Table of contents

This special feature is intended to present a comprehensive review of the present state and novel trends in the field of quantum measurement standards. Most of the present metrological research is concentrated on establishing and strengthening the links between the units and fundamental constants. This will be demonstrated in the nine articles in this feature.

The first four articles are devoted to time, frequency and length metrology. They describe quantum standards that are used or intended to be used for the realization of the SI base units of time and length, the second and the metre. The two units are related to each other by an adopted fixed value of the speed of light in vacuum and the second is at present defined by the energy difference (frequency) of a hyperfine transition in the ground state of caesium. The special feature starts with the discussion of caesium atomic clocks as direct realizations of the second and as a basis for other standards (e.g. the Josephson voltage quantum standard). Whereas Cs atomic clocks still provide us with the most accurate realization of the second, optical frequency standards based on cold trapped ions or cold atoms may eventually lead us to even lower uncertainty levels and may replace the present definition of the second by an optical transition. This situation is described in the contribution on optical frequency standards based on trapped single ions. Since optical frequency standards are also needed as wavelength standards in length metrology, the third contribution reviews the definition of the metre. It describes the different methods of realization, in particular by optical frequency standards including standards based on cold atoms. The use of optical frequency standards in time and length metrology requires the precise knowledge of their frequencies. Methods of optical frequency measurements based on various methods—including frequency comb generators—are discussed in the fourth article.

Turning to the electric units, the discovery of two macroscopic quantum effects, the Josephson effect—discovered in the early sixties—and the quantum Hall effect—discovered in 1980—allowed the linking of electric units to fundamental constants. By use of the Josephson effect quantized voltage values are realized as multiples of the product of a certain frequency and of the superconducting flux quantum (h/2e). Quantized resistance values are realized with the quantum Hall effect as submultiples of h/e², where this fundamental constant can also be interpreted as the quotient of the flux quantum of a normal conductor and the elementary charge. The metrological application became much easier by making use of the possibility of designing and manufacturing appropriate samples with microelectronic techniques. Since 1990 all calibrations of voltages or resistances worldwide have been based on these two quantum effects. Relative uncertainties of the order of 10^-9 are obtained. International comparisons have proved that the calibration results in different laboratories also agree within relative uncertainties of a few parts in 10⁹. Great attempts have also been made to realize a current quantum standard by counting single electrons. The Josephson effect and its application are described by Kohlmann and co-workers. The quantum Hall effect is only briefly described in this issue of Measurement Science and Technology since a comprehensive review article by Jeckelmann and Jeanneret appeared recently [1].

The unit of mass, the kilogram, is the only base unit in the SI that is derived from an artefact. Therefore one major task of today's metrological research is the linking of the unit of mass to a fundamental constant. There are two rival attempts: the comparison of mechanical and electrical power using the watt-balance and the counting of a large number of identical particles, like atoms or ions. The watt balance was pioneered by B Kibble. This experiment consists of two parts. A balance in equilibrium is opposed to a gravitational force and to a force on a coil fed by a current and opposed to a magnetic field. The latter force depends on geometrical dimensions. They can be determined electrically in the second part of the experiment, where the coil is moved through the magnetic field and the occurring inductive voltage is measured. This experiment links the kilogram with Planck's constant h. Counting of atoms or ions is described by Becker and Glaeser. Silicon atoms are the most suitable for growing large single crystals, in this case a sphere. The number of atoms is determined by precise measurements of the volume of an elementary cell and of the volume of the total sphere. One of the experimental problems is the appearance of voids in the crystal, another is the existence of various silicon isotopes with different masses. The latter aspect is avoided in the experiment where isotopic ions of gold or bismuth are accumulated. The number of ions is determined by integrating the ion current with time, while the current itself is measured using the Josephson and the quantum Hall effects. This experiment links the kilogram with a number of identical particles. On the basis of this experiment the definition of the unit of mass would be given through a definition of the Avogadro constant.

Reference

[1] Jeckelmann B and Jeanneret B 2001 The quantum Hall effect as an electrical resistance standard Rep. Prog. Phys.64 1603–55 (IOP Article)

For more than four decades, caesium atomic clocks have been the backbone in a variety of demanding applications in science and technology. Neither satellite based navigation systems, like the US Global Positioning System, nor the syntonization of telecommunication networks at the presently prescribed levels, would function without them. Recent years have brought major breakthroughs in the development, operation and mutual comparison of frequency standards based on the same hyperfine transition in caesium as used previously, but now incorporating the technique of laser cooling. Several cold-atom fountains have been developed. Mutual agreement to within about one part in 10¹⁵ has been demonstrated for two of them operated side by side, and also for two operated simultaneously, in the US and Germany. This paper gives an overview of currently available commercial caesium clocks and primary standards developed in national metrology institutes.

