Table of contents

A simple liquid nitrogen pump is described together with its application to sub-ambient temperature controlled experiments. Typical control in a real system was +or-1 degrees C in the range -90 degrees C to +30 degrees C; temperature profile systems are also described.

A technique for focusing a cold flashlamp prior to applying high voltage (HV) is described.

The problems associated with the use of solid state sensors in digital systems are reviewed and some solutions are offered which are intended to aid progress towards a methodology for the design and development of 'intelligent' sensor subsystems. The combination of silicon-based sensors with on-chip electronics, together with the processing power of modern microprocessors, offers great improvements in techniques of compensation, signal processing and data communications. As a result radical changes in the exploitation of sensing mechanisms are forecast.

The time required to exchange specimens in a UHV system can be reduced considerably by a sample airlock. A low-cost sample exchange system is described which was originally constructed for an Auger spectrometer, but may be applied to other UHV systems as well. The essential parts are a magnetically coupled transfer rod, a magazine for the simultaneous transport of six samples, a bayonet clip to fix the samples sequentially to a carousel sample holder and a tilting mechanism in the specimen stage to bring the transferred samples into an optimum position for the further experiment. The system can be adapted to the multiple sample transport between two completely separated UHV systems.

A four-probe electrometer system with driven shields is designed for measurements of resistance up to 10¹¹ Omega with a response time of the order of 1 ms. The system maintains the voltage drop across the measured resistance at a constant value for resistivity changes of four orders of magnitude.

Nonlinear effects due to the prong-wire thermal interaction effect are studied. The influence on the measured mean temperature and on the skewness is discussed.

A lock-in amplifier for temperature control in a Spacelab experiment is described. It is lightweight and compact. Its simple design utilises commonly available integrated circuits and requires a minimum of discrete parts. Using an AC Wheatstone bridge and thermistors for sensing elements it is capable of detecting temperature variations of +or-5 mu K. When incorporated into a controller it can keep a temperature constant with a +or-40 mu K range for a period of several months.

An apparatus and method for thin-film thickness measurements using the multiple-beam interferometric technique of Fizeau fringes are described. The apparatus is simple in construction and to use, inexpensive and portable. It can be used in the range 30-350 nm with an accuracy of 2 nm for film thickness below 100 nm rising for 350 nm films.

A solid-state humidity sensor based on the use of a hygroscopic aluminium oxide dielectric has been developed in two basic forms: an MIS (metal-insulator-silicon) capacitor and an MIS field-effect transistor. Anodisation of aluminium is used for the dielectric. Fabrication methods and results for the capacitor devices are reported. Response is typically within 12 s with +or-5% hysteresis. Batch reproducibility is generally within 5%.

Apparatus has been developed which enables absolute measurements of the heat capacity of samples of amorphous metals with a mass of 0.5 to 1 g to be made between liquid helium and liquid nitrogen temperatures. Between 1.5 and 10K the traditional pulse heating technique is used, whilst above 10K a variation of the adiabatic continuous warming technique yields more accurate results. Data collection and control of the equipment is performed by a microcomputer. Results obtained with a 3 g sample of high purity copper agree with accepted values to within +or-1% over the entire temperature range. The apparatus and technique could be useful whenever only small samples are available.

A simple method for wattmeter calibration, good also in the case of quadrature between current and voltage, is described. Essentially a mutual inductor is loaded with a non-inductive resistor. First the value of the power loss inside the resistance which loads the secondary of the mutual inductor is measured, and then, by calculation, the value which should be read on the wattmeter is obtained. In a practical case, using instruments with an accuracy of about 0.1% to measure the circuit parameters, in a range of frequencies from 1 to 400 Hz and phase angles between current and voltage to 88 degrees , the accuracy obtained for the wattmeter calibration is about +or-0.5%. The calibration accuracy essentially depends on the accuracy evaluation of the ratio between the value of the mutual inductance and the self inductance of the secondary winding. Another advantage of this method is that it can be used to calibrate the whole wattmeter, including the input attenuators, but making use of a suitably dimensioned mutual inductor.

