Table of contents

Telemetry techniques and developments for the immediate future are reviewed. Application areas considered are primarily industrial and scientific. Available media and communication methods are considered with reference to speed, reliability, practicality and cost. Attention is directed towards the emerging standard for an industrial field bus and the expectations for multiplexed optical systems.

The combined use of optical fibres and light scattering techniques for sensing, in the pursuit of new scientific and industrial instrumentation, is reviewed at an introductory level.

The gravitational constant G can be determined by an electrometric measurement using a group of current-bearing coils. An experiment to test this method has been performed.

A weighing method is described which uses coupling of an electromagnetic balance and the sample with permanent magnets. One magnet is fixed to the balance and attracts a second induced or permanent countermagnet which is attached to the sample and can move freely between two supports in a sealed nonferromagnetic sample cell. At the critical floating distance r_c of the two magnets, the effective mass m*(r_c) indicated by the balance is equal to the desired apparent mass m of the countermagnet. This unstable state of mass equality can be observed precisely by the balance itself, because the countermagnet moves from its support and either rises or falls very rapidly to the other support. The result of these events is an effective mass change from m*(r_c) to m*(r_c+or-d) indicated by the balance (where d is the distance moved by the countermagnet). In contrast with known magnetic coupling methods a stable state of free suspension of the countermagnet is consciously renounced in this simple and cheap weighing method.

Several sources of errors related to the Nutt time-interval digitiser are discussed, including the quantisation process, time jitter, non-linearity of time stretchers and phase noise of the reference clock. A model of the total measurement uncertainty is formulated. The authors also describe the main circuitry and test results of a microprocessor-controlled digitiser which has been developed to achieve a negligible quantisation error (1 ps resolution) and 1 ms range.

Five different digital signal processing techniques for estimating the dispersion properties of plasma or fluid instabilities are discussed. The implementation of each method on a minicomputer is described, and a comparison of their relative merits made by employing data obtained from a steady-state linear quadrupole plasma containment device.

A measuring system was constructed for the first time to evaluate the immediate availability of advanced batteries for an electric energy storage system. It was verified that the measuring system can be employed to determine the transient response of advanced batteries for abrupt load changes. Transient response for the charge-discharge (C/D) equipment is discussed. It was found that the transient response of the C/D equipment depends on the operating voltage and also on the internal resistance of the load (battery): the delay time of the output current was less than 100 ms at an operating voltage of between 12.5-63 V; the rise time of an output current was observed to be between 10 and 150 ms for an internal resistance between 10 and 200 m Omega . Second, the transient response of a 10 kW lead-acid battery is discussed. The rise and fall times of the load current were determined to be within 10 and 1 ms respectively with the use of this system. It was verified that the response of a large-scale secondary battery to abrupt load changes is fast enough to use the battery for an electric energy storage system.

A simple pneumatic sphere grinder is described, based on a commercially available grinding wheel with a circular recess or chamber in it. The sample is ground by propelling it about the chamber with compressed air. The spheres so ground have their magnetic properties measured in a vibrating sample magnetometer. The wall of the chamber is 40 mm thick and will withstand heavy abrasion by large samples of hard, dense magnetic alloy. The grinder will produce a 4.9 mm diameter sphere in 45 min from 1.4 g of alloy with a density of 7400 kg m^-3 and Vickers hardness 600.

A novel microwave structure which allows a plasma column to be sustained through the propagation of an electromagnetic surface wave is investigated. This plasma source is analysed in terms of a simple equivalent circuit which correctly predicts the transfer of power from the microwave generator to the plasma as a function of the tuning system of the launcher; it further shows that such a microwave structure is the waveguide equivalent of the surfatron. This device seems a particularly well adapted substitute for the surfatron at 2.45 GHz (or higher frequencies) when there is a need to operate at high (>500 W) microwave power levels or with large (>or=15 mm) tube diameters.

The authors have employed a transient measurement technique to study the thermal properties of insulators at low temperatures. Results of different analysis procedures disagree significantly unless the calculations include the thermal boundary resistance between the sample and the sample holder. By incorporating these effects, it is shown that the results can be made consistent. The improved model is corroborated by independent measurements of the thermal parameters using a two-heater, steady-state technique.