Optical frequency standards based on narrow absorptions in laser-cooled single trapped ions have recently begun to demonstrate stabilities that are competitive with cold atom fountain microwave standards. This paper presents a short review of the wider state-of-the-art development of these single cold trapped ion frequency standards, coupled with a more detailed account of recent results achieved at National Physical Laboratory in respect of single ion systems based on ⁸⁸Sr⁺, ⁸⁷Sr⁺ and ¹⁷¹Yb⁺. Narrow linewidth data for the optical clock quadrupole and octupole transitions respectively at 674 nm in ⁸⁸Sr⁺ and 467 nm in ¹⁷¹Yb⁺, are presented, together with a discussion of current systematics and future projections. The potential for optical clock operation is outlined.

After a short review of the history of the definition of the SI base unit of length—the metre—precise methods for its realization are discussed. The paper gives an overview of current realizations of the metre by frequency-stabilized lasers. Furthermore, a detailed description of a precise optical frequency standard based on cold neutral calcium atoms is presented together with an estimation of the relative uncertainty (2 × 10⁻¹⁴) of its frequency.

This paper reviews the history of the phase-coherent measurement of optical frequencies, its present status and future prospects. The development of frequency measurement of optical frequency standards with respect to the frequency of the Cs atomic clock is described.

This paper reviews the present state of modern Josephson voltage standards. The presentation focuses on conventional dc standards based on underdamped superconductor–insulator–superconductor junctions and programmable standards based on overdamped superconductor–insulator–normal conductor–insulator–superconductor junctions. The current developments of ac standards on the basis of pulse-driven arrays and single flux quantum-based voltage multipliers are briefly summarized.

The quantum Hall effect (QHE) provides an invariant reference for resistance linked to natural constants. It is used worldwide to maintain and compare the unit of resistance. The reproducibility reached today is almost two orders of magnitude better than the uncertainty of the determination of the ohm in the International System of Units. This article is a summary of a recently published review article which focuses mainly on the aspects of the QHE relevant for its metrological application.

This paper is mostly a review of the progress made at NIST in pursuing a capacitance standard based on the charge of the electron. We briefly introduce the Coulomb blockade, which is the basic physical phenomenon allowing control of single electrons, describe two types of single-electron tunnelling (SET) device and describe the metrology goals and payoffs achievable from SET devices. We then discuss the electron-counting capacitance standard (ECCS): the motivation, previous experimental work on various critical elements, present status and future prospects. This last part includes using the ECCS for a practical representation of capacitance, as well as pointing out that we can close the quantum metrology triangle without needing a large-value current standard. Finally, we briefly review other SET-based metrological applications.

The kilogram is the last remaining SI unit defined by an artefact—the platinum–iridium cylinder kept at the International Bureau of Weights and Measures (BIPM). The artefact standard is subject to, admittedly very small, irreversible changes caused both by wear and by accretion of mass from atmospheric pollutants. In addition, the unit of mass can only be disseminated by comparison with the artefact held by the BIPM. Comparisons of its mass with other similar and equally carefully preserved cylinders show relative changes, over many tens of years, of several parts in 10⁸. If a procedure was available to link mass to a fundamental constant, with a similar or better accuracy, the above limitations would be overcome, making the unit accessible to anyone caring to carry out the procedure and long term stability would be assured. Attempts to do this fall into two broad categories: the first relates the kilogram to atomic masses by measuring the number of atoms in a sample and the second relates the kilogram to Planck's constant, the metre and the second via the equivalence of mechanical and electrical energy. We summarize experiments in both these categories.

This paper summarizes the activities of the several national metrology institutes and one transnational institute in replacing the kilogram artefact by the mass of a certain number of atoms. This task is based on two different experiments: a very accurate determination of the Avogadro constant, N_A, and the accumulation of decelerated gold ions, which lead to the atomic mass of silicon and gold respectively. The relative uncertainties reached so far are in the first case two parts in 10⁷, and in the latter of the order of 1% due to the early state of the research work.

