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Page 811 of 936 Results 8101 - 8110 of 9356

Davor Živko, Vedran Bilas
Analysis of individual PEM fuel cell operating parameters for design of optimal measurement and control instrumentation

Number of applications, power span and implementation areas for PEM (proton exchange membrane) fuel cells are increasing daily. All these applications require adequate measurement and control that will ensure optimal functionality of PEM fuel cell. Number of measuring quantities, especially in high – power systems, is numerous so pricing of measurement and control equipment is appreciable. In this paper we have implemented simple PEM fuel cell model and analyzed individual influences of fuel cell operating parameters, like temperature, partial pressures and membrane humidity. We have found out that influence of each operating parameter on fuel cell’s power output is different. Therefore we can design measurement and control equipment with different complexity. With that we can ensure optimal control of fuel cell’s output power and efficiency without use of costly equipment.

Matei Mihaela Florentina, Mihai Petruta, Claudia Popescu, Mihai Octavian Popescu
Testing and shaping for the performance of a system, used for solar energy production

The solar energy, like a permanent source of energy, is the optimal solution, and also a very important solution, for our investigations, because this source could solve a part of the problems related with inaccesibility and the high costs. A long series of projects are developing now, taking into consideration the importance of this problem. At University Politehnica of Bucharest, like a result of the PV Enlargement project exists a photovoltaic station. The actual system is analysed and a lot of conclusions are made, concerning the efficiency (as a result of these investigations we obtain an nergy excess of 73%) and the improving variances.

Daniel Belega, Dominique Dallet
Measurement of the Sinewave RMS Value in Noncoherent Sampling Mode

In this paper, a method to estimate the rms value of a noncoherent sampled sinewave by means of the formula used in an AC analog electronic voltmeter is presented. Adding some modifications on the algorithm, the accuracy obtained in this case was improved. This method is well suited for real-time applications in which the measurement of the sinewave rms value with relative high accuracy is sufficiently. The performances of the method proposed are proved by means of computer simulation as well as experimental results.

C. A. Nieto de Castro
THERMAL CONDUCTIVITY OF MOLTEN MATERIALS. IS EXPERIMENT NECESSARY?

The knowledge on the thermal conductivity of molten materials (salts, metals, semiconductors, polymers) is very scarce, both from the experimental and theoretical points of view. Knowing of the difficulty in obtaining accurate experimental data for most liquids, the task is uncouthly more difficult when the measurements are to be performed at medium and high temperatures with materials that are corrosive, easily reacting and good heat transfer media. Convective and radiative heat transfer effects affect in particular thermal conductivity measurements at high temperatures. These facts also make difficult theoretical calculations using molecular/ionic theories and drastic approximations, both from the phenomenological side, to the force field between particles, restrict the validity of the results obtained. Computer simulations are a possible alternative to overcome these problems and its development in recent years is noteworthy, induced by the improvements in theory, algorithms and computer hardware. Their applications to the study of molten materials have been very limited and with results of questionable validity. However, it was recently possible to apply equilibrium molecular dynamics simulations to the calculation molten salts thermal conductivity with a reasonable success. In this lecture a short review of the field will be presented, with especially emphasis in the actual situation, challenges faced and foreseen solutions, including microgravity experiments.

Bob Hardy
TRUST BUT VERIFY – PRACTICAL APPROACHES TO HUMIDITY GENERATION AND MEASUREMENT

Construction of humidity generators is a common undertaking in many of today's national metrology institutes and commercial calibration laboratories. A generator based on theoretical methods and statistical uncertainty expectations is often trusted as a laboratories primary humidity reference. While it is important to follow sound design theory, and compute statistical estimates of the output from a generator, verification is an equally powerful tool that can help to make even the weakest of designs more trustworthy. Generator designs will be reviewed in an effort to avoid common obstacles while capitalizing on a few simple and practical improvements. Testing and verification will also be considered with a focus on condensation hygrometry.

