Shell Perturbations of an Acoustic Thermometer Determined from Speed of Sound in Gas Mixtures

With the goal of achieving a better understanding of gas–shell coupling perturbations in the acoustic resonators used at INRiM for the determination of the Boltzmann constant, we measured the variation of their acoustic and microwave resonances induced by changing the composition of a binary He–Ar mixture which filled the cavity at constant temperature and pressure. As a consequence of the progressive dilution of a sample of initially pure He with Ar, the radial acoustic modes of the resonator spanned decreasing frequency intervals, partially overlapping, for several modes. In addition to the expected breathing mode of the shell, the results evidenced the presence of several other shell resonances at lower and higher frequencies, confirming that the elastic response of the assembled resonator significantly differs from that of a simple spherical shell. Experimental results are reported for two resonators which differ in design, dimensions, and constructing material. In spite of their being preliminary and susceptible of significant improvement, these results favor the interpretation of acoustic thermometry measurements with pure gases.     

Developing a Microhumidity Generator for Methane-Ethane Gas Mixtures

Methods are examined for drying natural gases and checking the degree of dryness by means of an experimental equipment. The working conditions are defined for a generator giving trace concentrations in a methane-ethane gas mixture.     

Measuring Speed of Sound and Thermal Diffusivity in the Diamond-Anvil Cell

Transient grating spectroscopy in the diamond-anvil high-pressure cell permits, in favorable cases, determination of the equation of state and thermal transport properties of fluids at high temperatures and pressures. Measurements of the speed of sound and thermodynamic properties of aqueous Na₂SO₄to 3.4 GPa at 300°C and the thermal diffusivity of oxygen are reported as examples of the application of this technique.     

Photoacoustic Excitation of the Interior Surface of an Acoustic Resonator for Speed of Sound Measurement

A remote optical technique is proposed as a valid alternative to conventional electroacoustic excitation of sound inside an acoustic resonator. Efficient photoacoustic conversion of an amplitude-modulated light beam, impinging on the interior solid surface of the resonator wall, into a stationary acoustic field is achieved. The reported results were obtained in a cylindrical stainless steel resonator with a volume of about 572 cm³; the radiation source is the 514 nm line of an Ar⁺ laser. The laser beam can be focused on different points of the resonator internal surface. The signal frequency dependence is well interpreted in terms of gas-microphone detection theory. The precision obtained in the measurement of resonance frequency f _N and halfwidth g _N of the cavity modes is on the order of 10^-6 of f _N and is likely to be further improved by minor refinements of the experimental apparatus.     

Finite Element Simulation of Photoacoustic Pressure in a Resonant Photoacoustic Cell Using Lossy Boundary Conditions

The finite-element method (FEM) is used to simulate the photoacoustic signal in a cylindrical resonant photoacoustic cell. Simulations include loss effects near the cell walls that appear in the boundary conditions for the inhomogeneous Helmholtz equation governing the acoustic pressure. Reasonably good agreement is obtained between theoretical results and experimental data. However, it was anticipated that loss mechanisms other than viscous and thermal boundary losses occur and should be included. Nevertheless, the feasibility to use FEM together with the derived boundary conditions to simulate the photoacoustic signal was demonstrated and good agreement with experiments for the actual resonance frequency and the quality factor of the cell was obtained despite its complicated geometry.     

Concentration-Dependent Diffusion Coefficients of Binary Gas Mixtures Using a Loschmidt Cell with Holographic Interferometry

An improved experimental setup and data evaluation procedure are presented for a Loschmidt cell combined with interferometry to measure concentration-dependent binary diffusion coefficients. We overcome long-standing discrepancies about the concentration dependence found in the literature. The systematic analysis of the residuals from parameter estimation enabled the improvement of the experimental setup and the identification of relevant fitting parameters. In particular, we found that it is crucial to account for uncertainties (1) in the initial conditions, (2) in the thermal stability of the optical setup, and (3) in camera calibration. The improved experimental setup and data evaluation procedure are validated with diffusion measurements of the system helium–krypton. The concentration dependence of the diffusion coefficient is successfully determined from multiple experiments with gas mixtures of various initial compositions in the half-cells of the Loschmidt cell. The agreement with literature data and the excellent quality of fit allow for high confidence in the results. In Part II of this paper (Wolff et al., in Int J Thermophys, 2018,  https://doi.org/10.1007/s10765-018-2451-7), the improved measurement setup is combined with a refined diffusion model to determine concentration-dependent diffusion coefficients from single measurements of mixing pure gases.     

First Sound in Mixtures of Trapped Bosons and Fermions

We study the collisional density oscillations of a mixture of bosonic and fermionic potassium atoms confined in a spherical trap at zero temperature. We find that the lowest-lying fluctuations of the whole system occupy either a non-overlapping spatial domain or overlap with opposite velocities. For moderate number of particles, we compare these results to previous calculations within a random-phase approximation.     

Probing Sound Speed of an Optically-Trapped Bose Gas with Periodically Modulated Interactions by Bragg Spectroscopy

A Bose–Einstein condensate (BEC) with periodically modulated interactions (PMI) has emerged as a novel kind of periodic superfluid, which has been recently experimentally created using optical Feshbach resonance. In this paper, we are motivated to investigate the superfluidity of a BEC with PMI trapped in an optical lattice (OL). In particular, we explore the effects of PMI on the sound speed and the dynamical structure factor of the model system. Our numerical results, combined with the analytical results in both the weak-potential limit and the tight-binding limit, have shown that the PMI can strongly modify the sound speed of a BEC. Moreover, we have shown that the effects of PMI on the sound speed can be experimentally probed via the dynamic structure factor, where the excitation strength toward the first Bogoliubov band exhibits a marked difference from the non-PMI one. Our predictions of the effects of PMI on the sound speed can be tested using the Bragg spectroscopy.     