The NPL digital sampling AC wattmeters, which are used for calibration work, each require the generation of sample timing pulses suitably locked to the frequency of the AC which is to be measured. Repetitive waveforms representing the instantaneous test voltage and test current are digitised at equal intervals of time harmonically related to the period of the waveforms. A new NPL standard wattmeter calibrator (Mk III) incorporates improved sample timing generation. The Mk III uses a crystal digital oscillator to govern both the sample timing and the AC source frequency. This avoids reliance on timing interpolation uniformity within an input period; only digital frequency dividers are needed. Alternatively, commercial frequency synthesizers can be used, with a common master frequency, to give sampling and signal frequencies. Either alternative allows digital phase control, and reduces the use of special-purpose precision analogue electronics.

Hysteresis in the Seebeck coefficients of the thermocouple alloys Nicrosil and Nisil was studied over the temperature range, 200 to 1200 degrees C, for heating periods up to 300 h. The alloys were thermoelements in Nicrosil-sheathed mineral-insulated cable. For both alloys hysteresis peaks were found near 400 and 700 degrees C. In Nicrosil the lower temperature peak was small (<0.05 mu V K^-1) and the peak near 700 degrees C increased rapidly to about 0.3 mu V K^-1) for 50 h of heating and changed little thereafter. Both peaks in Nisil were about 0.2 mu V K^-1 in magnitude after 100 h of heating and increased steadily with time, reaching 0.3 mu V K^-1 at 300 H. The study showed that hysteresis was the main cause of instability in such thermocouples and suggested that hysteresis in Nisil could be greatly reduced during manufacture.

The authors have prepared a system for controlling the discharge of thousands of amperes into a conductive copper coil. The system is specially designed to generate high magnetic fields to be set up for an interval of a few seconds. An experimental prototype has been constructed capable of controlling discharges of 1000 A from a set of lead batteries. The production of the magnetic field was well as the measurement and control is done using a microprocessor. The stability obtained in a magnetic field of 2 T in 10³ mm³ is approximately 0.1%. These results can be extended to higher values of magnetic field, taking into account the limitations derived from the coil resistance and mechanical effects associated with these discharges, as well as the coil geometry. The techniques and material used in the construction of this system are available to any laboratory with a minimum of equipment.

A simple, inexpensive and portable apparatus has been designed for combined optical and ESR studies on electrochemically prepared conducting polymer films. It permits electrochemical studies to be performed as well as transferring samples to other types of measuring devices under controlled atmosphere. The apparatus presented may easily be adapted to other applications.

A feedback scheme is proposed to control the frequency of a light source in order to improve the long-term stability of a fibre-optic probe microphone. The light frequency is controlled so as to retain the time average constant irrespective of the measurand for the intensity change of the reflected light. An experimental model constructed on the proposed principle illustrates the feasibility of an extremely small probe microphone with excellent stability and repeatability as well as with high sensitivity.

The authors describe a device for the continuous measurement of micromolar concentrations of dissolved species in water. The device produces a collimated beam of ultraviolet light that alternates in wavelength over a wide range. When absorbing materials are placed in the beam, the alternation is converted to an amplitude modulation that may be detected by a phototube and lock-in amplifier. This arrangement has a noise level corresponding to an absorbance of 1*10^-4, or about 3*10^-8 mole l^-1 of sulphur(IV) in a 100 mm cell, if the integration time is one second. Some sample studies of the manganese catalysed oxidation of sulphur(IV) to sulphur(VI) are presented. Both the high and low sulphur concentration rate laws were observed for the manganese system with this apparatus. A dependence of the rate constant on the initial sulphur concentration in the 10^-4 to 10^-5 molar regime was also observed.

Industry has developed materials with ultra-low expansion coefficients of, in some cases, below 1*10^-8 degrees C^-1. This has produced a growing requirement for the measurement of thermal expansion and the assessment of the dimensional stability of such materials. A fully automatic absolute dilatometer system with a subnanometre resolution has been developed at the NPL to meet this need. The instrument and its performance are discussed, together with some results that have been obtained for steel, spectrosil and Zerodur.

The time dependence of pulsed-laser enhanced field evaporation is calculated for a variety of laser pulse shapes. It is shown that the decay of the pulse has an important effect on the time resolution obtained in the pulsed-laser atom probe.

An ordinary injection-locking system is modified such that the energy extracted from the line-narrowed oscillator is sufficient to lock a single-pass amplifier. A diffuser plate inserted between the oscillator and the amplifier increases the internal divergence to about 10 mrad. The performance of the system at 248 nm (KrF) is described.