In this study time-resolved proper orthogonal decomposition (POD) is proposed for the new technique of dynamic (digital) particle image velocimetry (DPIV), and applied to study the coherent structures at the near field of a round jet flow. Time-resolved POD is computed in the radial direction, the axial direction and the temporal domain, in which the turbulence can be easily measured by DPIV. As a result, large-scale vortical structures and their spatial and temporal evolution at the round jet mixing layer are uncovered.

A method for evaluating the dynamic response of force transducers against an oscillation force is proposed. In the method, a force transducer is connected to a mass by a spring, which generates the oscillating force after the mass is manually hit using a hammer. A pneumatic linear bearing is used to realize linear motion with sufficiently small friction acting on the mass, which is the moving part of the bearing. The inertial force acting on the mass is determined highly accurately by measuring the velocity of the mass using an optical interferometer.

This paper describes the development of a six-axis force/moment sensor and its control system for an intelligent robot's gripper. An intelligent robot's gripper should detect the forces F_x (x-direction force), F_y and F_z in the gripping direction and in the gravitational direction, and be controlled by the measured forces and the gripper's control system to safely grasp an unknown object. Also, it should detect the moments M_x (x-direction moment), M_y and M_z to accurately perceive the position of the object in the grippers. Therefore, an intelligent robot's gripper should be composed of a six-axis force/moment sensor which can measure the forces F_x, F_y and F_z, and the moments M_x, M_y and M_z simultaneously. It is required that each component sensor of the six-axis force/moment sensor generally has the same rated output, and the same capacity (e.g. F_x = F_y = F_z = 50 N, M_x = M_y = M_z = 5 N m) or different capacity (e.g. F_x = 40 N, F_y = 50 N, F_z = 60 N, M_x = 4 N m, M_y = 3 N m, M_z = 5 N m) in each force component and moment component in the gripper, depending on the particular application. The control system should have a fast processing speed for measuring the signals of the sensors and controlling the gripper without much delay time, and should be of small size. The grippers are made with a force sensor which can detect force in only one direction. The control system consists of a personal computer and an A/D, D/A converter board.

In this paper, a six-axis force/moment sensor using parallel-plate beams is designed and fabricated to make an intelligent robot's gripper; the gripper's control system is designed and manufactured using a digital signal processor.

A force sensor, for use with an artificial hand, needs to be small, robust, low power, cheap and easily interfaced to a controller using digital techniques. The prototype featured in this paper uses capacitance effects to measure the strain on an elastic polymer foam. Low power consumption results in a device that can be supplied from a miniature battery thereby requiring only signal wires to the controller. A non-linear model accurately describes the characteristic of the sensor, requiring the estimation of only three parameters. The device has been tested up to 20 N but is capable of measuring greater forces.

In this paper we present a novel method for determining the probing points for achieving efficient measurement and reconstruction of freeform surfaces. A B-spline is adopted for modelling the freeform surface. In the framework of Bayesian statistics, we develop a model selection strategy to obtain an optimal model structure for the freeform surface. Based on the selected model structure, a set of probing points is then determined where measurements are to be made. In order to obtain reliable parameter estimation for the B-spline model, we analyse the uncertainty of the model and use the statistical analysis of the Fisher information matrix to optimize the locations of the probing points needed in the measurements. Using a 'data cloud' of a surface acquired by a 3D vision system, we implemented the proposed method for reconstructing freeform surfaces. The experimental results show that the method is effective and promises useful applications in multi-sensor measurements including a vision guided coordinate measuring machine for reverse engineering.

We present the details of a new tunable optical filter, suitable for selecting different bands of wavelength in the UV, visible and IR regions. The filter comprises a ferrofluid-based emulsion cell (emulsion sandwiched between two transparent sheets), a miniature solenoid and a variable direct current source for changing the magnetic field inside the solenoid. By varying the magnetic field, one can tune the filter and select the desired wavelength. We discuss the working principle of the new tunable optical filter with a few examples.

We address the problem of estimating the time-of-flight (ToF) of a waveform that is disturbed heavily by additional reflections from nearby objects. These additional reflections cause interference patterns that are difficult to predict. The introduction of a model for the reflection in terms of a non-stationary auto-covariance function leads to a new estimator for the ToF of an acoustic tone burst. This estimator is a generalization of the well known matched filter. In many practical circumstances, for instance beacon-based position estimation in indoor situations, lack of knowledge of the additional reflections can lead to large estimation errors. Experiments show that the application of the new estimator can reduce these errors by a factor of about four. The cost of this improvement is an increase in computational complexity by a factor of about seven.