A simple theory has been developed to explain the behaviour of concave plate electrodes mounted either internally or externally to a pipe wall. The theory has been adequately confirmed by experimental results, and shows that the conductance and capacitance of the external electrode system are each functions of both fluid conductivity and permittivity and as these parameters increase the measured admittance tends to that of the capacity formed by the electrodes, the wall and the wall-fluid interface. Internally mounted electrodes are to be preferred for the measurement of fluid permittivity but externally mounted electrodes may be used where the effective fluid admittance is low.

An instrument for measuring the change in refractive index with temperature of solid infrared transmitting materials over the wavelength range 8 mu m to 13.5 mu m is described. A Littrow spectrometer arrangement is used with the prism sample mounted in an evacuated chamber with temperature control between -50 degrees C and 200 degrees C. The angular position of the spectrometer collimating mirror is servo-controlled to maintain the alignment of the outgoing and returning infrared beams from the prism. Measurement of the angular movement of this mirror then allows changes in the refractive index of the prism with temperature to be determined. The performance of the instrument is discussed in detail and results of measurements on germanium given; uncertainties in the values of the thermal coefficient of refractive index are considered unlikely to exceed +or-1.2%.

The quality factor kappa which describes the performance of the stroboscopic SEM detection system is first defined in terms of the signal-to-noise ratio (SNR). Then the relation between the SNR and the signal processing is measured in order to determine the kappa factor of the system for the DC beam with the spectrometer uninstalled. Also measured are the effects of electron beam pulsing and the installation of the spectrometer on the SNR. It is shown that the performance of the detection system can be well described by means of the kappa factor.

A very high sensitivity non-contacting vibration analysis system based upon a monomode fibre optic Mach Zehnder interferometer incorporating an open air path is described. The system was operated in both a closed loop limited tracking range (>10⁵) homodyne mode and an effective infinite tracking range novel pseudo-heterodyne mode. For periodic signals, amplitude detection sensitivities of approximately 2*10^-10 m at 10 Hz to 5*10^-11 m at 10 kHz have been demonstrated for both modes of operation.

There are industrial requirements for instrumentation of two and three component flows. The authors present an investigation into an ultrasound transducer suitable for determining gas concentration. A pair of ultrasound transducers are positioned opposite to each other, one on either side of the flow pipe. The transmitter is excited by pulses of power and pulsed ultrasound beams pass through the acoustic cavity between transmitter and receiver. The beam is randomly modulated in both amplitude and phase by either turbulent eddies in single phase fluids or suspended particles in two phases flows (e.g. sand in solid/liquid mixture, air bubbles in air/water mixture). The peak values of the output signal from the receiver are measured by a peak detector and, after time averaging, are functionally related to the concentration of the discontinuous phase. The main advantage of pulsed ultrasound sensors compared with the continuous wave (CW) ultrasound sensors is the avoidance of the deleterious effect of standing waves which occur in the acoustic cavity between transducers when CW is used. Therefore simpler and more reliable measurement systems can be built.

Starting from a discussion of known direct dynamic hot-wire probe calibration methods a calibration technique is described, by which the directional sensitivity of the probes is employed to determine the amplitude and phase response. Results obtained by this method indicate that an alteration of the dynamic response of hot-wire probes (constant-temperature mode) can occur for frequencies about 1 Hz, depending on the length-to-diameter ratio of the wire and the air velocity. Estimations of the energy balance of the hot-wire illustrate that the observed frequency response alteration is probably caused by the heat conduction end losses of the wire.

Mathematical details concerning a method for identifying individual dissociation channels of a triatomic molecule are presented. The technique utilises a transmission grid in front of a particle detector to limit the probability of dissociation products from reaching the detector.