On the accuracy of oxygen pressure measurements

Zirconia-based oxygen sensors are used in many applications and in research. However, the measurement of oxygen partial pressures with these devices is not necessarily correct. Reasons for deviation between sensor reading and nominal composition of the gas mixture are reviewed in this work. The sensor electrochemical permeability is introduced as a starting point. The impact of this oxygen leak on the composition of the surrounding atmosphere is evaluated from mass balances to all species in the system. This procedure shows that even the use of mixtures like CO+CO₂ to establish a given oxygen partial pressure has a limited range of applicability. This is due to the small buffer behavior of some of these mixtures. The gas phase transport of oxygen away from the sensor electrode is also studied. High concentration overpotentials are expected to develop under the circumstances where the buffer behavior of the gas mixture is small. In fact, high concentrations of the CO/CO₂ couple are required both to optimize the gas-mixture buffer effect and the oxygen transport in the gas phase. The latter instead of consisting exclusively of simple diffusion of free oxygen molecules might also involve combined diffusion and counter diffusion of both carbon oxides as oxygen carriers.     

Wide-range frequency method to obtain the transverse permeability from impedance measurements

In this work we present a method to obtain the transverse permeability of magnetic materials, valid for both cylindrical and rectangular geometry and a wide frequency range. The principle of this method is based on the dependence of the transverse permeability on the impedance through the skin-depth effect. We also present measurements of the transverse permeability from glass-coated microwires and a comparison with other well-established methods, namely approaches to low and high frequencies. From the complex permeability spectra we can obtain, for example, information about the domain-wall dynamics and the ferromagnetic resonance.     

The input impedance of the dielectric resonator antenna

Dielectric cylinders of very high permittivity have been used in the past as resonant cavities, but since the structure is not enclosed by metallic walls, electromagnetic fields do exist beyond the geometrical boundaries of the structure and part of the power is radiated. Through the proper choice of geometry and permittivity this radiation can become the dominant feature of the structure and become an efficient antenna for use at millimeter wave frequencies. Both experimental and theoretical investigations of a variety of these dielectric resonator antennas have been undertaken. In particular, the input impedance of a probe-fed cylindrical structure was examined in detail and a comparison of theoretical and experimental results was made.     

On the accuracy of maximum entropy reconstruction of high-resolution Z-contrast STEM images

The accuracy of maximum entropy reconstruction of Z-contrast STEM images has been evaluated with the effects of experimental variables and noise taken into account by the means of image simulation. As the specimen contains atom species of greatly different atomic numbers, special attention is given to the reliability of the position and composition of lighter atoms that are determined from Z-contrast images in the presence of heavier atoms. When the noise is moderate (SNR >2.5), the position of atom columns can be measured within an accuracy of 0.03 nm. With a higher signal-to-noise ratio (SNR >5) the composition of lighter atoms can be resolved reliably from the Z-contrast images. However, when image noise increases, the relative intensity of lighter atoms may deviate from the actual value in the specimen object function.     

From direct to absolute mass measurements: A study of the accuracy of ISOLTRAP

For a detailed study of the accuracy of the Penning trap mass spectrometer ISOLTRAP all expected sources of uncertainty were investigated with respect to their contributions to the uncertainty of the final result. In the course of these investigations, cross-reference measurements with singly charged carbon clusters ¹²C⁺ _n were carried out. The carbon cluster ions were produced by use of laser-induced desorption, fragmentation, and ionization of C₆₀ fullerenes and injected into and stored in the Penning trap system. The comparison of the cyclotron frequencies of different carbon clusters has provided detailed insight into the residual systematic uncertainty of ISOLTRAP and yielded a value of 8×10^-9. This also represents the current limit of mass accuracy of the apparatus. Since the unified atomic mass unit is defined as 1/12 of the mass of the ¹²C atom, it will be possible to carry out absolute mass measurements with ISOLTRAP in the future. Received 7 June 2002 Published online 6 November 2002 RID="a" ID="a"e-mail: a.kellerbauer@cern.ch RID="b" ID="b"Current address: Centre de Physique des Particules de Marseille, 13288 Marseille Cedex 9, France.     

On the accuracy of perturbative solutions to wave scattering from rough closed surfaces

The method of small perturbations is applied to the problem of plane-wave scattering from a soft circular surface with sinusoidal roughness. Rayleigh-theory and extinction-theorem perturbative solutions of arbitrary order are developed, and they are compared to each other as well as to an exact solution. A numerical study yields quantitative information about effects associated with the use of the Rayleigh hypothesis, about the merit of higher-order solutions, and about the reliability of the error criteria that measure a posteriori the conformity of perturbative solutions to the boundary condition. It is shown that the accuracy of perturbative solutions depends not only on the height, but also on the slope, of surface corrugations. Moreover, it is shown that the Rayleigh hypothesis does not affect the accuracy of far-field perturbative calculations. The mean extinction error is proposed as a measure for the error of the extinction-theorem perturbative solution.     

Matching impedance of a nonuniform transmission line: application to cochlear modeling

A generalization of the concept of characteristic impedance to nonuniform transmission lines leads to an impedance having a local character. This matching impedance depends on the solution of the transmission line equations, and cannot generally be obtained in analytical form. However, when the propagation properties of the line vary only slowly (as is the case in cochlear macromechanics), a convenient analytical approximation of the matching impedance can be derived by means of the Liouville-Green method.     