This paper reports the use of a plastic fibre sensor for detecting impact damage in carbon fibre epoxy cantilever beams by monitoring their damping response under free vibration loading conditions. The composite beams were impacted at impact energies up to 8 J. The residual strengths and stiffnesses of the damaged laminates were measured in order to relate reductions in their mechanical properties to changes in their damping characteristics. Here, optical fibre sensors were surface bonded to carbon fibre composite beams which were subjected to free vibration tests to monitor their dynamic response.

In the second part of this study, Ni–Ti shape memory alloy (SMA) wires were employed to control and modify the damping response of a composite beam. The SMA wires were initially trained to obtain the desired shape when activated. Here, the trained SMA wires were heated locally using a nickel/chromium wire that was wrapped around the trained region of the SMA. By using this method to activate the SMA wire (as opposed to direct electrical heating), it is possible to obtain localized actuation without heating the entire length of the wire. This procedure minimizes any damage to the host material that may result from local heat transfer between the SMA wire and the composite structure. In addition, the reduction in power requirements to achieve SMA activation permits the use of small-size power packs which can in turn lead to a potential weight reduction in weight-critical applications. The findings of this study demonstrate that a trained SMA offers a superior damping capability to that exhibited by an 'as-supplied' flat-annealed wire.

The method of direct measurement of vorticity (DMV) is used for extracting vorticity directly from digital particle images. It is the analogue of cross-correlation in particle image velocimetry (PIV) evaluation. There are two main purposes behind the present study: one is to compare the implementation of DMV with PIV in order to clarify the characteristics of the DMV technique; the other is to investigate the influence of the experimental parameters such as particle image size, the size of the sample pattern and the particle density on the precision of DMV. The investigation is performed by applying the DMV method to two kinds of pairs of synthetic digital particle recordings, which respectively represent a solid rotational and a parallel shearing flow. The statistical uncertainty errors are obtained by varying a single parameter at a time. The DMV method shows good measurement results for the stronger rotational flow. Limitations of the DMV technique with regard to the measurable intensity of the shear flow are discussed. The optimum experimental parameters for both flows are given.

Microfluidic devices with channel cross sections measuring 4 × 10 µm² instrumented with gold microelectrodes were used to sense flow rates of ionic solutions on the basis of electric impedance (EI) measured perpendicular to the flow. Negative pressures were applied to access ports of the microdevices to generate flow of saline solutions (physiologic concentrations 0.9%) through the micro-EI recording zone with flow rates between 2.4 and 4.8 µl min⁻¹. The EI spectra (100 Hz–20 MHz) recorded under flow conditions were compared with the no-flow condition. Changes in the magnitude of EI (at 350 Hz) for flow rates as low as 2.4 µl min⁻¹ were statistically significant compared with the no-flow condition. The observed dependence of EI on flow rate is attributed to the relative difference between the rate of migration of charge-balancing electrolyte ions to the electrode surface and the rate of removal of the same ions by forced convection. An electrochemical convection–diffusion model was used to study the observed dependence on flow. Simulations support the conceptual model that passing DC current from the gold electrodes into the ionic solution results in an increase in ionic concentration near the electrode surface (due to the inward migration of counter-balancing ions). When the fluid flow rates increase, these counter-balancing ions are replaced by the bulk solution, thereby lowering the average ionic concentration within the recording zone. This local concentration drop results in an increase in the real part of the impedance.

A new method has been developed to measure experimentally the break-up properties of bubbles. The technique is based on the application of a particle tracking velocimetry algorithm to high-speed video images not only to measure the velocity of the bubbles, but also to detect the break-up events. Thus the algorithm is able to associate every broken bubble with the daughter bubbles formed upon their corresponding break-up. Moreover, the lifetime, as well as the number and size of fragments resulting from the break-up process, can be measured for a large number of bubbles. Statistical processing of the information collected allows us to compute the break-up frequency and daughter size distribution of the bubbles as a function of the bubble size and the mean properties of the base flow. The method has been employed to study the break-up of a cloud of bubbles injected at the central axis of a turbulent water jet. Experimental results for the break-up frequency and daughter bubble size distribution are also presented to illustrate the performance of the technique.

