用双测点测量吸声面复反射系数的一种改进算法

在管道中两个测点处实时测量所得的数据作相关分析，可以测定试件的吸声性能，本文发展了一种复反射系数的改进算法，引入以功率谱为基础的Ｇ函数来代替流行的传递函数，可使随机误差明显降低，并可保证所得实验结果合理，可靠。     

沙质沉积物反射系数的宽带测量方法

海底作为海洋波导的重要边界,其反射系数对声传播起着至关重要的作用。为了快速而准确地获取海底反射系数,提出了一种基于脉冲压缩技术的测量方法,对接收信号进行压缩来提取反射波。该方法不仅可以克服实验过程中经常遇到的多途干扰,而且测量过程简单,可以同时获得测量频带内所有频点的反射系数,即实现了对反射系数的宽带测量。在实验室环境条件下,针对广泛覆盖于大陆架及沿海地区的沙质沉积物反射系数进行了测量,通过20~60 kHz频率范围内窄带和宽带测量结果的比较发现,宽带测量结果在频带边缘存在一定的偏差和起伏,但可以采用增大发射信号带宽提高压缩效果或用适当的窗函数压低旁瓣的方法来得到频率范围更宽、精度更高的测量结果。     

Traceable correction method for complex reflection coefficient using calculable air line impedance standards

A correction method for the measurement of complex reflection coefficients using vector network analyzers is presented. The method is based on the invariance property of the cross ratio of the bilinear transformation and is traceable to calculable air line impedance standards. The application of an accurate diameter measurement system (laser micrometer and air gauge system) allows the treatment of air lines as inhomogeneous transmission lines consisting of a cascade connection of a number of equally spaced line sections. Each line section is assumed to be homogeneous and characterized by the characteristic impedance and the propagation constant, which are traceable to the measured diameter values. The presented procedure is an extension and improvement of prior work using the same principle of correction, whereas the air lines were specified by nominal diameter and length values. The aim of this work is to enhance the accuracy of the correction or to reduce the requirements of the manufacturing tolerances of air lines. The derived theory is accompanied by a brief uncertainty consideration and by measurement results obtained by PC-7 air lines at frequencies up to 18 GHz and by numerical simulation results for a hypothetical air line.     

Using microwave multimeters to measure the complex reflection coefficient of waveguides

The application of a microwave multimeter in precision measurements of complex reflection coefficients is described. Experimental results are cited.Translated from Izmeritel'naya Tekhnika, No. 6, pp. 47–49, June, 1995.     

An interpolation scheme for precision intermediate frequency reflection coefficient measurement

The theoretical basis and the practical implementation of an objective approach to voltage reflection coefficient (VRC) interpolation in the development of radio frequency coaxial impedance standards are presented. Variants of the scheme apply to three impedances commonly used to calibrate vector reflectometers and vector network analyzers. The scheme permits impedance standards to be characterized in the intermediate frequency (IF) range (where, broadly speaking, IF refers to the frequency range 10 kHz to 100 MHz) in terms of their VRC values and associated measurement uncertainties. The scheme allows the use of commercially available devices (rather than specially constructed calculable devices) as impedance standards with VRC values traceable to national measurement standards. Measurement results are presented which illustrate the capabilities of the scheme.     

Development of an interferometer for measurement of the diffusion coefficient of miscible liquids

A common-path interferometer (CPI) system was developed to measure the diffusivity of transparent liquid pairs by real-time visualization of the concentration gradient profile. The CPI is an optical technique that can be used to measure changes in the gradient of the refractive index of transparent materials. The CPI is a shearing interferometer that shares the same optical path from a laser light source to the final imaging plane. Molecular diffusivity of liquids can be determined by use of physical relations between changes in the optical path length and the liquid phase properties. The data obtained by this interferometer are compared with similar results from other techniques. This demonstrates that the instrument is reliable for measurement of the diffusivity of miscible liquids and allows the system to be compact and robust. It can also be useful for studies in interface dynamics as well as other applications in a low-gravity environment.     

Viscosity measurement using drop coalescence in microgravity

We present in here validation studies of a new method for application in microgravity environment which measures the viscosity of highly viscous undercooled liquids using drop coalescence. The method has the advantage of avoiding heterogeneous nucleation at container walls caused by crystallization of undercooled liquids during processing. Homogeneous nucleation can also be avoided due to the rapidity of the measurement using this method. The technique relies on measurements from experiments conducted in near zero gravity environment as well as highly accurate analytical formulation for the coalescence process. The viscosity of the liquid is determined by allowing the computed free surface shape relaxation time to be adjusted in response to the measured free surface velocity for two coalescing drops. Results are presented from two sets of validation experiments for the method which were conducted on board aircraft flying parabolic trajectories. In these tests the viscosity of a highly viscous liquid, namely glycerin, was determined at different temperatures using the drop coalescence method described in here. The experiments measured the free surface velocity of two glycerin drops coalescing under the action of surface tension alone in low gravity environment using high speed photography. The liquid viscosity was determined by adjusting the computed free surface velocity values to the measured experimental data. The results of these experiments were found to agree reasonably well with the known viscosity for the test liquid used.     