Small amounts of hydrocarbon contaminants leads to a rapid degradation of the output power of UV CuII lasers, which is caused by UV-induced polymerisation of the hydrocarbons at the internal mirror surfaces. To avoid mirror contamination, a laser apparatus was built using ultrahigh vacuum (UHV) technology. The laser power first increases with the operating time, then drops again and stabilises after about 10 h to 60% of the maximum value. This means that the operating period of the mirror is enhanced by a factor of at least 50.

In this instrument, the echoes are added to a continuous wave having a frequency 1.5 times that of the echoes. Changes in ultrasonic velocity result in a change of the added waveform. The relative velocity changes are given by the relative frequency changes required to restore the waveform. A relative accuracy of 2 parts in 10⁶ can be easily achieved. This is an order of a magnitude better than the resolution of the pulse-echo overlap method.

The ultrasonic pulse-echo overlap method is modified to permit automatic recording of the ultrasonic round trip time. A transient recorder is used to capture selected echoes in the echo train. The circuitry used to select the correct trigger pulses for the transient recorder is controlled by the computer. If the transient recorder is triggered with a time interval equal to round trip time or to a fraction of that rate, then there is a perfect correlation between the captured echoes. The value of the correlation is used to generate a control voltage. There is no real limitation on the dynamic range of the system. Results are presented confirming that controllability is within +or-3 ns.

Dropouts can greatly affect the accuracy of flow measurements in the case of low-SNR Doppler signals. The dependence of the measurement accuracy on easily determined dropout parameters has been studied theoretically and experimentally. Based on a Gaussian narrowband representation of the Doppler signal, the dropout process is characterised by two parameters, tracking rate and unlock frequency and modelled as alternating renewal process. It is found that velocity estimates are poorest for unlock frequencies of the order of the signal bandwidth but improve at higher and lower values. Laminar flow experiments indicate that the transitions between lock and unlock are best described by a Poisson process. An application of this finding to the determination of the LDV scattering volume demonstrates that even under adverse tracking conditions useful volume-dimension estimates may be obtained.

A time-of-flight neutron reflectometer constructed for surface and interfacial studies, and installed at the ISIS pulsed neutron source, is described. One of its important design features is its inclined incident beam, since this allows both liquid- and solid-surface phenomena to be investigated. Measurements are presented to show the performance of the instrument, and new representative results, which include studies of liquid surfaces, Langmuir-Blodgett films, and thin-film multilayers, are included as illustrations of the scientific potential of the method.

Two ways of removing the effects of textures in Bragg peak spectra measured by time-of-flight techniques using a pulsed neutron source are described. Such a removal could be useful in the determination of phase-volume fractions in alloys and also in the determination of Debye-Waller factors in textured samples.

A 10-channel counter-timer circuit designed to monitor the rates of particle detectors is presented. It utilises two Am9513 'System Timing Controller' chips together with ancillary logic and is compatible with the IEEE-Std-696/S-100 computer bus. Input signals of up to 6.9 MHz on 10 twisted-pair lines can be counted with 16-bit accuracy. Alternatively, the circuit can be reconfigured by software to form two 80-bit counters. It is also capable of a wide range of time interval measurements, sequencing and digital waveform generation.

A compact microprocessor-based signal averager for processing ion mobility spectrometer (IMS) electrometer signals has been developed. The hardware requirements have been simplified and miniaturised by making use of a dual-ported RAM and an LCD graphics display. The system has proved to function satisfactorily and reliably; it is designed to be integrated into a portable IMS chemical sensor.

A silicon JFET preamplifier for use with SQUIDS and operating at 4.2 K and at 30 MHz has been developed. The silicon junction field effect transistors are Polish BF 245s. These worked satisfactory at 4.2 K. The noise of the cooling preamplifier is about 0.2 nV Hz^-12/. A schematic and a description of the preamplifier are given.

A schematic diagram of a C-V meter for semiconductor detectors of nuclear radiation is proposed. The comparatively simple network ensures high accuracy at a low measuring signal level, good temperature stability, a wide range of measured capacity and stray capacity compensation. High voltage can be applied to the detector and its reverse current can be quite high.

A surface coil for topical ESR spectroscopy is presented. The resonant structure has a toroidal shape cut along a chord to give an elliptical window. Samples placed immediately in front of this window can be observed up to a depth of about 10 mm. The device is useful in all cases in which sample size would make the use of traditional resonant structures impossible. Tests have been made to compare the sensitivity of the present resonator with that of previous designs.