The present paper describes an experimental technique of droplet sizing and velocity measurement for application to a luminous flame in spray combustion. The size measurement of unburnt fuel droplets in combustion is carried out by using an interferometric imaging method, while the corresponding velocity field is measured by particle tracking velocimetry (PTV) in combination with the rotary shutter to avoid the high intensity noise of the luminous flame in spray combustion. The measurements are successfully applied to the spray flow from a gun-type burner with and without combustion. The experimental results in spray combustion indicate that the smaller size of fuel droplets are almost burnt in the centre of the flame and the unburnt droplets of larger size remain in the outer region of the burner flow. It was found that the mean droplet velocity measured by the present PTV technique in combustion is almost independent of the droplet size and agrees closely with the gas velocity. However, the velocity magnitude with combustion is increased in comparison with the case without combustion, which suggests the influence of gas expansion at high temperatures.

A fibre optic system was developed to determine the fuel concentration near a spark plug using an infrared absorption method. The system was linked to an optical sensor installed in the spark plug, from which light could pass through the combustion chamber. By using this modified spark plug, successive measurements of the fuel concentration near the spark plug before ignition were performed in a spark-ignition engine burning homogeneously mixed methane–air. The fuel concentration was determined from the Lambert–Beer law by considering the dependence of the methane molar absorption coefficient on pressure and temperature. Three main conclusions were drawn from this study. First, the methane molar absorption coefficient was greater for lower pressures and decreased with increasing temperature and pressure above atmospheric pressure. The temperature and pressure effects were offset by each other, since the temperature effects were positive and the pressure effects were negative. Second, precise time-series data for the local fuel concentration were obtained by considering the in-cylinder pressure and temperature from an estimate of the methane molar absorption coefficient. And third, the measured air/fuel ratio near the spark plug before ignition agreed with the preset value when the developed optical sensor was used under motoring and firing conditions.

An infrared absorption method with a 3.392 µm He–Ne laser was used to determine the hydrocarbon fuel concentration near the spark plug in a spark-ignition engine. Iso-octane was used for the fuel. The pressure and temperature dependence of the molar absorption coefficient was clarified. The molar absorption coefficients of a multi-component fuel such as gasoline were estimated by using the coefficient of each component and considering the mass balance. A sensor was developed and installed in a spark plug, which was substituted in place of an ordinary spark plug in a spark-ignition engine. Light can pass from the sensor through the engine cylinder to measure the fuel concentration. The effects of liquid droplets inside the engine cylinder, mechanical vibrations and other gases such as H₂O and CO₂ on the measurement accuracy were considered. Four main conclusions were drawn from this study. First, the pressure and temperature effects on the molar absorption coefficient of liquid fuel vapour were determined independently in advance using a constant-volume vessel. The pressure and temperature dependence of the molar absorption coefficient was determined under engine firing conditions. Second, the molar absorption coefficients of a multi-component hydrocarbon fuel such as gasoline were estimated by considering the molar fraction of each component. Third, in situ measurements of the hydrocarbon fuel concentration in an actual engine were obtained using the spark plug sensor and the molar absorption coefficient of iso-octane. The concentration near the spark plug just before ignition was almost in agreement with the mean value that was obtained from the measurement of the flow rate made with a burette, which represented the mean value averaged over many cycles. And fourth, no liquid droplets were observed at near-idling conditions. The effects of other gases, such as CO, CO₂ and H₂O, can be neglected.

A user-friendly data acquisition and control system (DACS) for a chemical oxygen–iodine laser (COIL) has been developed. The system is capable of handling 117 analogue/digital channels for performing various operations such as on-line acquisition, control, display, safety measures and status indication of various subsystems. These operations are controlled either by control switches configured on a PC while not running or by a pre-determined sequence or timings during the run. The system is capable of real-time acquisition and on-line estimation of important diagnostic parameters for optimization of a COIL. The DACS system has been programmed using Advantech-GeniDAQ software. This software has also been used to convert the acquired data into graphical form. Using this DACS, more than 200 runs were given performed successfully.

The Wheatstone bridge in which the hot wire is embedded in normal constant-temperature hot-wire anemometers is never perfectly balanced, due to the finite value of the amplifier offset. In this paper we investigate the influence of this imbalance on the estimation of sensitivity coefficients for velocity and temperature fluctuations. The analysis is made using the anemometer model developed by Perry and Morrison (1971 J. Fluid Mech.47 577). It is shown that the absolute value of both non-dimensional sensitivity coefficients is always lower than in the ideal case of an anemometer which maintains the bridge perfectly balanced. The degree of discrepancy increases with increasing offset voltage and decreasing overheat ratio. However, it is shown that a direct calibration procedure takes this effect into account, thus ensuring an accurate determination of the sensitivity coefficients of the anemometer in practice. Experimental evidence confirming the theoretical results is presented.