Wide bandwidth measurement of complex permittivity of liquids usingcoplanar lines

A technique for measurement of complex permittivity of dielectric materials, using coplanar waveguide cells is presented. A complete solution of the forward and inverse problems, based on a quasi-TEM propagation model, is given. Unlike other methods, our method allows solution of the inverse problem without the use of iterative or graphical techniques. The model accounts for the various discontinuities along the line. Several advantages result from this approach. Measurement over a broad frequency range is possible, limited only by dispersive effects. It is suitable for solids not amenable to machining for cavity measurements, and there need not be any contact     

Tests of backscatter coefficient measurement using broadband pulses

An adaptation to a data reduction method is outlined for determining backscatter coefficients, eta, when broad-bandwidth pulses are employed. The accuracy of these eta values is assessed with well-characterized phantoms, for which backscatter coefficients based on their physical properties have been independently calculated. One phantom produces Rayleigh-like scattering, where the backscatter coefficient varies smoothly with frequency over the analysis bandwidth. A second phantom exhibits local maxima and minima in the scattering function versus frequency due to the presence of millimeter-sized graphite gel spheres in a gel background. The method was found to produce accurate results using time gate durations as small as 2 mus, although better accuracy is obtained for longer gate durations, particularly when the sample exhibits resonance peaks in backscatter versus frequency. Use of a Hamming window in place of a rectangular window extends the accuracy near the upper and lower limits of the frequency range.     

Metrological analysis of the algorithm of precision measurements of the complex reflection coefficient in waveguide channels

The block diagram of a system for precision measurement of the complex reflection coefficient in waveguide channels and a model of the measurement procedure are discussed. The results of metrological analysis obtained by computer simulation are reported. Translated from Izmeritel'naya Tekhnika, No. 2, pp. 49–52, February, 1999.     

Measurement of the complex refractive index of liquids in the infrared using spectroscopic attenuated total reflection ellipsometry: correction for depolarization by scattering

With spectroscopic ellipsometry one can measure the real and imaginary parts of the refractive index of a medium simultaneously. To determine this index in the infrared for a number of technical liquids, use was made of attenuated total internal reflection at the glass-liquid interface of a specially designed prism. This attenuated total reflection approach warrants minimal signal loss and is, for strongly absorbing liquids, the only way to measure the complex refractive index. A surprising phenomenon, observed when BK-7 prism glass was used, is scattering in the vicinity of the absorption wavelengths of the glass. A simple model that can be used to describe the relations among absorption, scattering, and depolarization was successfully used to correct the measurements. Refractive indices for demineralized water, Freon 113, heptane, benzene, gas oil, and crude oil in the wave number range from 5000 to 10,000 cm(-1) (1-2 μm) are presented.     

Acoustic methods for obtaining the pressure reflection coefficient from a buffer rod based measurement cell

The known acoustic methods for obtaining the pressure reflection coefficient from a buffer rod based measurement cell are presented, along with 2 new generic approaches for measuring the pressure reflection coefficient using 2 buffer rods enclosing the liquid to be characterized in a symmetrical arrangement. An acoustic transducer is connected to each of the buffer rods. The generic approaches are divided into a relative amplitude approach and a mixed amplitude approach. For the relative amplitude approach, families of 4, 5, or 6 echo signals can be used to obtain the pressure reflection coefficient. The mixed amplitude approach uses specific information about the transducers and/or the electronics sensitivities in receive mode to obtain the pressure reflection coefficient using families of 3, 4, 5, or 6 echo signals. Some of the new methods from the relative amplitude approach imply a reduced uncertainty relative to the previously known ABC method. The effect of the liquid attenuation, digitizer bit resolution, and the signal-to-noise ratio on the uncertainty characteristics of the pressure reflection coefficient are discussed, along with a discussion of the suitability of the various methods for different buffer materials.     

Viscosity of defined and undefined hydrocarbon liquids calculated using an extended corresponding-states model

We predict the viscosity of petroleum fractions using extended corresponding states. Our model builds upon the TRAPP procedure, which is the most advanced approach to predict transport properties of straight-chain nonpolar hydrocarbons and their mixtures. We perform comparisons with experimental viscosity data for pure hydrocarbons, treating them as nonstandard components; we find deviations of 10–15%. We also extend the model to predict the transport properties of petroleum fractions and compare with an experimental database of more than 80 crude oils, including highly aromatic petroleum fractions. The model predicts the viscosity of the crude oil fractions within experimental uncertainty.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Errors in measuring the reflection coefficient of ground

Conclusions The ground reflection factor can be determined in the range of 25–400 Mc from relative measurements of field strength in a vertical plane.In order to raise the accuracy of measurements it is necessary that the load impedance of the receiving dipole should be considerably larger than its input impedance and that its height should vary in the range of 0.5h₂2 (for h₁5=const).     

A method of determining the instrumental component of the error of high-precision equipment for measuring complex reflection coefficient

A method of constructing an adequate model of the instrumental component of the uneliminated systematic error of high-precision equipment for measuring complex reflection coefficient is considered, which enables the contradiction between the effort involved in realizing the model employed and the error in estimating the metrological characteristics of the equipment to be resolved. __________ Translated from Izmeritel’naya Tekhnika, No. 10, pp. 64–67, October, 2006.