Second Sound Contribution to Temperature Gradient Evolution in Superfluid Mixtures

Starting from the full system of hydrodynamic equations for helium isotopic mixtures, the problem of temperature and concentration relaxation is solved. The limiting case of ultralow temperatures, when the contribution of thermal excitations can be neglected, is considered. A comparison with the experimental data is carried out.      

An Improved Algorithm of Measurements of the Volume of Natural Gas in Gas Supply Using Measuring Systems with a Calculator

An algorithm for the measurement and calculation of the mean daily and average monthly volume of natural gas in a measuring system with a constricting device and a calculator, which takes into account the need to combine in time the channels for measuring the excess and barometric pressures, the gas temperature and density under standard conditions is proposed, which eliminates the systematic error inherent in existing methods of making measurements.     

Diffusion of Methylene Blue in Phantoms of Agar Using a Photoacoustic Technique

In this work, the kinetics of diffusion of methylene blue in agar aqueous solution is studied using a photoacoustic technique. Two agar phantoms solutions in water with a relation of mass/volume of 0.01% and 0.05% were analyzed. The study was performed using a modified Rosencwaig photoacoustic cell that is enclosed by transparent windows, on both sides. The sample is deposited directly on top of the upper window. A red light beam, at a fixed modulation frequency, is sent through the lower window illuminating the sample and inducing the photoacoustic effect inside the closed chamber of the cell. At the beginning of the experiment, a droplet of 100μL of agar solution is deposited; afterwards, the signal stabilizes, and 10μL of methylene blue aqueous solution (0.0125 g · mL^?1) is added to the surface of the agar. During the first seconds of the experiment, the photoacoustic signal amplitude increases followed by a gradual and long decay. Results for modulation frequencies in the range from 10Hz to 80Hz for both agar concentrations are presented. A simple theoretical approach is presented to analyze the experimental data. It is demonstrated that the kinetics of the process can be parameterized as a function of the changes of an effective optical absorption coefficient. From these results, the characteristic time, in which the dye diffusion process stabilizes, is obtained. It is found that this time is larger for samples with a higher agar concentration. These differences provide important results for biomedical sciences in which agar gels are used as phantoms resembling some of the properties of living organs and tissues.     

High-Resolution Shaft Speed Measurements Using a Microcomputer

A Motorola 6800 microcomputer has been applied to the measurement of instantaneous angular velocity at one degree intervals on a pair of coupled shafts. Velocity is determined by accurately measuring the time intervals between pulses produced by shaft encoders. Variations in relative velocity can be detected when precision timing is available. Using an essentially software approach time can be measured in 30-?s increments. The number of instructions in the timing loop can be reduced by adding a small amount of hardware to permit parallel data acquisition. Time increments of 12?s were obtained using inputs from two shafts. The results of measurements on a pair of shafts coupled by noncircular gears is included as an example.     

A New Development of Photoacoustic Detection for Microchip-CE Using a Simple Pick-up Device

A new approach to laser-based non-fluorescence detection was developed for microchip-capillary electrophoresis (microchip-CE) utilizing an electrical pick-up device. An intensity-modulated laser beam was irradiated on the microchip, and an acoustic wave was then generated by the periodic thermal expansion. Detection of signals was performed by the measurement of the induced electromagnetic wave with the use of a pickup device. The signal magnitude showed a linear relationship with the laser power and applied voltage, while the acoustic signal displayed a linear dependence on the concentration of the sample over a wide range. The separation of dye mixtures is achieved by the use of this new detection method for microchip-CE.     

Second Sound Measurements Near the Tricritical Point in 3He-4He Mixtures

In this paper we discuss a pulsed second sound experiment, aimed at determining accurately the critical exponent , and the predicted logarithmic correction to scaling, for the superfluid density along a tricritical path in the ³ He- ⁴ He phase diagram. We present an accurate estimate for the limits for closest approach to the tricritical point, as set by gravitationally induced sample inhomogeneities and finite size effects, and discuss some of the complications associated with measurements close to the tricritical point.     

Pulsed Nonlinear Sound in Superfluid 4He and 4He-3He Mixtures

Sound modes in ⁴ He and ⁴ He- ³ He mixtures which arise out of the two-fluid equations are made up up a vector convective flow and scalar temperature changes. A method for modeling nonlinear pulses of sound with geometric approximations to the vector and scalar components has recently been applied towards understanding nonlinear second sound near the lambda point. ⁶ The same method may be used, in general, for modeling linear and nonlinear sounds in Helium II. We demonstrate with a model for nonlinear second sound pulses in ³ He- ⁴ He mixtures and compare the results to experimental observations.     

10MW高温气冷堆蒸汽发生器实验参数测量采集系统

在对10MW高温气冷堆蒸气发生器流体流动稳定性进行理论分析的基础上,设计了该实验的参数测量系统。整个实验系统为全尺寸1:1工程模拟。通过对其分析,完成了动态信号的有效捕捉。实现微机的并联处理,实时监视系统设备的运行参数;完成了关键参数专用流量测量和在特殊环境下信号引出结构的设计。经过长时间的实验检验,其系统性能稳定可靠。