A dynamic calibration system was introduced to evaluate the dynamic performance of fine-wire thermocouples. It provided a high-temperature step using a rocket plume with dual-aromatic-magnesium-1 propellant. Experiments were carried out in a rocket motor static state experiment laboratory. Two kinds of fine-wire thermocouple, tungsten–rhenium 5%/tungsten–rhenium 20% and nickel–chromium/nickel–silicon (type K), were studied at different positions in the jet plume in each test. Spectrum analysis was conducted to study the temperature data for the jet plume. The working frequency bands were determined for these two kinds of fine-wire thermocouple. These results serve as references for compensating for the dynamic performances of fine-wire thermocouples when they are used in measuring temperature fluctuation.

The planar liquid-laser-induced fluorescence (PLLIF) technique has been known to be a useful tool for the measurement of the spray mass distributions for various spray injectors because it can obtain two-dimensional images with high spatial resolutions without any intrusion on the spray field. In the cases of dense sprays, however, it has been known that the extinctions of the incident laser beam or fluorescence signal and the secondary emission can cause errors in quantifying the spray mass distributions. Since a like-doublet injector, which is commonly used in liquid rocket engines, has a locally concentrated spray zone at the spray centre, we investigated the applicability of the PLLIF technique for this injector. From the experimental results, we found out that the extinctions of the incident laser beam and fluorescence signal are not significant because the concentrated spray zone is narrow. Also, we found out an optimal incident laser power which can avoid a nonlinear increase of fluorescence signal at the spray centre as well as obtain a high signal-to-noise ratio, and we measured the spray mass concentration of the like-doublet injector spray using the optimal laser power. In order to assess the accuracy of the PLLIF data, we converted the spray mass concentration into the mass flux distribution and compared it with the data obtained by a mechanical patternator and phase Doppler particle analyser. From the result that the PLLIF data showed good agreement with those of the mechanical patternator, we concluded that the PLLIF technique can be successfully applied to measuring the mass distributions of the like-doublet injectors.

Based on a transient thermal model of a point heat pulse, a dual-thermistor probe, in which two thermistor beads serve as point heater and temperature sensor respectively, has been developed for absolute measurement of thermal diffusivity and conductivity. An experimental system with high sensitivity for temperature change is designed to perform the measurement. Experimental results of samples are in agreement with the recommended values. The validation of the one-dimensional point source heat transfer is discussed, and the deviation from the theoretical model is negligibly small for the measurement. The estimated measurement uncertainties are 3.5% and 5.3% for thermal diffusivity and conductivity respectively.

It is difficult for outdoor apparel manufacturers to interpret the technical information provided by fabric suppliers concerning fabric 'breathability' properties because different methods and test conditions are used. In addition, fabrics with hydrophilic components change their properties under different humidity conditions. The purpose of this study was to measure the water vapour permeability and evaporative resistance of 26 different waterproof, windproof and breathable shell fabrics using five standard test methods. The water vapour transmission rate (WVTR) was measured using the ASTM E 96 upright and inverted cup tests with water, the JIS L 1099 desiccant inverted cup test and the new ASTM F 2298 standard using the dynamic moisture permeation cell (DMPC). The evaporative resistance was measured using the ISO 11092 sweating hot plate test. The WVTRs were consistently highest when measured with the desiccant inverted cup, followed by the inverted cup, DMPC and upright cup. The upright cup was significantly correlated with the DMPC (0.97), and the desiccant inverted cup was correlated to the sweating hot plate (−0.91).

The thermal response of the attenuation bands of an optical fibre long period grating was monitored over a temperature range of 4.2–280 K. A linear dependence of the central wavelength of the band, of gradient 0.2 nm K⁻¹, was observed over the range 77–280 K. A measurable wavelength shift was observed at temperatures as low as 20 K.

A planar jet impinging on an edge under feedback control is studied by simultaneous flow visualization, control and particle image velocimetry (PIV) measurement to understand the feedback control mechanism of edge tone. The instantaneous velocity signal from the visualized dye images is used as a feedback signal for the flow control, while the whole flow field is measured simultaneously by analysing the tracer particle images by PIV. A comparative study of the flow field with and without feedback control is carried out using this technique and the attenuation of the flow oscillation of the jet flow is observed for certain combinations of the phase lag and feedback gain. Attenuation of the jet oscillation by feedback control is also observed in the velocity and vorticity distributions at various phases of jet oscillation in the flow field. It is found that the velocity fluctuations created by the jet oscillation are cancelled by those produced by the control flow near the jet nozzle, which satisfies the opposite phase condition of the flow.