Analytical expression for the input impedance of a microstrip probe in waveguide

We present a closed-form expression for the the input impedance of a microstrip probe in a rectangular waveguide. The probe extends only part way across the waveguide and is therefore compatible with RF components that require an open circuit at low frequencies. Our analysis is based on the spectral-domain method and is able to take into account the orientation of the antenna with respect to the direction of propagation. We have examined the validity of our model by carrying out extensive impedance measurements at 5GHz. In those cases where the probe did not extend more than half way across the waveguide, excellent agreement was obtained. We show that the bandwidth of a probe that stretches only part way cross the waveguide is very much greater than the bandwidth of a probe that stretches all of the way across the waveguide and that is earthed at both ends. Moreover, the input resistance is lower and more suited to submillimetre-wave detectors such as SIS tunnel junctions. Our expression suggests that it should be possible to develop low-impedance, wideband probes for nearlydouble-height waveguide, and this implies that the upper frequency limit to which probes and waveguides can be manufactured can be extended well into the THz frequency range. A related, and often neglected consideration, is that the ohmic loss associated with an oversized waveguide is very much smaller than the ohmic loss associated with a reduced-height waveguide.     

A method for calculation of the input impedance of a microstrip electric dipole

We reduce the problem of the current distribution over the surface of a microstrip dipole in the form of a thin, perfectly conducting strip on a dielectric substrate, which is metalized on one side, to a singular integral equation with the Cauchy kernel. The complex-valued current distributions on the dipole surface are presented along with the dependences of the input impedance on the dipole-arm length normalized to the wavelength for different values of the dielectric permittivity of the substrate. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 12, pp. 1061–1070, December 2008.     

On the accuracy of thickness measurements in impact-echo testing of finite concrete specimens--numerical and experimental results

In impact-echo testing of finite concrete structures, reflections of Rayleigh and body waves from lateral boundaries significantly affect time-domain signals and spectra. In the present paper we demonstrate by numerical simulations and experimental measurements at a concrete specimen that these reflections can lead to systematic errors in thickness determination. These effects depend not only on the dimensions of the specimen, but also on the location of the actual measuring point and on the duration of the detected time-domain signal.     

Automated multifrequency measurements of the complex impedance of fast ion conductors

An automated apparatus based on phase synchronous detection has been assembled for measurements of the complex impedance of fast ion conductors. Procedures for calibration and for data acquisition and analysis are described. The relative advantages and disadvantages of this method are compared with ac bridge and pulse methods. The method is illustrated with measurements of the sodium beta-aluminas.     

On the quantization of the hall impedance

The quantized Hall impedence is shown to relate to the ratio of two period integrals which are identified as the familiar period integrals of flux and electric charge if their domain of definition is extended to the realm of spacetime.     

Dynamic displacement field reconstruction through a limited set of measurements: Application to plates

The paper presents a procedure for the identification of the full-field dynamic response of a structure from a limited set of experimental measurements. An iterative technique based on modal decomposition maps the displacement field of the vibrating structure by using experimental data in conjunction with the numerical model of the considered structure. Algebraic relationships between experimental measurements and equivalent modal loads allow the identification of the full-field dynamic response from few experimental data. This procedure is detailed for a plate structure subjected to a harmonic concentrated load.     

Using impedance measurements to detect and quantify the effect of air leaks on the attenuation of earplugs

The impedance of a simple artificial ear occluded with an earplug and bypassed with narrow air leaks was measured along with the attenuation of sound through the air leaks. A lumped element model is suggested for the simple occluded artificial ear with an air leak. The suggested model was adapted to the impedance measurements and the attenuation was predicted from the model. The attenuation predictions were compared to the attenuation measurements and were found to be within [-3.5,+3] dB of the measured attenuation over the frequency range of 50-1000 Hz and an attenuation range of -2-38 dB. The average difference between the measured and predicted attenuation for four different leaks in the frequency range of 50-1000 Hz was -0.7 dB, indicating a very slight underestimation of the attenuation.     

Positron diffusion in solids and the reconstruction of inhomogeneous defect distributions from lifetime measurements

The time dependent diffusion trapping equations for positrons implanted into inhomogeneous solids are analyzed. This problem is of central importance in the study of polycrystalline materials and for the application of pulsed positron beams to defect studies in materials research. The main problem in previous investigations was the necessity to solve the time-dependent diffusion equation. It prevented analytical treatment in all but the simplest applications. For the first time this difficulty is eliminated by invoking a new concept, the observable local annihilation characteristics for local implantation of positrons into the thermalized ensemble. It will be shown that the local annihilation characteristics are governed by field equations which reduce to the well known quantities of the standard trapping model in the case of homogeneous defect distributions. Furthermore, inhomogeneous defect distributions are uniquely determined from the field equations provided the local annihilation characteristics are known. Analytical solutions are derived and applied successfully to recent experimental results for a selection of simple, but realistic problems. The formal procedure includes internal drift fields and could be extended to cover also the epithermal period of positron thermalization, if necessary.