kV is described. This uses stable voltage reference supplies in place of the resistors in the reference arms of the bridge. The system has been implemented at the National Physical Laboratory to replace the previous manual system without impairing the uncertainty of the measurements.

A method is proposed for predicting the value of nonlinear corrections in electro-optical distance measurement (EODM) and also predicting the return signal amplitude of the EODM instrument. The method includes both IR source and IR detector inhomogeneities. Exemplifying results, obtained by a computer simulation process, are provided and capabilities of the method are discussed.

A new method of measuring magnetostriction is described. The method employs a fibre-optic dilatometer in which the changes in sample length vary the optical power transmitted across a narrow gap in the optical fibre. The method is especially suitable for making routine measurements on samples in the form of thin ribbons, as only the minimum mechanical load is imposed on the sample; nor is it necessary to attach any ancillary components to the sample. Illustrative results indicate a sensitivity of about 10^-7 when measuring magnetostriction as a function of applied field. For measurements of saturation magnetostriction only, the sensitivity is about 10^-8.

A design is given for a low-noise limited oscillator whose inherent sensitivity is then sacrificed for the practical convenience of having remotely controlled varicap diode tuning and small diameter probes. A series of seven probes, each tunable over more than an octave, covers 1.36 to 60 MHz in overlapping ranges.

The design and application of an inexpensive logging multichannel averager is described. The device records the presence or absence of events within 4096 consecutive 25 ns intervals, here called channels, and uses channel by channel summation to construct a temporal profile. The device operates under the software control of a personal computer and its use in the study of decaying plasmas is discussed.

The authors report the design and construction of a very simple vibrating reed apparatus with automatic frequency locking capability where the resonance frequency and the internal friction can be recorded continuously as a function of temperature. The apparatus is particularly suitable for studies down to liquid helium temperatures or below.

The plane heat pulse technique of Pierrus and Sigalas (1985) has been generalised to configurations where the sample stack is made out of two materials. At least one of the sample slabs that make up the sample stack is made out of standard, easy to handle material of known thermal conductivity and diffusivity. The plane heater is attached to this slab. Another slab is made out of the material under investigation. This configuration eliminates the need to attach a heater to the face of a slab. made out of a poisonous material or of a material that is chemically unstable in air. The theory necessary for performing the experiments is presented. Subsequently it was tested on cells made out of NaCl-NaBr and NaCl-NaI and proven to work.

Describes an update of a earlier apparatus which measures the critical current of superconducting wires and tapes as a function of magnetic field and temperature. The critical element is a compact probe of 25 mm diameter which can be inserted into the bore of one of the several high-field magnets. The design and critical features of the probe which incorporates high-resolution thermometry and accurate temperature control in the demanding environment of high sample currents and large magnetic fields, are discussed. Data obtained on a sample of multifilamentary V₃Ga wireare presented as an illustration of the probe's capabilities. Analytical modelling and consistency measurements suggest that the accuracy in the critical current data presented can be given by an uncertainty in temperature of +or-0.2 K for 4.2 K<T<or=T_c and +or-0.05 K for T<or=4.2 K.

A novel electromagnetically-driven switch is described, which is suited to the interruption of large, low-voltage DC currents, and which may also find applications in the switching of AC currents at current zero.

A method for measuring both the real and imaginary parts of complex shear-wave impedance Z_s* for low viscosity liquids using a planar AT-quartz resonator (thickness shear) in an RF bridge network between 2 and 34 MHz, is described. This technique employs a PLL circuit and permits continuous quantitative recording of the sample impedance (probe volume below 1 ml). The mechanical vibration amplitude at the transducer-liquid interface is in the Angstrom range, preventing any heating and structural damage effects. Incremental Z_s-resolution is better than 50 kg m^-2 s^-1 ( approximately 1% of water impedance at 6 MHz), as is demonstrated by test measurements on different newtonian liquids. These measurements show that Z_s* values determine quantitatively the shear quartz resonator parameters. This method permits measurements of interface and surface layer properties.

The SIRO₂ oxygen sensor comprises a solid electrolyte pellet or disc sealed to an alumina tube. It is particularly suitable for demanding in situ industrial applications, examples of which are discussed. With novel electrodes, operation is possible at temperatures as low as 420 degrees C.

An automatic apparatus has been set up for electrical profile measurements. After measuring sheet resistance and Hall voltage before and after the stripping of silicon by anodic oxidation, the authors calculate the concentration and the mobility of free carriers using a microcomputer that also enables the automation of the process. Comparison of the results with other methods is also presented.

By making use of a bridge circuit initially proposed by A. Serner (1937) for the measurement of impedances at low frequencies and the properties of transmission lines, a crossed coils NMR probe was built for 100 MHz with all its controls separated from the probe itself.

A short description is given of a homodyne X-band ESR spectrometer and its operating mode for electronic spin-lattice relaxation time measurements at low temperatures. The spin-lattice relaxation times are determined using a modified saturation-recovery method. With this method the sensitivity of the apparatus is about 10¹⁶ spins, similar to that obtained using superheterodyne detection.