In vision inspection applications, for perspective projection there is a distortion in the position of an ellipse's centre, which is inappropriate for high-precision vision inspection applications. Based on perspective projection and spatial analytical geometry, the coordinates of the spatial ellipse's image centre are described and the practical image coordinates of the spatial ellipse's centre are also given in detail. Later, a position-distortion model of the ellipse's centre is established precisely for perspective projection. Meanwhile, a numerical simulation of the position distortion is completed, and some trends in its dependence on various geometrical parameters are identified. Finally, a practical structured-light based 3D vision system provides a validity check for the position-distortion model proposed in this paper. The work in this paper is significant for some relevant applications of machine vision.

The authors present a new non-intrusive experimental procedure based on laser techniques for the measurement of mechanical properties of tendons. The procedure is based on the measurement of the first resonance frequency of the tendon by laser Doppler vibrometry during in vitro tensile experiments, with the final aim of establishing a measurement procedure to perform the mechanical characterization of tendons by extracting parameters such as the resonance frequency, also achievable during in vivo investigation. The experimental procedure is reported, taking into account the need to simulate the physiological conditions of the Achilles tendon, and the measurement technique used for the non-invasive determination of tendon cross-sectional area during tensile vibration tests at different load levels is described. The test procedure is based on a tensile machine, which measures longitudinal tendons undergoing controlled load conditions. Cross-sectional area is measured using a new non-contact procedure for the measurement of tendon perimeter (repeatability of 99% and accuracy of 2%). For each loading condition, vibration resonance frequency and damping, cross-sectional area and tensile force are measured, allowing thus a mechanical characterization of the tendon. Tendon stress–strain curves are reported. Stress–strain curves have been correlated to the first vibration resonance frequency and damping of the tendon measured using a single-point laser Doppler vibrometer. Moreover, experimental results have been compared with a theoretical model of a vibrating cord showing discrepancies. In vitro tests are reported, demonstrating the validity of the method for the comparison of different aged rabbit tendons.

The present study describes a newly developed sensor capable of simultaneously measuring fine-scale velocity and concentration gradients in a 4.8 × 6 × 4 mm³ volume. The sensor combines the established technologies of photoionization, to obtain concentration, and hot-wire (x-array) anemometry, to obtain two components of the velocity vector. Spatial gradients are obtained by finite difference across the measurement volume. A novelty of the sensor lies in the ability to capture instantaneous time series of most of the terms in the scalar variance transport equation. The frequency response of the sensor is approximately 325 Hz for concentration and 1.2 kHz for velocity. The present study focuses primarily on the development and characterization of the probe. Major sources of measurement uncertainty are discussed, including instrumentation noise, calibration errors, flow distortion and spatiotemporal resolution. The propagation of uncertainty, relevant to the terms in the scalar variance transport equation, is also briefly discussed.

Mean flow measurements taken in fully developed turbulent pipe flow over a wide Reynolds number range are used to evaluate current methods of correcting Pitot probe data. Based on this evaluation, a new form for the displacement correction is proposed which appears to be more accurate over a wider range of conditions than those currently available. The difficulty of obtaining the true near-wall velocity profile near the wall is explored.

The water vapour spectrum in the 1–2 µm near-infrared region is systematically analysed to find the best absorption transitions for sensitive measurement of H₂O concentration and temperature in combustion environments using a single tunable diode laser with typical distributed feedback single-mode scanning range (1 cm⁻¹). The use of a single laser, even with relatively narrow tuning range, can offer distinct advantages over wavelength-multiplexing techniques. The strategy and spectroscopic criteria for selecting optimum wavelength regions and absorption line combinations are discussed. It should be stressed that no single figure of merit can be derived to simplify the selection process, and the optimum line pair should be chosen case by case. Our investigation reveals that the 1.8 µm spectral region is especially promising, and we have identified 10 of the best water line pairs in this spectral region for temperature measurements in flames. Based on these findings, a pair of H₂O transitions near 1.8 µm was targeted for the design and development of an initial single-laser sensor for simultaneously measuring H₂O concentration and temperature in atmospheric-pressure flames. As part of the sensor development effort, fundamental spectroscopic parameters including the line strength, line-centre frequency and lower state energies of the probed transitions were measured experimentally to improve the current databases. We conclude with demonstration results in a steady and a forced atmospheric-pressure laboratory combustor.