Howard W. Yoon, Charles E. Gibson, David W. Allen, Robert D. Saunders, Maritoni Litorja, Steven W. Brown, George P. Eppeldauer, Keith R. Lykke
THE REALIZATION AND THE DISSEMINATION OF THE DETECTOR-BASED KELVIN

In the International Temperature Scale of 1990 (ITS-90), temperatures above the freezing temperature of silver are determined with radiation thermometers calibrated using spectral radiance ratios to one of the Ag-, Au- or Cu-freezing temperature blackbodies and the Planck radiance law. However, due to the use of spectral radiance rats, the temperature uncertainties of the ITS-90 increase as the square of the temperature ratios. Recent acoustic- gas thermometry measurements have also shown that the underlying thermodynamic temperatures used in the radiance ratios in determining the Ag- and Au-fixed point temperatures could be in error. Since the establishment of ITS-90, much progress has been made in the development of radiation thermometers and blackbody sources. Cryogenic electrical-substitution radiometry is widely used in detector and radiometer calibrations, and stable, high-temperature metal-carbon eutectic blackbodies are under development. Radiation thermometers can be calibrated for absolute radiance responsivity, and blackbody temperatures determined from measurement of optical power without the use of any fixed points thus making possible direct dissemination of thermodynamic temperatures. We show that these temperatures can be measured with lower final uncertainties than the ITS-90 derived temperatures. We have shown that these “Absolute Pyrometers” can be used to determine the thermodynamic temperatures of the ITS-90 fixed points as well as also being used in bilateral comparisons of temperature scales. Many leading national measurement institutes are already utilizing detector- based temperatures in establishing spectroradiometric source scales. We believe, that due to these developments, the international temperature scale should be revised so that a thermodynamic temperature scale can be directly disseminated.

P. Marcarino, G. Bonnier
TEMPERATURE AMPLIFIER BY MEANS OF COUPLED GAS-CONTROLLED HEAT-PIPES

BNM-INM and IMGC-CNR have been working during many years in the field of Gas-Controlled Heat-Pipes (GCHPs). These devices have been specifically developed for accurate temperature measurements, with ever improving thermal characteristics during the last 30 years. A GCHP is based on the thermodynamic properties of the liquid-vapor transition of a given working fluid under a controlled pressure. The measuring zones of the more recent GCHPs, connected to an accurate pressure controlled line, present a temperature uniformity and stability at the millikelvin level in a very large range of temperature. The basic principles of GCHPs are reviewed and the most important results achieved by using this device in thermometry are presented. By applying the same pressure to several GCHPs using different working fluids, the temperature in one GCHP is thermodynamically related to the temperature in another GCHP. It means that any temperature of a given working fluid, i.e. between 240 °C and 400 °C for a mercury GCHP, is able to be "amplified" in order to establish an unique and very reproducible higher temperature in another working fluid, i.e. between 660 °C and 962 °C for a sodium GCHP. This instrument, called “Temperature Amplifier” (TA), allows a considerable improvement in the calibration process of SPRTs at high temperature. Indeed, the operating temperature of the reference SPRT in the low temperature GCHP can be limited to 400 °C, and, consequently, the stability and reproducibility in the high temperature GCHP are largely improved. The experimental results lead to the possibility to use the TA above the Aluminum point as a possible alternative instrument in a future temperature scale.

Joachim Fischer, Bernd Fellmuth, Joachim Seidel, Wolfgang Buck
TOWARDS A NEW DEFINITION OF THE KELVIN: WAYS TO GO

The present definition of the kelvin links the unit of temperature with a material property. It would be more consistent with the current approaches to other base units to fix the value of the Boltzmann constant k, in- stead. For this purpose, k must first be determined with distinctly lower uncertainty. This paper considers ex- perimental methods having potential to contribute to the determination of k.

Shang Wei Lu, Li Tao, Cheng Yong Pei, Wang Yan, Qian Zhao Jun
50 kNm TORQUE STANDARD MACHINE

The paper introduces the working principle of dead-weight balance type 50kNm torque standard machine , which was developed recently. The paper describes in detail the key technique , etc. such as the knife edge supporting technique with high load , the force magnifing technique , the alignment technique and the computer control automatic loading technique , etc. The paper analyses the uncertainty of the machine and the compared and verifyed data with LNE . The uncertainty of the 50kNm torque standard machine arrives at 0.023%(tp=3.1).

Page 811 of 936 Results 8101 - 8110 of 9356