A design for a novel fibre-optic pH sensor with a response time of a few seconds is discussed in detail. The fabrication and properties are described of a sensing material, which comprises an ion exchange membrane in which is bound a sulphophthalein dye. The properties of the fibre-optic component, a coaxial directional coupler which senses changes in the refractive index of the polymer, are also presented. Measurements on a combination of the two are analysed to assess the practicality of a sensor capable of discriminating against interfering index variations.

Two branching-ratio intensity calibration systems for VUV spectrometers are described. Both of them allow calibration at a single wavelength with a single plasma discharge.

A conventional TEM was extended to a time-resolving instrument by installing a pulsed microchannel plate as a low-cost time-resolving image detector and a pulsed electron beam shutter. The technique provides electron micrographs and diffraction pictures of non-recurring events on the nanosecond/micrometre scale within a commercial TEM.

A two-cell laser system is reported which allows the modified Hansch-type dye stage to operate at two widely separated wavelengths. Broadband tunability is only limited by the potential incompatibility of the absorption bands of the two organic dyes employed.

The technique of ultrasonic metering of flow velocity is rapidly progressing in various aspects of its scientific and technical applications. The accuracy of these measurements is very high, but not as high as could be desired. The author presents some results of a search for sources of error. The simplest case is analysed; uniform flow of a homogeneous fluid measured with small ultrasonic probes. The analysis proceeds with a decomposition of the sound path into two components, which are named 'carry-off' and 'vibrational'. An exact formula is derived, which resembles the well known Lorentz transformation formula.

The temperature compensation of a thermistor flowmeter is accomplished digitally, thus simplifying the compensation problem by replacing required hardware with software. The resistance-temperature curve of the thermistor is a least-squares fit to a logarithmic function whose coefficients are stored in computer memory. Signals representing the temperature of the heated thermistor and the ambient temperature are digitised and converted to temperature values based on the stored resistance-temperature curve. In this case the signals are obtained from a single thermistor switched between the high and low overheat modes. The thermistor temperature T, the ambient temperature T_a, and the electrical power input are used to form the dissipation factor (K=P/(T-T_a)) which calibrates the flowmeter when plotted against velocity. Results from calibration experiments on a thermistor flowmeter designed for use in human coronary arteries are presented to demonstrate the viability of the method.

Instantaneous local temperatures in fluids are often measured by passing a small constant current through a fine metallic cold wire. A new circuit for supplying current to such wires is described in detail. In addition to ease of construction and operation, the circuit features wide bandwidth and very low temperature drift. Drift performance is compared with one other available circuit, and measurements taken in a heated, laminar, oscillatory pipe flow are presented as an application.

A novel type of temperature control system for liquid-crystal cells is presented. The basic principle involves the use of commercially available indium-tin-oxide (ITO)-coated glass substrates which serve as the heat source and the temperature-measurement resistor. Compared with conventional heating cells, the system offers compactness and very fast thermal response times, together with good temperature stability and large apertures which are necessary for optical set-ups.

A new technique for the production of rapidly solidified ribbons is presented: centrifuge melt spinning (CMS). CMS improves conventional melt spinning by reaching cooling rates higher than 10⁶ K s^-1, the highest value reported for already existing melt spinning machines. A rotating crucible which contains a molten alloy is employed. Due to the centrifugal force, the alloy is expelled from the crucible to solidify onto the surface of a rotating substrate. The cooling substrate is a copper rim which rotates in the opposite direction. As the alloy stream impacts on the rim surface, a molten puddle is formed, from which the rapidly solidified ribbon is extracted, as a result of the counter-rotation of the crucible and the substrate. CMS reaches cooling rates as high as 10⁸ K s^-1, due to improved thermal contact of the molten alloy puddle and the cooling substrate, and to a beneficial combination of crucible and substrate velocities, thus yielding high extraction velocities of the ribbons from the melt.

The construction is described of a three-electrode dielectric sample holder incorporating a BNC plug as a design aid and it is shown that fringe field effects can be made negligible.

The best near-infrared detectors produce frequent and massive noise pulses due to their sensitivity to cosmic rays. Algorithmic signal processing techniques which eliminate the pulses are described.

A double-layer scintillator detector has been designed for the detection of beta particles with a minimal response to gamma radiation, for use in thickness measurements. Measurements indicate that the use of the new detector in a ⁹⁰Sr beta -particle thickness gauge allows the limit of measurable thickness to be raised by approximately 25% over the corresponding thickness using the conventional discriminator method.

Several methods of measuring vibrations in monolayer films were used to evaluate the effectiveness of various designs of vibration isolation tables. Both a recording seismograph and a Langmuir film balance interfaced to a microcomputer (IBM-PC) were used to monitor vibrations in the film for each of the designs which were studied. A correlation between the film balance measurements and the seismographic results was shown. It was concluded that the film balance itself can be used as a sensitive indicator of environmental vibrations in monolayer films. A preferred method of vibration isolation was determined from these results.