When using a scanning Doppler laser vibrometer (SLDV) an important amount of user interaction is required to perform a calibration between the required scanning mirror angles and the coordinates of the grid points which are to be measured. Apart from the possibility of leading to incorrect (or inaccurate) laser beam positioning, the user interaction is a problem when the SLDV is used in-line (for instance as a quality control tool in the production process). In this paper, a method is developed to perform the position calibration in a fully automatic manner. The method is validated with the aid of two SLDV measurement examples: a sheet with a rectangular mesh, and an electronic circuit board.

A novel multi-point optical fibre sensor for ultraviolet detection is presented. The sensor is a single piece of large-core polymer optical fibre which has had sections of its cladding stripped and replaced with phosphor-doped epoxies. These photoluminescent coatings act as the new cladding. Two points on the fibre have been coated with a different phosphor, each providing a different emission spectrum (red and green) when stimulated by ultraviolet radiation. The spectral emission intensity is dependent on the strength of the incident ultraviolet radiation. Part of this emission is coupled to the fibre's core through the evanescent wave of guided modes within the core. The fibre's output is monitored using a low-cost miniature spectrometer. The spectrometer output is used to interpret the different phosphor emission spectra and thus determine localized axial locations on the fibre of increased or decreased ultraviolet intensity. Results are included in the form of spectra (400–900 nm) under different ultraviolet stimulation conditions. An expanded version of this sensor has applications in monitoring ultraviolet water and waste-water sterilization units.

A compact high-speed beam chopper for ultrafast-pulsed-laser applications is designed and tested. To achieve high-speed operation, the chopper blade is positioned on the focal plane of a pair of confocal parabolic mirrors. When the pulse train of an ultrafast Ti:sapphire laser is focused and re-collimated by these mirrors, it becomes modulated by the rotating chopper blade. By using 360 slots on the chopper blade and a precision-balanced high-speed DC electric motor, optical modulation rates up to 100 kHz are achieved. The application of this system to noise reduction in ultrafast-laser experiments is investigated through interferometric autocorrelation measurements, where it is found that optical modulation rates above 60 kHz provide the lowest noise level.

A microcontroller-based multi-sensor temperature measurement and control system that uses a steady-state one-dimensional heat-flow technique for absolute determination of thermal conductivity of a rigid poor conductor using the guarded hot-plate method is described. The objective of this project was to utilize the latest powerful, yet inexpensive, technological developments, sensors, data acquisition and control system, computer and application software, for research and teaching by example. The system uses an ST6220 microcontroller and LM335 temperature sensors for temperature measurement and control. The instrument interfaces to a computer via the serial port using a Turbo C++ programme. LM335Z silicon semiconductor temperature sensors located at different axial locations in the heat source were calibrated and used to measure temperature in the range from room temperature (about 293 K) to 373 K. A zero and span circuit was used in conjunction with an eight-to-one-line data multiplexer to scale the LM335 output signals to fit the 0–5.0 V full-scale input of the microcontroller's on-chip ADC and to sequentially measure temperature at the different locations. Temperature control is achieved by using software-generated pulse-width-modulated signals that control power to the heater. This article emphasizes the apparatus's instrumentation, the computerized data acquisition design, operation and demonstration of the system as a purposeful measurement system that could be easily adopted for use in the undergraduate laboratory. Measurements on a 10 mm thick sample of polyurethane foam at different temperature gradients gave a thermal conductivity of 0.026 ± 0.004 W m⁻¹ K⁻¹.

Fibre drop analysis is a new multi-analysis technology for measuring the physical and chemical properties of liquids. One parameter that can be measured by the fibre drop analyser is surface tension. The conventional method for surface tension measurement is the constant volume delivery mode, in which the uncertainty is mainly caused by evaporation. We present here a remnant drop (RD) method to improve the uncertainty in measuring volatile liquids. The method measures the mass and vibration frequency of the RD and calculates the surface tension by comparing the measured liquid with a reference liquid. A RD model is presented to analyse the surface tension. The experimental results show that the method improved the accuracy of surface tension measurements by about 60% for a volatile liquid (e.g. ether).

On line 9 of the second column of page 655 the citation of reference [12] is incorrect. The article cited at this point should be:

Mobasseri B G 1995 Automatic target scoring system using machine vision Machine Vision and Applications8 20–30